WO2025157235A1

WO2025157235A1 - Method, apparatus, and medium for video processing

Info

Publication number: WO2025157235A1
Application number: PCT/CN2025/074461
Authority: WO
Inventors: Lei Zhao; Kai Zhang; Li Zhang
Original assignee: Douyin Vision Co Ltd; ByteDance Inc
Current assignee: Douyin Vision Co Ltd; ByteDance Inc
Priority date: 2024-01-24
Filing date: 2025-01-23
Publication date: 2025-07-31
Anticipated expiration: 2026-07-24

Abstract

Embodiments of the present disclosure provide a solution for video processing. A method for video processing is proposed. In the method, for a conversion between a current video unit of a video and a bitstream of the video, a plurality of intra modes for a current video unit of the video is determined. The current video unit is coded in a geometric partitioning mode (GPM). A plurality of transform kernel sets is determined based on the plurality of intra modes. A transform kernel set of the plurality of transform kernel sets comprises at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set. The conversion is performed based on the plurality of transform kernel sets.

Description

METHOD, APPARATUS, AND MEDIUM FOR VIDEO PROCESSING

FIELDS

Embodiments of the present disclosure relates generally to video processing techniques, and more particularly, to low-frequency non-separable transform (LFNST) and/or non-separable primary transform (NSPT) .

BACKGROUND

In nowadays, digital video capabilities are being applied in various aspects of peoples’ lives. Multiple types of video compression technologies, such as MPEG-2, MPEG-4, ITU-TH. 263, ITU-TH.264/MPEG-4 Part 10 Advanced Video Coding (AVC) , ITU-TH. 265 high efficiency video coding (HEVC) standard, versatile video coding (VVC) standard, have been proposed for video encoding/decoding. However, coding efficiency of video coding techniques is generally expected to be further improved.

SUMMARY

Embodiments of the present disclosure provide a solution for video processing.

In a first aspect, a method for video processing is proposed. The method comprises: determining, for a conversion between a current video unit of a video and a bitstream of the video, a plurality of intra modes for the current video unit, the current video unit being coded in a geometric partitioning mode (GPM) ; determining a plurality of transform kernel sets based on the plurality of intra modes, a transform kernel set of the plurality of transform kernel sets comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and performing the conversion based on the plurality of transform kernel sets. The method in accordance with the first aspect of the present disclosure can improve the coding efficiency and coding quality.

In a second aspect, another method for video processing is proposed. The method comprises: determining, for a conversion between a current video unit of a video and a bitstream of the video, a target intra mode for the current video unit; determining at least one transform kernel set for inter coding based on the target intra mode, the at least one transform kernel set comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and performing the conversion based on the at least one transform kernel set. The method in accordance with the second aspect of the present disclosure can improve the coding efficiency and coding quality.

In a third aspect, another method for video processing is proposed. The method comprises: determining, for a conversion between a current video unit of a video and a bitstream of the video, at least one merge list for the current video unit for an inter transform, the inter transform comprising at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) ; and performing the conversion based on the at least one merge list. The method in accordance with the third aspect of the present disclosure can improve the coding efficiency and coding quality.

In a fourth aspect, an apparatus for video processing is proposed. The apparatus comprises a processor and a non-transitory memory with instructions thereon. The instructions upon execution by the processor, cause the processor to perform a method in accordance with the first, second, or third aspect of the present disclosure.

In a fifth aspect, a non-transitory computer-readable storage medium is proposed. The non-transitory computer-readable storage medium stores instructions that cause a processor to perform a method in accordance with the first, second, or third aspect of the present disclosure.

In a sixth aspect, another non-transitory computer-readable recording medium is proposed. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: determining a plurality of intra modes for a current video unit of the video, the current video unit being coded in a geometric partitioning mode (GPM) ; determining a plurality of transform kernel sets based on the plurality of intra modes, a transform kernel set of the plurality of transform kernel sets comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and generating the bitstream based on the plurality of transform kernel sets.

In a seventh aspect, another non-transitory computer-readable recording medium is proposed. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: determining a plurality of intra modes for a current video unit of the video, the current video unit being coded in a geometric partitioning mode (GPM) ; determining a plurality of transform kernel sets based on the plurality of intra modes, a transform kernel set of the plurality of transform kernel sets comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; generating the bitstream based on the plurality of transform kernel sets; and storing the bitstream in a non-transitory computer-readable recording medium.

In an eighth aspect, another non-transitory computer-readable recording medium is proposed. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: determining a target intra mode for a current video unit of the video; determining at least one transform kernel set for inter coding based on the target intra mode, the at least one transform kernel set comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and generating the bitstream based on the at least one transform kernel set.

In a ninth aspect, a method for storing a bitstream of a video is proposed. The method comprises: determining a target intra mode for a current video unit of the video; determining at least one transform kernel set for inter coding based on the target intra mode, the at least one transform kernel set comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; generating the bitstream based on the at least one transform kernel set; and storing the bitstream in a non-transitory computer-readable recording medium.

In a tenth aspect, a method for storing a bitstream of a video is proposed. The method comprises: determining at least one merge list for a current video unit of the video for an inter transform, the inter transform comprising at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) ; and generating the bitstream based on the at least one merge list.

In an eleventh aspect, a method for storing a bitstream of a video is proposed. The method comprises: determining at least one merge list for a current video unit of the video for an inter transform, the inter transform comprising at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) ; generating the bitstream based on the at least one merge list; and storing the bitstream in a non-transitory computer-readable recording medium.

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Through the following detailed description with reference to the accompanying drawings, the above and other objectives, features, and advantages of example embodiments of the present disclosure will become more apparent. In the example embodiments of the present disclosure, the same reference numerals usually refer to the same components.

Fig. 1 illustrates a block diagram that illustrates an example video coding system, in accordance with some embodiments of the present disclosure;

Fig. 2 illustrates a block diagram that illustrates a first example video encoder, in accordance with some embodiments of the present disclosure;

Fig. 3 illustrates a block diagram that illustrates an example video decoder, in accordance with some embodiments of the present disclosure;

Fig. 4 illustrates the positions of spatial and temporal neighboring blocks used in AMVP/merge candidate list construction;

Fig. 5 illustrates the positions of non-adjacent candidate in ECM;

Fig. 6A illustrates the control point based 4 parameter affine motion model;

Fig. 6B illustrates the control point based 6 parameter affine motion model;

Fig. 7 illustrates the affine MVF per subblock;

Fig. 8 illustrates the locations of inherited affine motion predictors;

Fig. 9 illustrates the control point motion vector inheritance;

Fig. 10 illustrates the locations of candidates position for constructed affine merge mode;

Fig. 11 illustrates the spatial neighbors for deriving inherited affine merge candidates and the spatial neighbors for deriving constructed affine merge candidates;

Fig. 12 illustrates from non-adjacent neighbors to constructed affine merge candidates;

Fig. 13 illustrates an example of generating an HAPC;

Fig. 14 illustrates the illustration of regression based affine merge candidate derivation;

Fig. 15 illustrates the template matching performs on a search area around initial MV;

Fig. 16 illustrates the template and the corresponding reference template;

Fig. 17 illustrates the template and reference template for block with sub-block motion using the motion information of the subblocks of current block;

Fig. 18 illustrates deriving sub-CU motion field obtained by applying a motion shift based on the neighboring motion information;

Fig. 19 illustrates the top and left neighboring blocks used in CIIP weight derivation;

Fig. 20 illustrates the CIIP_PDPC flowchart of the extended CIIP mode using PDPC;

Fig. 21A and Fig. 21B illustrate the division method for angular modes;

Fig. 22 illustrates the subblock templates generation of SbTMVP;

Fig. 23 illustrates the diamond regions in the search area;

Fig. 24 illustrates examples of the GPM splits grouped by identical angles;

Fig. 25 illustrates the uni-prediction MV selection for geometric partitioning mode;

Fig. 26 illustrates the exemplified generation of a bending weight w_0 using geometric partitioning mode;

Fig. 27 illustrates the spatial GPM candidates;

Fig. 28 illustrates the GPM template;

Fig. 29 illustrates the ramp function for the weights for GPM blending based on the displacement (d) from a predicted sample position to the GPM partitioning boundary and the blending area size (τ) ;

Fig. 30 illustrates the Low-Frequency Non-Separable Transform (LFNST) process;

Fig. 31 illustrates the SBT position, type and transform type;

Fig. 32 illustrates the ROI for LFNST16;

Fig. 33 illustrates the ROI for LFNST8;

Fig. 34 illustrates the discontinuity measure;

Fig. 35 illustrates a flowchart of a method for video processing in accordance with embodiments of the present disclosure;

Fig. 36 illustrates a flowchart of a method for video processing in accordance with embodiments of the present disclosure;

Fig. 37 illustrates a flowchart of a method for video processing in accordance with embodiments of the present disclosure; and

Fig. 38 illustrates a block diagram of a computing device in which various embodiments of the present disclosure can be implemented.

Throughout the drawings, the same or similar reference numerals usually refer to the same or similar elements.

DETAILED DESCRIPTION

Principle of the present disclosure will now be described with reference to some embodiments. It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present disclosure, without suggesting any limitation as to the scope of the disclosure. The disclosure described herein can be implemented in various manners other than the ones described below.

In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs.

References in the present disclosure to “one embodiment, ” “an embodiment, ” “an example embodiment, ” and the like indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an example embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

It shall be understood that although the terms “first” and “second” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the listed terms.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a” , “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” , “comprising” , “has” , “having” , “includes” and/or “including” , when used herein, specify the presence of stated features, elements, and/or components etc., but do not preclude the presence or addition of one or more other features, elements, components and/or combinations thereof.
Example Environment

Fig. 1 is a block diagram that illustrates an example video coding system 100 that may utilize the techniques of this disclosure. As shown, the video coding system 100 may include a source device 110 and a destination device 120. The source device 110 can be also referred to as a video encoding device, and the destination device 120 can be also referred to as a video decoding device. In operation, the source device 110 can be configured to generate encoded video data and the destination device 120 can be configured to decode the encoded video data generated by the source device 110. The source device 110 may include a video source 112, a video encoder 114, and an input/output (I/O) interface 116.

The video source 112 may include a source such as a video capture device. Examples of the video capture device include, but are not limited to, an interface to receive video data from a video content provider, a computer graphics system for generating video data, and/or a combination thereof.

The video data may comprise one or more pictures. The video encoder 114 encodes the video data from the video source 112 to generate a bitstream. The bitstream may include a sequence of bits that form a coded representation of the video data. The bitstream may include coded pictures and associated data. The coded picture is a coded representation of a picture. The associated data may include sequence parameter sets, picture parameter sets, and other syntax structures. The I/O interface 116 may include a modulator/demodulator and/or a transmitter. The encoded video data may be transmitted directly to destination device 120 via the I/O interface 116 through the network 130A. The encoded video data may also be stored onto a storage medium/server 130B for access by destination device 120.

The destination device 120 may include an I/O interface 126, a video decoder 124, and a display device 122. The I/O interface 126 may include a receiver and/or a modem. The I/O interface 126 may acquire encoded video data from the source device 110 or the storage medium/server 130B. The video decoder 124 may decode the encoded video data. The display device 122 may display the decoded video data to a user. The display device 122 may be integrated with the destination device 120, or may be external to the destination device 120 which is configured to interface with an external display device.

The video encoder 114 and the video decoder 124 may operate according to a video compression standard, such as the High Efficiency Video Coding (HEVC) standard, Versatile Video Coding (VVC) standard and other current and/or further standards.

Fig. 2 is a block diagram illustrating an example of a video encoder 200, which may be an example of the video encoder 114 in the system 100 illustrated in Fig. 1, in accordance with some embodiments of the present disclosure.

The video encoder 200 may be configured to implement any or all of the techniques of this disclosure. In the example of Fig. 2, the video encoder 200 includes a plurality of functional components. The techniques described in this disclosure may be shared among the various components of the video encoder 200. In some examples, a processor may be configured to perform any or all of the techniques described in this disclosure.

In some embodiments, the video encoder 200 may include a partition unit 201, a prediction unit 202 which may include a mode select unit 203, a motion estimation unit 204, a motion compensation unit 205 and an intra-prediction unit 206, a residual generation unit 207, a transform unit 208, a quantization unit 209, an inverse quantization unit 210, an inverse transform unit 211, a reconstruction unit 212, a buffer 213, and an entropy encoding unit 214.

In other examples, the video encoder 200 may include more, fewer, or different functional components. In an example, the prediction unit 202 may include an intra block copy (IBC) unit. The IBC unit may perform prediction in an IBC mode in which at least one reference picture is a picture where the current video block is located.

Furthermore, although some components, such as the motion estimation unit 204 and the motion compensation unit 205, may be integrated, but are represented in the example of Fig. 2 separately for purposes of explanation.

The partition unit 201 may partition a picture into one or more video blocks. The video encoder 200 and the video decoder 300 may support various video block sizes.

The mode select unit 203 may select one of the coding modes, intra or inter, e.g., based on error results, and provide the resulting intra-coded or inter-coded block to a residual generation unit 207 to generate residual block data and to a reconstruction unit 212 to reconstruct the encoded block for use as a reference picture. In some examples, the mode select unit 203 may select a combination of intra and inter prediction (CIIP) mode in which the prediction is based on an inter prediction signal and an intra prediction signal. The mode select unit 203 may also select a resolution for a motion vector (e.g., a sub-pixel or integer pixel precision) for the block in the case of inter-prediction.

To perform inter prediction on a current video block, the motion estimation unit 204 may generate motion information for the current video block by comparing one or more reference frames from buffer 213 to the current video block. The motion compensation unit 205 may determine a predicted video block for the current video block based on the motion information and decoded samples of pictures from the buffer 213 other than the picture associated with the current video block.

The motion estimation unit 204 and the motion compensation unit 205 may perform different operations for a current video block, for example, depending on whether the current video block is in an I-slice, a P-slice, or a B-slice. As used herein, an “I-slice” may refer to a portion of a picture composed of macroblocks, all of which are based upon macroblocks within the same picture. Further, as used herein, in some aspects, “P-slices” and “B-slices” may refer to portions of a picture composed of macroblocks that are not dependent on macroblocks in the same picture.

In some examples, the motion estimation unit 204 may perform uni-directional prediction for the current video block, and the motion estimation unit 204 may search reference pictures of list 0 or list 1 for a reference video block for the current video block. The motion estimation unit 204 may then generate a reference index that indicates the reference picture in list 0 or list 1 that contains the reference video block and a motion vector that indicates a spatial displacement between the current video block and the reference video block. The motion estimation unit 204 may output the reference index, a prediction direction indicator, and the motion vector as the motion information of the current video block. The motion compensation unit 205 may generate the predicted video block of the current video block based on the reference video block indicated by the motion information of the current video block.

Alternatively, in other examples, the motion estimation unit 204 may perform bi-directional prediction for the current video block. The motion estimation unit 204 may search the reference pictures in list 0 for a reference video block for the current video block and may also search the reference pictures in list 1 for another reference video block for the current video block. The motion estimation unit 204 may then generate reference indexes that indicate the reference pictures in list 0 and list 1 containing the reference video blocks and motion vectors that indicate spatial displacements between the reference video blocks and the current video block. The motion estimation unit 204 may output the reference indexes and the motion vectors of the current video block as the motion information of the current video block. The motion compensation unit 205 may generate the predicted video block of the current video block based on the reference video blocks indicated by the motion information of the current video block.

In some examples, the motion estimation unit 204 may output a full set of motion information for decoding processing of a decoder. Alternatively, in some embodiments, the motion estimation unit 204 may signal the motion information of the current video block with reference to the motion information of another video block. For example, the motion estimation unit 204 may determine that the motion information of the current video block is sufficiently similar to the motion information of a neighboring video block.

In one example, the motion estimation unit 204 may indicate, in a syntax structure associated with the current video block, a value that indicates to the video decoder 300 that the current video block has the same motion information as the another video block.

In another example, the motion estimation unit 204 may identify, in a syntax structure associated with the current video block, another video block and a motion vector difference (MVD) . The motion vector difference indicates a difference between the motion vector of the current video block and the motion vector of the indicated video block. The video decoder 300 may use the motion vector of the indicated video block and the motion vector difference to determine the motion vector of the current video block.

As discussed above, video encoder 200 may predictively signal the motion vector. Two examples of predictive signaling techniques that may be implemented by video encoder 200 include advanced motion vector prediction (AMVP) and merge mode signaling.

The intra prediction unit 206 may perform intra prediction on the current video block. When the intra prediction unit 206 performs intra prediction on the current video block, the intra prediction unit 206 may generate prediction data for the current video block based on decoded samples of other video blocks in the same picture. The prediction data for the current video block may include a predicted video block and various syntax elements.

The residual generation unit 207 may generate residual data for the current video block by subtracting (e.g., indicated by the minus sign) the predicted video block (s) of the current video block from the current video block. The residual data of the current video block may include residual video blocks that correspond to different sample components of the samples in the current video block.

In other examples, there may be no residual data for the current video block for the current video block, for example in a skip mode, and the residual generation unit 207 may not perform the subtracting operation.

The transform processing unit 208 may generate one or more transform coefficient video blocks for the current video block by applying one or more transforms to a residual video block associated with the current video block.

After the transform processing unit 208 generates a transform coefficient video block associated with the current video block, the quantization unit 209 may quantize the transform coefficient video block associated with the current video block based on one or more quantization parameter (QP) values associated with the current video block.

The inverse quantization unit 210 and the inverse transform unit 211 may apply inverse quantization and inverse transforms to the transform coefficient video block, respectively, to reconstruct a residual video block from the transform coefficient video block. The reconstruction unit 212 may add the reconstructed residual video block to corresponding samples from one or more predicted video blocks generated by the prediction unit 202 to produce a reconstructed video block associated with the current video block for storage in the buffer 213.

After the reconstruction unit 212 reconstructs the video block, loop filtering operation may be performed to reduce video blocking artifacts in the video block.

The entropy encoding unit 214 may receive data from other functional components of the video encoder 200. When the entropy encoding unit 214 receives the data, the entropy encoding unit 214 may perform one or more entropy encoding operations to generate entropy encoded data and output a bitstream that includes the entropy encoded data.

Fig. 3 is a block diagram illustrating an example of a video decoder 300, which may be an example of the video decoder 124 in the system 100 illustrated in Fig. 1, in accordance with some embodiments of the present disclosure.

The video decoder 300 may be configured to perform any or all of the techniques of this disclosure. In the example of Fig. 3, the video decoder 300 includes a plurality of functional components. The techniques described in this disclosure may be shared among the various components of the video decoder 300. In some examples, a processor may be configured to perform any or all of the techniques described in this disclosure.

In the example of Fig. 3, the video decoder 300 includes an entropy decoding unit 301, a motion compensation unit 302, an intra prediction unit 303, an inverse quantization unit 304, an inverse transformation unit 305, and a reconstruction unit 306 and a buffer 307. The video decoder 300 may, in some examples, perform a decoding pass generally reciprocal to the encoding pass described with respect to video encoder 200.

The entropy decoding unit 301 may retrieve an encoded bitstream. The encoded bitstream may include entropy coded video data (e.g., encoded blocks of video data) . The entropy decoding unit 301 may decode the entropy coded video data, and from the entropy decoded video data, the motion compensation unit 302 may determine motion information including motion vectors, motion vector precision, reference picture list indexes, and other motion information. The motion compensation unit 302 may, for example, determine such information by performing the AMVP and merge mode. AMVP is used, including derivation of several most probable candidates based on data from adjacent PBs and the reference picture. Motion information typically includes the horizontal and vertical motion vector displacement values, one or two reference picture indices, and, in the case of prediction regions in B slices, an identification of which reference picture list is associated with each index. As used herein, in some aspects, a “merge mode” may refer to deriving the motion information from spatially or temporally neighboring blocks.

The motion compensation unit 302 may produce motion compensated blocks, possibly performing interpolation based on interpolation filters. Identifiers for interpolation filters to be used with sub-pixel precision may be included in the syntax elements.

The motion compensation unit 302 may use the interpolation filters as used by the video encoder 200 during encoding of the video block to calculate interpolated values for sub-integer pixels of a reference block. The motion compensation unit 302 may determine the interpolation filters used by the video encoder 200 according to the received syntax information and use the interpolation filters to produce predictive blocks.

The motion compensation unit 302 may use at least part of the syntax information to determine sizes of blocks used to encode frame (s) and/or slice (s) of the encoded video sequence, partition information that describes how each macroblock of a picture of the encoded video sequence is partitioned, modes indicating how each partition is encoded, one or more reference frames (and reference frame lists) for each inter-encoded block, and other information to decode the encoded video sequence. As used herein, in some aspects, a “slice” may refer to a data structure that can be decoded independently from other slices of the same picture, in terms of entropy coding, signal prediction, and residual signal reconstruction. A slice can either be an entire picture or a region of a picture.

The intra prediction unit 303 may use intra prediction modes for example received in the bitstream to form a prediction block from spatially adjacent blocks. The inverse quantization unit 304 inverse quantizes, i.e., de-quantizes, the quantized video block coefficients provided in the bitstream and decoded by entropy decoding unit 301. The inverse transform unit 305 applies an inverse transform.

The reconstruction unit 306 may obtain the decoded blocks, e.g., by summing the residual blocks with the corresponding prediction blocks generated by the motion compensation unit 302 or intra-prediction unit 303. If desired, a deblocking filter may also be applied to filter the decoded blocks in order to remove blockiness artifacts. The decoded video blocks are then stored in the buffer 307, which provides reference blocks for subsequent motion compensation/intra prediction and also produces decoded video for presentation on a display device.

Some exemplary embodiments of the present disclosure will be described in detailed hereinafter. It should be understood that section headings are used in the present document to facilitate ease of understanding and do not limit the embodiments disclosed in a section to only that section. Furthermore, while certain embodiments are described with reference to Versatile Video Coding or other specific video codecs, the disclosed techniques are applicable to other video coding technologies also. Furthermore, while some embodiments describe video coding steps in detail, it will be understood that corresponding steps decoding that undo the coding will be implemented by a decoder. Furthermore, the term video processing encompasses video coding or compression, video decoding or decompression and video transcoding in which video pixels are represented from one compressed format into another compressed format or at a different compressed bitrate.
1 Brief Summary

This disclosure is related to video coding technologies. Specifically, it is about combined prediction method in video coding. The ideas may be applied individually or in various combination, to any video coding standard or non-standard video codec.
2 Introduction

The exponential increasing of multimedia data poses a critical challenge for video coding. To satisfy the increasing demands for more efficient compression technology, ITU-T and ISO/IEC have developed a series of video coding standards in the past decades. In particular, the ITU-T produced H. 261 and H. 263, ISO/IEC produced MPEG-1 and MPEG-4 visual, and the two organizations jointly developed the H. 262/MPEG-2 Video, H. 264/MPEG-4 Advanced Video Coding (AVC) , H. 265/HEVC and the latest VVC standards. Since H. 262/MPEG-2, hybrid video coding framework is employed wherein in intra/inter prediction plus transform coding are utilized.

Fig. 4 illustrates the positions of spatial and temporal neighboring blocks used in AMVP/merge candidate list construction.
2.1 MVP in video coding

Inter prediction aims to remove the temporal redundancy between adjacent frames, which serves as an indispensable component in the hybrid video coding framework. Specifically, inter prediction makes use of the contents specified by motion vector (MV) as the predicted version of the current to-be-coded block, thus only residual signals and motion information are transmitted in the bitstream. To reduce the cost for MV signaling, motion vector prediction (MVP) came into being as an effective mechanism to convey motion information. Early strategies simply use the MV of a specified neighboring block or the median MV of neighboring blocks as MVP. In H. 265/HEVC, competing mechanism was involved where the optimal MVP is selected from multiple candidates through rate distortion optimization (RDO) . In particular, advanced MVP (AMVP) mode and merge mode are devised with different motion information signaling strategy. With the AMVP mode, a reference index, a MVP candidate index referring to an AMVP candidate list and motion vector difference (MVD) is signaled. Regarding the merge mode, only a merge index referring to a merge candidate list is signaled, and all the motion information associated with the merge candidate is inherited. Both AMVP mode and merge mode need to construct MVP candidate list, and the details of the construction process for these two modes are described as follows.

AMVP mode: AMVP exploits spatial-temporal correlation of motion vector with neighboring blocks, which is used for explicit transmission of motion parameters. For each reference picture list, a motion vector candidate list is constructed by firstly checking availability of left, above temporally neighboring positions, removing redundant candidates and adding zero vector to make the candidate list to be constant length. For spatial motion vector candidate derivation, two motion vector candidates are eventually derived based on motion vectors of blocks located in five different positions as depicted in Fig. 4. The five neighboring blocks located at B0, B 1, B2, and A0, A1 are classified into two groups, where Group A includes the three above spatial neighboring blocks and Group B includes the two left spatial neighboring blocks. The two MV candidates are respectively derived with the first available candidate from Group A and Group B in a predefined order. For temporal motion vector candidate derivation, one motion vector candidate is derived based on two different collocated positions (bottom-right (C0) and central (C1) ) checked in order, as depicted in Fig. 4. To avoid redundant MV candidates, duplicated motion vector candidates in the list are abandoned. If the number of potential candidates is smaller than two, additional zero motion vector candidates are added to the list. Fig. 5 illustrates the positions of non-adjacent candidate in ECM.

Merge mode: Similar to AMVP mode, MVP candidate list for merge mode comprises of spatial and temporal candidates as well. For spatial motion vector candidate derivation, at most four candidates are selected with order A1, B1, B0, A0 and B2 after performing availability and redundant checking. For temporal merge candidate (TMVP) derivation, at most one candidate is selected from two temporal neighboring blocks (C0 and C1) . When there are not enough merge candidates with spatial and temporal candidates, combined bi-predictive merge candidates and zero MV candidates are added to MVP candidate list. Once the number of available merge candidates reaches the signaled maximally allowed number, the merge candidate list construction process is terminated.

In VVC, the construction process for merge mode is further improved by introducing the history-based MVP (HMVP) , which incorporates the motion information of previously coded blocks which may be far away from current block. In VVC, HMVP merge candidates are appended to merge list after the spatial MVP and TMVP. In this method, the motion information of a previously coded block is stored in a table and used as MVP for the current CU. The table with multiple HMVP candidates is maintained with first-in-first-out strategy during the encoding/decoding process. Whenever there is a non-subblock inter-coded CU, the associated motion information is added to the last entry of the table as a new HMVP candidate.

During the standardization of VVC, Non-adjacent MVP was proposed to facilitate better motion information derivation by exploiting the non-adjacent area. In ECM software, Non-adjacent MVP are inserted between TMVP and HMVP, where the distances between non-adjacent spatial candidates and current coding block are based on the width and height of current coding block as depicted in Fig. 5.
2.2 Affine motion compensated prediction

In HEVC, only translation motion model is applied for motion compensation prediction (MCP) . While in the real world, there are many kinds of motion, e.g., zoom in/out, rotation, perspective motions and the other irregular motions. In VVC, a block-based affine transform motion compensation prediction is applied. As shown Fig. 6A and Fig. 6B, the affine motion field of the block is described by motion information of two control point (4-parameter) or three control point motion vectors (6-parameter) .

For 4-parameter affine motion model, motion vector at sample location (x, y) in a block is derived as:

For 6-parameter affine motion model, motion vector at sample location (x, y) in a block is derived as:

Where (mv0x, mv0y) is motion vector of the top-left corner control point, (mv1x, mv1y) is motion vector of the top-right corner control point, and (mv2x, mv2y) is motion vector of the bottom-left corner control point.

To simplify the motion compensation prediction, block based affine transform prediction is applied. To derive motion vector of each 4×4 luma subblock, the motion vector of the center sample of each subblock, as shown in Fig. 7, is calculated according to above equations, and rounded to 1/16 fraction accuracy. Then the motion compensation interpolation filters are applied to generate the prediction of each subblock with derived motion vector. The subblock size of chroma-components is also set to be 4×4. The MV of a 4×4 chroma subblock is calculated as the average of the MVs of the top-left and bottom-right luma subblocks in the collocated 8x8 luma region. Fig. 7 illustrates the affine MVF per subblock.

As done for translational motion inter prediction, there are also two affine motions inter prediction modes: affine merge mode and affine AMVP mode.
2.2.1 Affine merge prediction

Affine merge mode can be applied for CUs with both width and height larger than or equal to 8. In this mode the CPMVs of the current CU is generated based on the motion information of the spatial neighboring CUs. There can be up to five CPMVP candidates and an index is signalled to indicate the one to be used for the current CU. In VVC, the following three types of CPVM candidate are used to form the affine merge candidate list:
– Inherited affine merge candidates that extrapolated from the CPMVs of the neighbor CUs;
– Constructed affine merge candidates CPMVPs that are derived using the translational MVs of the
neighbor CUs;
– Zero MVs.

In VVC, there are maximum two inherited affine candidates, which are derived from affine motion model of the neighboring blocks, one from left neighboring CUs and one from above neighboring CUs. The candidate blocks are shown in Fig. 31. For the left predictor, the scan order is A0->A1, and for the above predictor, the scan order is B0->B1->B2. Only the first inherited candidate from each side is selected. No pruning check is performed between two inherited candidates. When a neighboring affine CU is identified, its control point motion vectors are used to derive the CPMVP candidate in the affine merge list of the current CU. As shown in Fig. 9, if the neighbor left bottom block A is coded in affine mode, the motion vectors v₂ , v₃ and v₄ of the top left corner, above right corner and left bottom corner of the CU which contains the block A are attained. When block A is coded with 4-parameter affine model, the two CPMVs of the current CU are calculated according to v₂, and v₃. In case that block A is coded with 6-parameter affine model, the three CPMVs of the current CU are calculated according to v₂ , v₃ and v₄.

Fig. 8 illustrates the locations of inherited affine motion predictors. Fig. 9 illustrates the control point motion vector inheritance.

Constructed affine candidate means the candidate is constructed by combining the neighbor translational motion information of each control point. The motion information for the control points is derived from the specified spatial neighbors and temporal neighbor shown in Fig. 33. CPMVk (k=1, 2, 3, 4) represents the k-th control point. For CPMV1, the B2->B3->A2 blocks are checked and the MV of the first available block is used. For CPMV2, the B1->B0 blocks are checked and for CPMV3, the A1->A0 blocks are checked. For TMVP is used as CPMV4 if it’s available.

After MVs of four control points are attained, affine merge candidates are constructed based on those motion information. The following combinations of control point MVs are used to construct in order:
{CPMV1, CPMV2, CPMV3} , {CPMV1, CPMV2, CPMV4} , {CPMV1, CPMV3, CPMV4} ,
{CPMV2, CPMV3, CPMV4} , {CPMV1, CPMV2} , {CPMV1, CPMV3} .

The combination of 3 CPMVs constructs a 6-parameter affine merge candidate and the combination of 2 CPMVs constructs a 4-parameter affine merge candidate. To avoid motion scaling process, if the reference indices of control points are different, the related combination of control point MVs is discarded. Fig. 10 illustrates the locations of candidates position for constructed affine merge mode.

After inherited affine merge candidates and constructed affine merge candidate are checked, if the list is still not full, zero MVs are inserted to the end of the list.
2.2.2 Affine merge prediction

Affine AMVP mode can be applied for CUs with both width and height larger than or equal to 16. An affine flag in CU level is signalled in the bitstream to indicate whether affine AMVP mode is used and then another flag is signalled to indicate whether 4-parameter affine or 6-parameter affine. In this mode, the difference of the CPMVs of current CU and their predictors CPMVPs is signalled in the bitstream. The affine AMVP candidate list size is 2 and it is generated by using the following four types of CPVM candidate in order:
– Inherited affine AMVP candidates that extrapolated from the CPMVs of the neighbor CUs
– Constructed affine AMVP candidates CPMVPs that are derived using the translational MVs of the
neighbor CUs
– Translational MVs from neighboring CUs
– Zero MVs

The checking order of inherited affine AMVP candidates is same to the checking order of inherited affine merge candidates. The only difference is that, for AMVP candidate, only the affine CU that has the same reference picture as in current block is considered. No pruning process is applied when inserting an inherited affine motion predictor into the candidate list.

Constructed AMVP candidate is derived from the specified spatial neighbors shown in Fig. 33. The same checking order is used as done in affine merge candidate construction. In addition, reference picture index of the neighboring block is also checked. The first block in the checking order that is inter coded and has the same reference picture as in current CUs is used. There is only one When the current CU is coded with 4-parameter affine mode, and mv0 and mv1 are both available, they are added as one candidate in the affine AMVP list. When the current CU is coded with 6-parameter affine mode, and all three CPMVs are available, they are added as one candidate in the affine AMVP list. Otherwise, constructed AMVP candidate is set as unavailable. Fig. 11 illustrates (a) the spatial neighbors for deriving inherited affine merge candidates, and (b) the spatial neighbors for deriving constructed affine merge candidates.

If affine AMVP list candidates are still less than 2 after valid inherited affine AMVP candidates and constructed AMVP candidate are inserted, mv0, mv1 and mv2 will be added, in order, as the translational MVs to predict all control point MVs of the current CU, when available. Finally, zero MVs are used to fill the affine AMVP list if it is still not full.
2.2.3 Affine merge prediction

In ECM-6.0, 3 additional Affine merge and AMVP candidate derivation methods are integrated, which are Non-adjacent spatial candidates, History-parameter-based candidates, Regression based affine candidates and Pixel based affine motion compensation.
2.2.3.1 Non-adjacent spatial candidates

In ECM-6.0, non-adjacent spatial neighbors are investigated to provided candidates for both Affine merge and Affine AMVP. The pattern of obtaining non-adjacent spatial candidates is shown in (a) of Fig. 11. Same as the non-adjacent regular merge candidates, the distances between non-adjacent spatial candidates and current coding block are also defined based on the width and height of current CU.

The motion information of the non-adjacent spatial neighbors in Fig. 11 is utilized to generate additional inherited and constructed affine merge candidates. Specifically, to generate inherited candidates, the non-adjacent spatial neighbors are checked based on their distances to the current block, i.e., from near to far. At a specific distance, only the first available neighbor which is coded with Affine mode from each side (e.g., the left and above) of the current block is included. As indicated in (a) of Fig. 11, the checking of the neighbors on the left and above sides are performed from bottom-to-up and right-to-left, respectively. For constructed candidates, as shown in (b) of Fig. 11, the positions of one left and above non-adjacent spatial neighbors are firstly determined independently; After that, the location of the top-left neighbor can be determined accordingly to form a rectangular virtual block together with the left and above non-adjacent neighbors. Fig. 12 illustrates from non-adjacent neighbors to constructed affine merge candidates. The motion information of the three non-adjacent neighbors is used to form the CPMVs at the top-left (A) , top-right (B) and bottom-left (C) of the virtual block, which is projected to the current CU to generate the corresponding constructed candidates, as shown in Fig. 12.
2.2.3.2 History-parameter-based affine candidates

History-parameter-based affine model inheritance (HAMI) allows the affine model to be inherited from a previously affine-coded block which may not be neighboring to the current block. A history-parameter table (HPT) is established. An entry of HPT stores a set of affine parameters: a, b, c and d, each of which is represented by a 16-bit signed integer. Entries in HPT is categorized by reference list and reference index. Five reference indices are supported for each reference list in HPT. In a formular way, the category of HPT (denoted as HPTCat) is calculated as
HPTCat (RefList, RefIdx) = 5×RefList + min (RefIdx, 4) (3)
wherein RefList and RefIdx represents a reference picture list (0 or 1) and a reference index, respectively.
For each category, at most seven entries can be stored, resulting in 70 entries totally in HPT. At the beginning of each CTU row, the number of entries for each category is initialized as zero. After decoding an affine-coded CU with reference list RefListcur and RefIdxcur, the affine parameters are utilized to update entries in the category HPTCat (RefListcur, RefIdxcur) in a way similar to HMVP table updating.

A history-affine-parameter-based candidate (HAPC) is derived from a neighbouring 4×4 block denoted as A0, A1, B0, B1 or B2 in Fig. 13 and a set of affine parameters stored in a corresponding entry in HPT. The MV of a neighbouring 4×4 block served as the base MV. In a formulating way, the MV of the current block at position (x, y) is calculated as:

where (mvhbase, mvvbase) represents the MV of the neighbouring 4×4 block, (xbase, ybase) represents the
center position of the neighbouring 4×4 block. (x, y) can be the top-left, top-right and bottom-left corner of the current block to obtain the corner-position MVs (CPMVs) for the current block, or it can be the center of the current block to obtain a regular MV for the current block.

Fig. 13 illustrates an example of generating an HAPC. The affine parameters {a0, b0, c0, d0} are directly fetched from one entry of category HPTIdx (RefListA0, refIdx0A0) in HPT. The affine parameters from HPT, with the center position of A0 as the base position, and the MV of block A0 as the base MV, are used together to derive the CPMVs for an affine merge HAPC, or an affine AMVP HAPC. They can also be used to derive MVs located at the center of the current block, as regular merge candidates. A HAPC can be put into the sub-block-based merge candidate list, the affine AMVP candidate list or the regular merge candidate list. As a response to new HAPCs being introduced, the size of sub-block-based merge candidate list is increased from five to ten and twelve for random access and low-delay B configurations, respectively. Besides, the size of regular merge candidate list is increased from ten to eleven for random access configurations to accommodate the newly added regular merge candidates.
2.2.3.3 Regression based affine candidate

In ECM-6.0, the regression based affine merge candidates are derived and added to the affine merge list. Subblock motion field from a previously coded affine CU and motion information from adjacent subblocks of a current CU are used as the input to the regression process to derive proposed affine candidates.

The previously coded affine CU can be identified from scanning through non-adjacent positions and the affine HMVP table. Adjacent subblock information of current CU is fetched from 4x4 sub-blocks represented by the grey zone as depicted in Fig. 14. For each sub-block, given a reference list, the corresponding motion vector and center coordinate of the sub-block may be used.

For each affine CU, up to 2 affine candidates can be derived. One with adjacent subblock information and one without. All the linear-regression-generated candidates are pruned and collected into one candidate sub-group, TM cost based ARMC process is applied when ARMC is enabled. Afterwards, up to N linear-regression-generated candidates are added to the affine merge list when N affine CUs are found. Fig. 14 illustrates the illustration of regression based affine merge candidate derivation.
2.2.3.4 Pixel based affine motion compensation

With pixel based affine motion compensation, minimum affine subblock size is set to 1x1 for luma component when OBMC is not applied, minimum subblock size is always set to 1x1 for chroma components.
2.3 Template matching merge/AMVP mode in ECM

Template matching (TM) merge/AMVP mode is a decoder-side MV derivation method to refine the motion information of the current CU by finding the closest match between a template (i.e., top and/or left neighboring blocks of the current CU) in the current picture and a block (i.e., same size to the template) in a reference picture. As illustrated in Fig. 15, a better MV is to be searched around the initial motion of the current CU within a [–8, +8] -pel search range. Fig. 15 illustrates the template matching performs on a search area around initial MV.

In AMVP mode, an MVP candidate is determined based on the template matching error to pick up the one which reaches the minimum difference between the current block and the reference block templates, and then TM performs only for this particular MVP candidate for MV refinement. TM refines this MVP candidate, starting from full-pel MVD precision (or 4-pel for 4-pel AMVR mode) within a [–8, +8] -pel search range by using iterative diamond search. The AMVP candidate may be further refined by using cross search with full-pel MVD precision (or 4-pel for 4-pel AMVR mode) , followed sequentially by half-pel and quarter-pel ones depending on AMVR mode. This search process ensures that the MVP candidate still keeps the same MV precision as indicated by adaptive motion vector resolution (AMVR) mode after TM process.

In the merge mode, similar search method is applied to the merge candidate indicated by the merge index. TM merge may perform all the way down to 1/8-pel MVD precision or skipping those beyond half-pel MVD precision, depending on whether the alternative interpolation filter (that is used when AMVR is of half-pel mode) is used according to merged motion information. Besides, when TM mode is enabled, template matching may work as an independent process or an extra MV refinement process between block-based and subblock-based bilateral matching (BM) methods, depending on whether BM can be enabled or not according to its enabling condition check. When BM and TM are both enabled for a CU, the search process of TM stops at half-pel MVD precision and the resulted MVs are further refined by using the same model-based MVD derivation method as in DMVR.
2.4 Adaptive reorder of merge candidates (ARMC)

Inspired by the spatial correlation between reconstructed neighboring pixels and the current coding block, adaptive reorder of merge candidates (ARMC) was proposed to refine the candidates order in a given candidate list. The underlying assumption is that the candidates with less template matching cost have higher probability to be chosen through RDO process, hence should be placed in front positions within the list to reduce the signaling cost.

The reordering method is applied to regular merge mode, template matching (TM) merge mode, and affine merge mode (excluding the SbTMVP candidate) . For the TM merge mode, merge candidates are reordered before the refinement process.

After a merge candidate list is constructed, merge candidates are divided into several subgroups. The subgroup size is set to 5. Merge candidates in each subgroup are reordered ascendingly according to cost values based on template matching. For simplification, merge candidates in the last but not the first subgroup are not reordered.

The template matching cost is measured by the sum of absolute differences (SAD) between samples of a template of the current block and their corresponding reference template. The template comprises a set of reconstructed samples neighboring to the current block, while reference template is located by the same motion information of the current block, as illustrated in Fig. 16. When a merge candidate utilizes bi-directional prediction, the reference samples of the template of the merge candidate are also generated by bi-prediction.

For subblock-based merge candidates with subblock size equal to Wsub *Hsub, the above template comprises several sub-templates with the size of Wsub × K, and the left template comprises several sub-templates with the size of K × Hsub. As shown in Fig. 17. the motion information of the subblocks in the first row and the first column of current block is used to derive the reference samples of each sub-template.
2.5 Subblock-based temporal motion vector prediction (SbTMVP)

VVC supports the subblock-based temporal motion vector prediction (SbTMVP) method. Similar to the TMVP, SbTMVP takes advantage of the motion field in the collocated picture to facilitate more precise MVP derivation. The same collocated picture used by TMVP is used for SbTMVP. SbTMVP differs from TMVP mainly in two aspects. Firstly, SbTMVP enables sub-CU level motion prediction whereas TMVP predicts motion at CU level; Secondly, compared with TMVP that fetches the temporal MV from the collocated block in the collocated picture (the collocated block is the bottom-right or center block relative to the current CU) , SbTMVP applies a motion shift before fetching the temporal motion information from the collocated picture, where the motion shift is obtained by re-using the MV from one of the spatial neighboring blocks of the current CU.

Fig. 18 illustrates the derivation process of the sub-block level motion field for SbTMVP. In particular, the motion information of left-bottom sub-block A1 is firstly fetched, if either of the MVs in reference list0 and list1 points to the collocated frame, then the corresponding MV will be identified as motion shift. Otherwise, zero mv will be used as motion shift.

Once the motion shift is determined, the specified region in the collocated frame is employed to derive sub-block level motion field. Assuming A1’ motion is used as motion shift as depicted in Fig. 18. Then for each sub-CU, the motion information of its corresponding block (the smallest motion grid that covers the center sample) in the collocated picture is fetched to provide motion information, where MV scale operation is firstly performed to align the reference frames of the temporal motion vectors to those of the current CU.

Fig. 17 illustrates the template and reference template for block with sub-block motion using the motion information of the subblocks of current block. Fig. 18 illustrates deriving sub-CU motion field obtained by applying a motion shift based on the neighboring motion information.

In VVC and ECM, in addition to CU level MVP candidate list, a sub-CU level MVP candidate list is also constructed to provide more precise motion prediction for the current CU, which comprises the motion fields produced by both SbTMVP and AFFINE methods. In particular, only one SbTMVP candidate is included and is always placed in the first entry of the constructed sub-CU level MVP candidate list, whereas multiple AFFINE candidates are included in the list after performing template matching-based reordering, where those with smaller costs are placed in fronter positions.
2.6 Combined inter and intra prediction (CIIP)

In VVC, when a CU is coded in merge mode, if the CU contains at least 64 luma samples (that is, CU width times CU height is equal to or larger than 64) , and if both CU width and CU height are less than 128 luma samples, an additional flag is signalled to indicate if the combined inter/intra prediction (CIIP) mode is applied to the current CU. As its name indicates, the CIIP prediction combines an inter prediction signal with an intra prediction signal. The inter prediction signal in the CIIP mode P_inter is derived using the same inter prediction process applied to regular merge mode; and the intra prediction signal P_intra is derived following the regular intra prediction process with the planar mode. Then, the intra and inter prediction signals are combined using weighted averaging, where the weight value is calculated depending on the coding modes of the top and left neighbouring blocks (depicted in Fig. 19) as follows:
– If the top neighbor is available and intra coded, then set isIntraTop to 1, otherwise set isIntraTop to 0;
– If the left neighbor is available and intra coded, then set isIntraLeft to 1, otherwise set isIntraLeft to 0;
– If (isIntraLeft + isIntraTop) is equal to 2, then wt is set to 3;
– Otherwise, if (isIntraLeft + isIntraTop) is equal to 1, then wt is set to 2;
– Otherwise, set wt to 1.

The CIIP prediction is formed as follows:
P_CIIP=( (4-wt) *P_inter+wt*P_intra+2)＞＞2 (5)

Fig. 19 illustrates the top and left neighboring blocks used in CIIP weight derivation.
2.7 CIIP with PDPC blending

In ECM, the CIIP mode is extended. In this extended mode (CIIP_PDPC) , the prediction of the regular merge mode is refined using the above (Rx, -1) and left (R-1, y) reconstructed samples. This refinement inherits the position dependent prediction combination (PDPC) scheme. The flowchart of the prediction of the CIIP_PDPC mode can be depicted as in Fig. 20, where WT and WL are the weighted values which depend on the sample position in the block as defined in PDPC.

The CIIP_PDPC mode is signaled together with CIIP mode. When CIIP flag is true, another flag, namely CIIP_PDPC flag, is further signaled to indicate whether to use CIIP_PDPC. Fig. 20 illustrates the CIIP_PDPC flowchart of the extended CIIP mode using PDPC.
2.8 Combination of CIIP with TIMD and TM merge

In ECM CIIP mode, the prediction samples may be generated by weighting an inter prediction signal predicted using CIIP-TM merge candidate and an intra prediction signal predicted using TIMD derived intra prediction mode. The method is only applied to coding blocks with an area less than or equal to 1024.

The TIMD derivation method is used to derive the intra prediction mode in CIIP. Specifically, the intra prediction mode with the smallest SATD values in the TIMD mode list is selected and mapped to one of the 67 regular intra prediction modes.

In addition, it is also proposed to modify the weights (wIntra, wInter) for the two tests if the derived intra prediction mode is an angular mode. For near-horizontal modes (2 <= angular mode index < 34) , the current block is vertically divided as shown in Fig. 21A; for near-vertical modes (34 <= angular mode index <= 66) , the current block is horizontally divided as shown in Fig. 21B.

The (wIntra, wInter) for different sub-blocks are shown in Table 1. Fig. 21A and Fig. 21B illustrate the division method for angular modes.
Table 1. The modified weights used for angular modes.

With CIIP-TM, a CIIP-TM merge candidate list is built for the CIIP-TM mode. The merge candidates are refined by template matching. The CIIP-TM merge candidates are also reordered by the ARMC method as regular merge candidates. The maximum number of CIIP-TM merge candidates is equal to two.
2.9 Combined intra block copy and intra prediction

Combined intra block copy and intra prediction (IBC-CIIP) is a coding tool for a CU which uses IBC and intra prediction to obtain two prediction signals, and the two prediction signals are weighted summed to generate the final prediction as follows:
P=(w_ibc*P_ibc+ ( (1＜＜shift) -w_ibc) *P_intra+ (1＜＜ (shift-1) ) ) >>shift (6)
wherein P_ibc and P_intra denote the IBC prediction signal and intra prediction signal. (w_ibc, shift) are set equal
to (13, 4) and (1, 1) for IBC merge mode and IBC AMVP mode.

An intra prediction mode (IPM) candidate list is used to generate the intra prediction signal, and the IPM candidate list size is pre-defined as 2. An IPM index is signalled to indicate which IPM is used. Fig. 22 illustrates the subblock templates generation of SbTMVP.
2.10 Temporal motion derivation in ECM

In VVC, the Temporal Motion Vector Prediction (TMVP) for the AMVP and merge mode is derived by fetching the motion information from the center or the bottom-right of the collocated block in a signaled collocated picture. Similarly, for the Subblock-based Temporal Motion Vector Prediction (SbTMVP) mode, the motion information from the left neighboring position is used as a motion shift, which is then employed to obtain TMVPs at sub-CU level.

In ECM, to further improve the coding efficiency of TMVP, two aspects are modified. Firstly, two collocated pictures are utilized which are the two reference frames with the least POC distance relative to the to-be-coded frame. Secondly, the motion shift to locate TMVP is adaptively determined from multiple locations according to template costs. More specifically, two motion shift candidate lists are constructed respectively for the two collocated frames. The motion shifts with the minimum template matching cost are used to derive SbTMVP or TMVP candidates. At most 4 SbTMVP candidates are included in the sub-block-based merge list. The SbTMVP candidate with the least template matching cost derived from the first collocated frame is placed in the first entry without reordering, while other SbTMVP candidates are sorted together with affine candidates. In addition, the prediction direction of each subblock template is determined based on the center subblock. As illustrated in Fig. 22, if the center subblock is uni-predicted, then all the subblock templates are uni-predicted, and vice versa. If the motion vector of corresponding adjacent subblock at the determined reference list is not available for a subblock template, zero MV is used for that subblock template.
2.11 Multi-pass decoder-side motion vector refinement (DMVR)

A multi-pass decoder-side motion vector refinement is integrated in ECM. In the first pass, bilateral matching (BM) is applied to the coding block. In the second pass, BM is applied to each 16x16 subblock within the coding block. In the third pass, MV in each 8x8 subblock is refined by applying bi-directional optical flow (BDOF) . The refined MVs are stored for both spatial and temporal motion vector prediction.
2.11.1 First pass –Block based bilateral matching MV refinement

In the first pass, a refined MV is derived by applying BM to a coding block. Similar to decoder-side motion vector refinement (DMVR) , in bi-prediction operation, a refined MV is searched around the two initial MVs (MV0 and MV1) in the reference picture lists L0 and L1. The refined MVs (MV0_pass1 and MV1_pass1) are derived around the initiate MVs based on the minimum bilateral matching cost between the two reference blocks in L0 and L1.

BM performs local search to derive integer sample precision intDeltaMV. The local search applies a 3×3 square search pattern to loop through the search range [–sHor, sHor] in horizontal direction and [–sVer, sVer] in vertical direction, wherein, the values of sHor and sVer are determined by the block dimension, and the maximum value of sHor and sVer is 8.

The bilateral matching cost is calculated as: bilCost = mvDistanceCost + sadCost. When the block size cbW *cbH is greater than 64, mean-removal SAD (MRSAD) cost function is applied to remove the DC effect of distortion between reference blocks. When the bilCost at the center point of the 3×3 search pattern has the minimum cost, the intDeltaMV local search is terminated. Otherwise, the current minimum cost search point becomes the new center point of the 3×3 search pattern and continue to search for the minimum cost, until it reaches the end of the search range.

The existing fractional sample refinement is further applied to derive the final deltaMV. The refined MVs after the first pass is then derived as:
MV0_pass1 = MV0 + deltaMV (7)
MV1_pass1 = MV1 –deltaMV
2.11.2 Second pass –Subblock based bilateral matching MV refinement

In the second pass, a refined MV is derived by applying BM to a 16×16 grid subblock. For each subblock, a refined MV is searched around the two MVs (MV0_pass1 and MV1_pass1) , obtained on the first pass, in the reference picture list L0 and L1. The refined MVs (MV0_pass2 (sbIdx2) and MV1_pass2 (sbIdx2) ) are derived based on the minimum bilateral matching cost between the two reference subblocks in L0 and L1.

For each subblock, BM performs full search to derive integer sample precision intDeltaMV. The full search has a search range [–sHor, sHor] in horizontal direction and [–sVer, sVer] in vertical direction, wherein, the values of sHor and sVer are determined by the block dimension, and the maximum value of sHor and sVer is 8.

The bilateral matching cost is calculated by applying a cost factor to the SATD cost between two reference subblocks, as: bilCost = satdCost *costFactor. The search area (2*sHor + 1) * (2*sVer + 1) is divided up to 5 diamond shape search regions shown on Fig. 23. Each search region is assigned a costFactor, which is determined by the distance (intDeltaMV) between each search point and the starting MV, and each diamond region is processed in the order starting from the center of the search area. In each region, the search points are processed in the raster scan order starting from the top left going to the bottom right corner of the region. When the minimum bilCost within the current search region is less than a threshold equal to sbW *sbH, the int-pel full search is terminated, otherwise, the int-pel full search continues to the next search region until all search points are examined. Additionally, if the difference between the previous minimum cost and the current minimum cost in the iteration is less than a threshold that is equal to the area of the block, the search process terminates. Fig. 23 illustrates the diamond regions in the search area.

The existing VVC DMVR fractional sample refinement is further applied to derive the final deltaMV (sbIdx2) . The refined MVs at second pass is then derived as:
MV0_pass2 (sbIdx2) = MV0_pass1 + deltaMV (sbIdx2) (8)
MV1_pass2 (sbIdx2) = MV1_pass1 –deltaMV (sbIdx2)
2.11.3 Third pass –Subblock based bi-directional optical flow MV refinement

In the third pass, a refined MV is derived by applying BDOF to an 8×8 grid subblock. For each 8×8 subblock, BDOF refinement is applied to derive scaled Vx and Vy without clipping starting from the refined MV of the parent subblock of the second pass. The derived bioMv (Vx, Vy) is rounded to 1/16 sample precision and clipped between -32 and 32.

The refined MVs (MV0_pass3 (sbIdx3) and MV1_pass3 (sbIdx3) ) at third pass are derived as:
MV0_pass3 (sbIdx3) = MV0_pass2 (sbIdx2) + bioMv (9)
MV1_pass3 (sbIdx3) = MV0_pass2 (sbIdx2) –bioMv

In all aforementioned sub-clauses, when wrap around motion compensation is enabled, the motion vectors shall be clipped with wrap around offset taken into consideration.
2.12 Geometric partitioning mode (GPM)

In VVC, a geometric partitioning mode is supported for inter prediction. The geometric partitioning mode is signalled using a CU-level flag as one kind of merge mode, with other merge modes including the regular merge mode, the MMVD mode, the CIIP mode and the subblock merge mode. In total 64 partitions are supported by geometric partitioning mode for each possible CU size w×h=2^m×2ⁿ with m, n ∈ {3…6} excluding 8x64 and 64x8.

When this mode is used, a CU is split into two parts by a geometrically located straight line (Fig. 24) . The location of the splitting line is mathematically derived from the angle and offset parameters of a specific partition. Each part of a geometric partition in the CU is inter-predicted using its own motion; only uni-prediction is allowed for each partition, that is, each part has one motion vector and one reference index. The uni-prediction motion constraint is applied to ensure that same as the conventional bi-prediction, only two motion compensated prediction are needed for each CU. Fig. 24 illustrates examples of the GPM splits grouped by identical angles.

If geometric partitioning mode is used for the current CU, then a geometric partition index indicating the partition mode of the geometric partition (angle and offset) , and two merge indices (one for each partition) are further signalled. The number of maximum GPM candidate size is signalled explicitly in SPS and specifies syntax binarization for GPM merge indices. After predicting each of part of the geometric partition, the sample values along the geometric partition edge are adjusted using a blending processing with adaptive weights. This is the prediction signal for the whole CU, and transform and quantization process will be applied to the whole CU as in other prediction modes. Finally, the motion field of a CU predicted using the geometric partition modes is stored.
2.12.1 Uni-prediction candidate list construction

The uni-prediction candidate list is derived directly from the merge candidate list constructed according to the extended merge prediction process. Denote n as the index of the uni-prediction motion in the geometric uni-prediction candidate list. The LX motion vector of the n-th extended merge candidate, with X equal to the parity of n, is used as the n-th uni-prediction motion vector for geometric partitioning mode. These motion vectors are marked with “x” in Fig. 25. In case a corresponding LX motion vector of the n-the extended merge candidate does not exist, the L (1 -X) motion vector of the same candidate is used instead as the uni-prediction motion vector for geometric partitioning mode. Fig. 25 illustrates the uni-prediction MV selection for geometric partitioning mode.
2.12.2 Blending along the geometric partitioning edge

After predicting each part of a geometric partition using its own motion, blending is applied to the two prediction signals to derive samples around geometric partition edge. The blending weight for each position of the CU are derived based on the distance between individual position and the partition edge.

The distance for a position (x, y) to the partition edge are derived as:

where i, j are the indices for angle and offset of a geometric partition, which depend on the signaled geometric
partition index. The sign of ρ_x, j and ρ_y, j depend on angle index i.

The weights for each part of a geometric partition are derived as following:
wIdxL (x, y) =partIdx ? 32+d (x, y) : 32-d (x, y) (14)

w₁(x, y) =1-w₀ (x, y) (16)

The partIdx depends on the angle index i. One example of weigh w₀ is illustrated in Fig. 26.
2.12.3 Motion field storage for geometric partitioning mode

Mv1 from the first part of the geometric partition, Mv2 from the second part of the geometric partition and a combined Mv of Mv1 and Mv2 are stored in the motion filed of a geometric partitioning mode coded CU.

The stored motion vector type for each individual position in the motion filed are determined as:
sType = abs (motionIdx) < 32 ? 2∶ (motionIdx≤0 ? (1 -partIdx ) : partIdx ) (17)
where motionIdx is equal to d (4x+2, 4y+2) . The partIdx depends on the angle index i.

If sType is equal to 0 or 1, Mv0 or Mv1 are stored in the corresponding motion field, otherwise if sType is equal to 2, a combined Mv from Mv0 and Mv2 are stored. The combined Mv are generated using the following process:
1) If Mv1 and Mv2 are from different reference picture lists (one from L0 and the other from L1) , then
Mv1 and Mv2 are simply combined to form the bi-prediction motion vectors.
2) Otherwise, if Mv1 and Mv2 are from the same list, only uni-prediction motion Mv2 is stored.
2.13 Spatial geometric partitioning mode (SGPM)

SGPM is an intra mode that resembles the inter coding tool of GPM, where the two prediction parts are generated from intra predicted process. In this mode, a candidate list is built with each entry containing one partition split and two intra prediction modes as shown in Fig. 27.26 partition modes and 3 intra prediction modes are used to form the combinations. the length of the candidate list is set equal to 16. The selected candidate index is signalled. Fig. 27 illustrates the spatial GPM candidates.

The list is reordered using template (Fig. 28) where SAD between the prediction and reconstruction of the template is used for ordering. The template size is fixed to 1. Fig. 28 illustrates the GPM template.

For each partition mode, an IPM list is derived for each part using the same intra-inter GPM list derivation. The IPM list size is set to 3. In the list, TIMD derived mode is replaced by 2 derived modes with horizontal and vertical orientations.

The SGPM mode is applied with a restricted blocks size: 4<=width<=64, 4<=height<=64, width<height*8, height<width*8, width*height>=32.

A PPS flag is coded to indicate whether no blending of two intra predictions is allowed. When this PPS flag is set to false, the following adaptive blending is also used for spatial GPM, where blending depth τ is derived as follows:
· If min (width, height) ==4, 1/2 τ is selected
· else if min (width, height) ==8, τ is selected
· else if min (width, height) ==16, 2 τ is selected
· else if min (width, height) ==32, 4 τ is selected
· else, 8 τ is selected.

Otherwise (the PPS flag is set to true) , 1/4 τ is always used for spatial GPM coded blocks to make sure no blending is used when SGPM block has partition angle completely horizontal or vertical, and much narrower blending width is used when SGPM block has other partition angles. It is noted that the flag is set to true in current Common Test Conditions (CTC) for the screen content videos.
2.14 Geometric partitioning mode (GPM) with adaptive blending

In VVC, the final prediction samples are generated with by blending the prediction of the two prediction signals using weighted average. Two integer blending matrices (W0 and W1) are used. The weights in the GPM blending matrices are derived from the ramp function based on the displacement from a predicted sample position to the GPM partitioning boundary. The blending area size is fixed to two (2 samples on each side of the GPM partition split boundary) .

The blending process in ECM is improved by adding four extra blending area sizes (quarter, half, double, and quadrupole of the existing area size) as shown in Fig. 29. A CU level flag is coded to signal the selected blending area size is signalled. Furthermore, the extended weighting precision is utilized, in which the maximum value of the weighs is changed from 8 (in VVC) to 32 to accommodate the extended blending area sizes. Fig. 29 illustrates the ramp function for the weights for GPM blending based on the displacement (d) from a predicted sample position to the GPM partitioning boundary and the blending area size (τ) .
2.15 Large block-size transforms with high-frequency zeroing

In VVC, large block-size transforms, up to 64×64 in size, are enabled, which is primarily useful for higher resolution video, e.g., 1080p and 4K sequences. High frequency transform coefficients are zeroed out for the transform blocks with size (width or height, or both width and height) equal to 64, so that only the lower-frequency coefficients are retained. For example, for an M×N transform block, with M as the block width and N as the block height, when M is equal to 64, only the left 32 columns of transform coefficients are kept. Similarly, when N is equal to 64, only the top 32 rows of transform coefficients are kept. When transform skip mode is used for a large block, the entire block is used without zeroing out any values. In addition, transform shift is removed in transform skip mode. The VTM also supports configurable max transform size in SPS, such that encoder has the flexibility to choose up to 32-length or 64-length transform size depending on the need of specific implementation.
2.16 Multiple transform selection (MTS) for core transform

In addition to DCT-II which has been employed in HEVC, a Multiple Transform Selection (MTS) scheme is used for residual coding both inter and intra coded blocks. It uses multiple selected transforms from the DCT8/DST7. The newly introduced transform matrices are DST-VII and DCT-VIII. Table 2 shows the basis functions of the selected DST/DCT.
Table 2 -Transform basis functions of DCT-II/VIII and DSTVII for N-point input

In order to keep the orthogonality of the transform matrix, the transform matrices are quantized more accurately than the transform matrices in HEVC. To keep the intermediate values of the transformed coefficients within the 16-bit range, after horizontal and after vertical transform, all the coefficients are to have 10-bit.

In order to control MTS scheme, separate enabling flags are specified at SPS level for intra and inter, respectively. When MTS is enabled at SPS, a CU level flag is signalled to indicate whether MTS is applied or not. Here, MTS is applied only for luma. The MTS signaling is skipped when one of the below conditions is applied.

The position of the last significant coefficient for the luma TB is less than 1 (i.e., DC only) .

The last significant coefficient of the luma TB is located inside the MTS zero-out region.

If MTS CU flag is equal to zero, then DCT2 is applied in both directions. However, if MTS CU flag is equal to one, then two other flags are additionally signalled to indicate the transform type for the horizontal and vertical directions, respectively. Transform and signalling mapping table as shown in Table 3. Unified the transform selection for ISP and implicit MTS is used by removing the intra-mode and block-shape dependencies. If current block is ISP mode or if the current block is intra block and both intra and inter explicit MTS is on, then only DST7 is used for both horizontal and vertical transform cores. When it comes to transform matrix precision, 8-bit primary transform cores are used. Therefore, all the transform cores used in HEVC are kept as the same, including 4-point DCT-2 and DST-7, 8-point, 16-point and 32-point DCT-2. Also, other transform cores including 64-point DCT-2, 4-point DCT-8, 8-point, 16-point, 32-point DST-7 and DCT-8, use 8-bit primary transform cores.
Table 3 -Transform and signalling mapping table

To reduce the complexity of large size DST-7 and DCT-8, High frequency transform coefficients are zeroed out for the DST-7 and DCT-8 blocks with size (width or height, or both width and height) equal to 32. Only the coefficients within the 16x16 lower-frequency region are retained.

As in HEVC, the residual of a block can be coded with transform skip mode. To avoid the redundancy of syntax coding, the transform skip flag is not signalled when the CU level MTS_CU_flag is not equal to zero. Note that implicit MTS transform is set to DCT2 when LFNST or MIP is activated for the current CU. Also the implicit MTS can be still enabled when MTS is enabled for inter coded blocks.
2.17 Low-frequency non-separable transform (LFNST)

In VVC, LFNST is applied between forward primary transform and quantization (at encoder) and between de-quantization and inverse primary transform (at decoder side) . In LFNST, 4x4 non-separable transform or 8x8 non-separable transform is applied according to block size (Fig. 30) . For example, 4x4 LFNST is applied for small blocks (i.e., min (width, height) < 8) and 8x8 LFNST is applied for larger blocks (i.e., min (width, height) > 4) . Fig. 30 illustrates the Low-Frequency Non-Separable Transform (LFNST) process.

Application of a non-separable transform, which is being used in LFNST, is described as follows using input as an example. To apply 4x4 LFNST, the 4x4 input block X

is first represented as a vector

The non-separable transform is calculated aswhereindicates the transform coefficient vector, and T is a 16x16 transform matrix. The 16x1 coefficient vectoris subsequently re-organized as 4x4 block using the scanning order for that block (horizontal, vertical or diagonal) . The coefficients with smaller index will be placed with the smaller scanning index in the 4x4 coefficient block.
2.18 Reduced Non-separable transform

LFNST (low-frequency non-separable transform) is based on direct matrix multiplication approach to apply non-separable transform so that it is implemented in a single pass without multiple iterations. However, the non-separable transform matrix dimension needs to be reduced to minimize computational complexity and memory space to store the transform coefficients. Hence, reduced non-separable transform (or RST) method is used in LFNST. The main idea of the reduced non-separable transform is to map an N (N is commonly equal to 64 for 8x8 NSST) dimensional vector to an R dimensional vector in a different space, where N/R (R < N) is the reduction factor. Hence, instead of NxN matrix, RST matrix becomes an R×N matrix as follows:

where the R rows of the transform are R bases of the N dimensional space. The inverse transform matrix for RT
is the transpose of its forward transform. For 8x8 LFNST, a reduction factor of 4 is applied, and 64x64 direct matrix, which is conventional 8x8 non-separable transform matrix size, is reduced to16x48 direct matrix. Hence, the 48×16 inverse RST matrix is used at the decoder side to generate core (primary) transform coefficients in 8×8 top-left regions. When16x48 matrices are applied instead of 16x64 with the same transform set configuration, each of which takes 48 input data from three 4x4 blocks in a top-left 8x8 block excluding right-bottom 4x4 block. With the help of the reduced dimension, memory usage for storing all LFNST matrices is reduced from 10KB to 8KB with reasonable performance drop. In order to reduce complexity LFNST is restricted to be applicable only if all coefficients outside the first coefficient sub-group are non-significant. Hence, all primary-only transform coefficients have to be zero when LFNST is applied. This allows a conditioning of the LFNST index signalling on the last-significant position, and hence avoids the extra coefficient scanning in the current LFNST design, which is needed for checking for significant coefficients at specific positions only. The worst-case handling of LFNST (in terms of multiplications per pixel) restricts the non-separable transforms for 4x4 and 8x8 blocks to 8x16 and 8x48 transforms, respectively. In those cases, the last-significant scan position has to be less than 8 when LFNST is applied, for other sizes less than 16. For blocks with a shape of 4xN and Nx4 and N > 8, the proposed restriction implies that the LFNST is now applied only once, and that to the top-left 4x4 region only. As all primary-only coefficients are zero when LFNST is applied, the number of operations needed for the primary transforms is reduced in such cases. From encoder perspective, the quantization of coefficients is remarkably simplified when LFNST transforms are tested. A rate-distortion optimized quantization has to be done at maximum for the first 16 coefficients (in scan order) , the remaining coefficients are enforced to be zero.
2.19 LFNST transform selection

There are totally 4 transform sets and 2 non-separable transform matrices (kernels) per transform set are used in LFNST. The mapping from the intra prediction mode to the transform set is pre-defined as shown in Table 4. If one of three CCLM modes (INTRA_LT_CCLM, INTRA_T_CCLM or INTRA_L_CCLM) is used for the current block (81 <= predModeIntra <= 83) , transform set 0 is selected for the current chroma block. For each transform set, the selected non-separable secondary transform candidate is further specified by the explicitly signalled LFNST index. The index is signalled in a bit-stream once per Intra CU after transform coefficients.
Table 4 -Transform selection table

2.20 LFNST index Signaling and interaction with other tools

Since LFNST is restricted to be applicable only if all coefficients outside the first coefficient sub-group are non-significant, LFNST index coding depends on the position of the last significant coefficient. In addition, the LFNST index is context coded but does not depend on intra prediction mode, and only the first bin is context coded. Furthermore, LFNST is applied for intra CU in both intra and inter slices, and for both Luma and Chroma. If a dual tree is enabled, LFNST indices for Luma and Chroma are signaled separately. For inter slice (the dual tree is disabled) , a single LFNST index is signaled and used for both Luma and Chroma.

Considering that a large CU greater than 64x64 is implicitly split (TU tiling) due to the existing maximum transform size restriction (64x64) , an LFNST index search could increase data buffering by four times for a certain number of decode pipeline stages. Therefore, the maximum size that LFNST is allowed is restricted to 64x64. Note that LFNST is enabled with DCT2 only. The LFNST index signaling is placed before MTS index signaling.

The use of scaling matrices for perceptual quantization is not evident that the scaling matrices that are specified for the primary matrices may be useful for LFNST coefficients. Hence, the uses of the scaling matrices for LFNST coefficients are not allowed. For single-tree partition mode, chroma LFNST is not applied.
2.21 Subblock transform (SBT)

In VTM, subblock transform is introduced for an inter-predicted CU. In this transform mode, only a sub-part of the residual block is coded for the CU. When inter-predicted CU with cu_cbf equal to 1, cu_sbt_flag may be signaled to indicate whether the whole residual block or a sub-part of the residual block is coded. In the former case, inter MTS information is further parsed to determine the transform type of the CU. In the latter case, a part of the residual block is coded with inferred adaptive transform and the other part of the residual block is zeroed out.

When SBT is used for an inter-coded CU, SBT type and SBT position information are signaled in the bitstream. There are two SBT types and two SBT positions, as indicated in Fig. 31. For SBT-V (or SBT-H) , the TU width (or height) may equal to half of the CU width (or height) or 1/4 of the CU width (or height) , resulting in 2: 2 split or 1: 3/3: 1 split. The 2: 2 split is like a binary tree (BT) split while the 1: 3/3: 1 split is like an asymmetric binary tree (ABT) split. In ABT splitting, only the small region contains the non-zero residual. If one dimension of a CU is 8 in luma samples, the 1: 3/3: 1 split along that dimension is disallowed. There are at most 8 SBT modes for a CU.

Position-dependent transform core selection is applied on luma transform blocks in SBT-V and SBT-H (chroma TB always using DCT-2) . The two positions of SBT-H and SBT-V are associated with different core transforms. More specifically, the horizontal and vertical transforms for each SBT position is specified in Fig. 31. For example, the horizontal and vertical transforms for SBT-V position 0 is DCT-8 and DST-7, respectively. When one side of the residual TU is greater than 32, the transform for both dimensions is set as DCT-2. Therefore, the subblock transform jointly specifies the TU tiling, cbf, and horizontal and vertical core transform type of a residual block.

The SBT is not applied to the CU coded with combined inter-intra mode. Fig. 31 illustrates the SBT position, type and transform type.
2.22 Maximum Transform Size and Zeroing-out of Transform Coefficients

Both CTU size and maximum transform size (i.e., all MTS transform kernels) are extended to 256, where the maximum intra coded block can have a size of 128x128. The maximum CTU size is set to 256 for UHD sequences and it is set to 128, otherwise. In the primary transformation process, there is no normative zeroing out operation applied on transform coefficients. However, if LFNST is applied, the primary transform coefficients outside the LFNST region are normatively zeroed-out.
2.23 Enhanced MTS for intra coding

In the current VVC design, for MTS, only DST7 and DCT8 transform kernels are utilized which are used for intra and inter coding.

Additional primary transforms including DCT5, DST4, DST1, and identity transform (IDT) are employed. Also MTS set is made dependent on the TU size and intra mode information. 16 different TU sizes are considered, and for each TU size 5 different classes are considered depending on intra-mode information. For each class, 4 different transform pairs are considered, the same as that of VVC. Note, although a total of 80 different classes are considered, some of those different classes often share exactly same transform set. So there are 58 (less than 80) unique entries in the resultant LUT.

For angular modes, a joint symmetry over TU shape and intra prediction is considered. So, a mode i (i > 34) with TU shape AxB will be mapped to the same class corresponding to the mode j= (68 –i) with TU shape BxA. However, for each transform pair the order of the horizontal and vertical transform kernel is swapped. For example, for a 16x4 block with mode 18 (horizontal prediction) and a 4x16 block with mode 50 (vertical prediction) are mapped to the same class. However, the vertical and horizontal transform kernels are swapped. For the wide-angle modes the nearest conventional angular mode is used for the transform set determination. For example, mode 2 is used for all the modes between -2 and -14. Similarly, mode 66 is used for mode 67 to mode 80.

MTS index [0, 3] is signalled with 2 bit fixed-length coding.
2.24 Secondary Transformation: LFNST extension with large kernel

The LFNST design in VVC is extended as follows:
· The number of LFNST sets (S) and candidates (C) are extended to S=35 and C=3, and the LFNST set
(lfnstTrSetIdx) for a given intra mode (predModeIntra) is derived according to the following formula:
ο For predModeIntra < 2, lfnstTrSetIdx is equal to 2
ο lfnstTrSetIdx = predModeIntra, for predModeIntra in [0, 34]
ο lfnstTrSetIdx = 68 –predModeIntra, for predModeIntra in [35, 66]
· Three different kernels, LFNST4, LFNST8, and LFNST16, are defined to indicate LFNST kernel sets,
which are applied to 4xN/Nx4 (N≥4) , 8xN/Nx8 (N≥8) , and MxN (M, N≥16) , respectively.

The kernel dimensions are specified by:
(LFSNT4, LFNST8*, LFNST16*) = (16x16, 32x64, 32x96) .

The forward LFNST is applied to top-left low frequency region, which is called Region-Of-Interest (ROI) . When LFNST is applied, primary-transformed coefficients that exist in the region other than ROI are zeroed out, which is not changed from the VVC standard.

The ROI for LFNST16 is depicted in Fig. 32. It consists of six 4x4 sub-blocks, which are consecutive in scan order. Since the number of input samples is 96, transform matrix for forward LFNST16 can be Rx96. R is chosen to be 32 in this contribution, 32 coefficients (two 4x4 sub-blocks) are generated from forward LFNST16 accordingly, which are placed following coefficient scan order.
Fig. 32 illustrates the ROI for LFNST16.

The ROI for LFNST8 is shown in Fig. 33. The forward LFNST8 matrix can be Rx64 and R is chosen to be 32. The generated coefficients are located in the same manner as with LFNST16. Fig. 33 illustrates the ROI for LFNST8.

The mapping from intra prediction modes to these sets is shown in Table 5:
Table 5. Mapping of intra prediction modes to LFNST set index

2.25 Non-Separable Primary Transform for Intra Coding (NSPT)

DCT-II+LFNST is replaced by NSPT for the block sizes 4x4, 4x8, 8x4 and 8x8. The NSPTs follows the design of LFNST, i.e. 3 candidates and 35 sets, chosen based on the intra mode. The kernel sizes are as follows:
· NSPT4x4: 16x16
· NSPT4x8 /NSPT8x4: 32x20
· NSPT8x8: 64x32

Therefore, 12 and 32 coefficients are zeroed-out for NSPT4x8/NSPT8x4 and NSPT8x8 respectively.
2.26 Sign prediction

The basic idea of the coefficient sign prediction method is to calculate reconstructed residual for both negative and positive sign combinations for applicable transform coefficients and select the hypothesis that minimizes a cost function.

To derive the best sign, the cost function is defined as discontinuity measure across block boundary shown on Fig. 34. It is measured for all hypotheses, and the one with the smallest cost is selected as a predictor for coefficient signs. Fig. 34 illustrates the discontinuity measure.

The cost function is defined as a sum of absolute second derivatives in the residual domain for the above row and left column as follows:

where R is reconstructed neighbors, P is prediction of the current block, and r is the residual hypothesis. The
term (-R_-1+2R₀-P₁) can be calculated only once per block and only residual hypothesis is subtracted.
2.27 LFNST/NSPT for inter coding

In ECM-12.0, LFNST/NSPT is applied to inter coding, where a virtual intra prediction mode (VIPM) is employed. Each VIPM represents a kind of feature, for example, a specific direction. The VIPM is derived by applying a DIMD-like process, which is also used in MIP and IntraTMP, to the inter prediction block. The horizontal and vertical gradients of pixels inside the prediction block are calculated with Sobel operator and the horizontal and vertical gradients of each pixel are used to derive a direction. The amplitudes of the gradient of each pixel are accumulated for the corresponding direction. The intra prediction mode corresponding to the strongest accumulation is selected as the VIPM. With the VIPM, the transform set of LFNST/NSPT can be derived. For inter coding, the kernel mapping method is the same as that for intra LFNST/NSPT.

The distribution of inter LFNST/NSPT index is different from that of intra LFNST/NSPT index. Therefore the signaling of inter LFNST/NSPT index is proposed to be different from that of intra LFNST/NSPT index. The intra LFNST/NSPT index binarization employs two context coded bins for each symbol, while it is proposed to use truncated unary code with different context models for inter LFNST/NSPT index coding.
3 Problems

Existing inter LFNST/NSPT method has the following problems:

In ECM-12.0, only one intra mode is derived to determine the inter LFNST/NSPT set, while multiple intra modes may be utilized to provide more abundant and accurate LFNST/NSPT set selection.

In ECM-12.0, a LFNST/NSPT candidate index has to be signalled or parsed for a inter LFNST/NSPT-coded block, which may not be necessary with an inter LFNST/NSPT merge list, where the inter LFNST/NSPT candidate or index may be indicated by a merge index.
4 Detailed solutions

In this disclosure, it is proposed to further improve CIIP by allowing subblock-based prediction as inter component. In particular, intra prediction and subblock-based inter prediction can be blended to form the CIIP prediction.

The detailed embodiments below should be considered as examples to explain general concepts. These embodiments should not be interpreted in a narrow way. Furthermore, these embodiments can be combined in any manner.

The terms ‘video unit’ or ‘coding unit’ or ‘block’ may represent a coding tree block (CTB) , a coding tree unit (CTU) , a coding block (CB) , a CU, a PU, a TU, a PB, a TB. The term “subblock-based coding tools” may represent affine, SbTMVP, and the corresponding variants, and etc.

In this disclosure, regarding “ablock coded with mode N” , here “mode N” may be a prediction mode (e.g., MODE_INTRA, MODE_INTER, MODE_PLT, MODE_IBC, and etc. ) , or a coding technique (e.g., DIMD, TIMD, PDPC, CCLM, CCCM, GLM, intraTMP, AMVP, SMVD, Merge, BDOF, PROF, DMVR, AMVR, TM, Affine, CIIP, GPM, spatial GPM, SGPM, GPM inter-inter, GPM intra-intra, GPM inter-intra, MHP, GEO, TPM, MMVD, BCW, HMVP, SbTMVP, LIC, OBMC, ALF, deblocking, SAO, bilateral filter, LMCS, and the corresponding variants, and etc. ) .

It is noted that the terminologies mentioned below are not limited to the specific ones defined in existing standards. Any variance of the coding tool is also applicable.
Intra mode list construction for inter LFNST/NSPT
1. For inter LFNST/NSPT, an intra mode may be firstly derived based on at least one of the following
modes/methods:
a) TIMD mode (e.g., the best/second best/N-th best intra mode yielding the least TM cost)
b) DIMD mode on the prediction block (e.g., the best/second best/N-th best intra mode yielding the
highest accumulated amplitudes of the gradient)
c) DIMD of a neighbouring reconstructed block
d) The coding mode of an adjacent/non-adjacent intra-coded block
e) Intra propagation mode
f) Certain fixed intra mode, i.e., planar, DC, or an intra mode with arbitrary angular index.
g) An intra angular mode with ±M (M>0, e.g., M=1) index offset upon an existing one.
2. An intra mode list may be constructed for inter LFNST/NSPT.
a) In one example, the following intra modes may be included in the intra mode list with a certain order:
i. TIMD mode (e.g., the best/second best/N-th best intra mode yielding the least TM cost)
ii. DIMD mode on the prediction block (e.g., the best/second best/N-th best intra mode yielding the
highest accumulated amplitudes of the gradient)
iii. DIMD of a neighbouring reconstructed block
iv. The coding mode of an adjacent/non-adjacent intra-coded block
v. Intra propagation mode
vi. Certain fixed intra mode, i.e., planar, DC, or an intra mode with arbitrary angular index.
vii. An intra angular mode with ±M (M>0, e.g., M=1) index offset upon an existing one.
b) At most K (K>=1) intra modes may be included in the intra mode list.
i. In one example, the construction of the list terminates if the capacity of the list reach K.
c) In one example, the intra mode list may be further reordered based on certain metric.
i. In one example, the metric may be TM cost.
3. The construction of intra mode list may be constructed along with pruning process.
a) In one example, a candidate may be determined as redundant if there is one with the same intra mode
index already exists in the list.
4. An index indicating specific candidate in the intra mode list may be signalled in the bitstream.
a) The index may be coded with truncated Rice (TR) code, the truncated binary (TB) code, the k-th
order Exp-Golomb (EGk) code or the fixed-length (FL) code.
b) In one example, the intra mode indicated by the index may be used to map the corresponding inter
LFNST/NSPT information, e.g., the LFNST set (lfnstTrSetIdx) .
i. In one example, a LFNST/NSPT candidate index may be further signalled to indicate the
candidate within the LFNST/NSPT set (lfnstTrSetIdx) .
5. Whether to/How to construct intra mode list may be dependent on the coding information of at least one
adjacent/non-adjacent blocks.
a) In one example, the information may be whether a block is intra-coded or not.
b) In one example, the information may be whether a block use inter/intra LFNST/NSPT or not.
6. Whether to/How to construct intra mode list may be dependent on the coding information of the current
block.
a) In one example, the information may be the coding mode/block width/height/area/ratio of width and
height/the smaller (or larger) value of width and height/quantization parameter/whether temporal layer index (Tid) is larger or smaller than a predefined constant/encoder signalled/both encoder and decoder derived value.
b) In one example, the information may be whether current slice satisfies low-delay condition.
c) In one example, the information may be whether current block belongs to screen content.
7. On the signalling of inter LFNST/NSPT syntax.
a) In one example, an index indicating whether inter LFNST/NSPT is used or not may be signalled. If
this syntax indicates LFNST/NSPT is used, another syntax indicating the index with the intra mode index may be signalled.
i. In one example, a first syntax may be signalled, if the value of the first syntax satisfies certain
condition (e.g., larger than 0) , a second syntax indicating intra mode index may be signalled.
1) In one example, the first syntax may indicate the LFNST/NSPT candidate index, and/or the
second syntax may indicate the intra mode that may be used to derive the LFNST/NSPT set index (lfnstTrSetIdx) .
Inter LFNST/NSPT merge list construction
8. At least one merge list may be constructed for inter LFNST/NSPT.
a) In one example, a merge candidate in the list may comprise all or partial information relating to inter
LFNST/NSPT.
i. In one example, the information may include:
1) the LFNST/NSPT set index (lfnstTrSetIdx)
2) the LFNST/NSPT candidate index within the LFNST/NSPT set.
b) In one example, an inter LFNST/NSPT merge candidate may be collected from adjacent and/or non-
adjacent locations of the current block.
i. In one example, an inter LFNST/NSPT merge candidate may be collected from the locations
specified by the MV or BV of the current coding block.
ii. In one example, an inter LFNST/NSPT merge candidate may be collected from a history table
with stored inter LFNST/NSPT information associated with previously coded blocks.
iii. In one example, an inter LFNST/NSPT merge candidate may be collected from temporal
locations.
c) In one example, the inter LFNST/NSPT merge candidates are checked and/or included into the merge
list in a pre-defined order.
i. In one example, the candidates specified by the MV or BV of the current coding block are checked
before adjacent blocks.
ii. In one example, adjacent candidates are checked before non-adjacent candidates.
iii. In one example, the checking order is in accordance with the distance relative to the
corresponding locations.
iv. In one example, a location with less distance relative to the current location has higher priority to
be checked and/or included in the inter LFNST/NSPT merge list.
d) The availability of an inter LFNST/NSPT merge candidate in a certain location may be determined
based on the following information:
i. Whether the coding block associated with the location is intra-coded.
1) In one example, specifically, whether LFNST/NSPT candidate index equals to certain value
(e.g., 0) .
ii. Whether the coding block associated with the location used inter/intra LFNST/NSPT.
iii. In one example, if the coding information of a certain location satisfies certain condition, the
corresponding inter/intra LFNST/NSPT information may be included in the merge list.
e) In one example, the checking process terminates when the number of available candidates reaches the
maximum allowed value.
9. In one example, a history table with stored inter LFNST/NSPT information associated with previously
coded blocks may be maintained and/or updated on-the-fly.
10. The construction of inter LFNST/NSPT merge list may be constructed along with pruning process.
a) In one example, a candidate may be determined as redundant if there is one with the same LFNST set
index (lfnstTrSetIdx) and/or LFNST candidate index already exists in the list.
11. An index indicating specific candidate in the inter LFNST/NSPT merge list may be signalled in the
bitstream.
a) The index may be coded with truncated Rice (TR) code, the truncated binary (TB) code, the k-th
order Exp-Golomb (EGk) code or the fixed-length (FL) code.
b) In one example, the inherited inter LFNST/NSPT information indicated by the index may be used to
conduct inter LFNST/NSPT.
12. On the signalling of inter LFNST/NSPT syntax.
a) In one example, an index indicating whether inter LFNST/NSPT is used or not may be signalled. If
this syntax indicates LFNST/NSPT is used, another syntax indicating the index within the inter LFNST/NSPT merge list may be signalled, and/or the corresponding LFNST/NSPT set (lfnstTrSetIdx) and/or LFNST/NSPT candidate of the inherited information may be directly used for inter LFNST/NSPT.
b) In one example, alternatively, a first syntax indicating whether inter LFNST/NSPT is used or not may
be signalled. If this syntax indicates LFNST/NSPT is used, a second syntax indicating whether merge mode is used or not may be signalled. If inter LFNST/NSPT merge mode is used, a third syntax indicating the index within the inter LFNST/NSPT merge list may be signalled. Otherwise, if inter LFNST/NSPT merge mode is not used, a fourth indicating the candidate within the LFNST/NSPT set (lfnstTrSetIdx) may be signalled thereafter.
i. In one example, specifically, if inter LFNST/NSPT merge mode is not used, an intra mode may
be derived based on Bullet 1, which may then be mapped to the LFNST/NSPT set index (lfnstTrSetIdx) , and/or the fourth syntax may be used to specify the concrete kernel.
c) In one example, alternatively, a first syntax indicating whether inter LFNST/NSPT is used or not may
be signalled. If this syntax indicates LFNST/NSPT is used and/or satisfies certain condition (e.g., equals to certain value (e.g., 0) ) , a second syntax indicating the candidate within the LFNST/NSPT set (lfnstTrSetIdx) may be signalled thereafter. Otherwise, if LFNST/NSPT is used but not satisfy certain condition, the value (or transformed value) of the first syntax may be used as the index within the inter LFNST/NSPT merge list, and/or no more inter LFNST/NSPT syntax needs to signalled any longer, and/or the inherited inter LFNST/NSPT information may be used for inter LFNST/NSPT.
i. In one example, specifically, if the first syntax equals to certain value (e.g., 1) , an intra mode may
be derived based on Bullet 1, which may then be mapped to the LFNST/NSPT set index (lfnstTrSetIdx) , and/or the second may be used to specify the concrete kernel.
Inter LFNST/NSPT for GPM
13. A different inter LFNST/NSPT processing may be applied to GPM-coded block.
a) In one example, a GPM mode may be, e.g, regular GPM, GPM inter-inter, GPM inter-intra, GPM
MMVD, GPM TM, GPM IBC, GPM IntraTMP, etc.
b) In one example, more than one intra modes may be derive for a GPM-coded block.
i. In one example, the HOG (or accumulated amplitudes of the gradient, in a similar way as DIMD)
of the two partitions on the prediction signal is respectively calculated, and the intra mode yielding the highest amplitude in each HOG is obtained (i.e., 2 intra modes in total) .
ii. In one example, the HOG (or accumulated amplitudes of the gradient, in a similar way as DIMD)
of the whole block on the prediction signal is calculated, and the intra modes yielding the highest two amplitude in HOG are obtained.
c) In one example, an index indicating the intra mode index may be signalled for GPM-coded block.
General aspects
14. In above examples, the video unit may refer to the video unit may refer to color component/sub-
picture/slice/tile/coding tree unit (CTU) /CTU row/groups of CTU/coding unit (CU) /prediction unit (PU) /transform unit (TU) /coding tree block (CTB) /coding block (CB) /prediction block (PB) /transform block (TB) /ablock/sub-block of a block/sub-region within a block/any other region that contains more than one sample or pixel.
15. Whether to and/or how to apply the disclosed methods above may be signalled at sequence level/group of
pictures level/picture level/slice level/tile group level, such as in sequence header/picture header/SPS/VPS/DPS/DCI/PPS/APS/slice header/tile group header.
16. Whether and/or how to apply the above methods may depend on the following information:
a) A message signalled in the DPS/SPS/VPS/PPS/APS/picture header/slice header/tile group header/
Largest coding unit (LCU) /Coding unit (CU) /LCU row/group of LCUs/TU/PU block/Video coding unit
b) Position of CU/PU/TU/block/Video coding unit
c) Block dimension of current block and/or its neighbouring blocks
d) Block shape of current block and/or its neighbouring blocks
e) coded mode of a block, e.g., IBC or non-IBC inter mode or non-IBC subblock mode
f) Indication of the color format (such as 4: 2: 0, 4: 4: 4)
g) Coding tree structure
h) Slice/tile group type and/or picture type
i) Color component (e.g., may be only applied on chroma components or luma component)
j) Temporal layer ID
k) Profiles/Levels/Tiers of a standard
l) Whether current slice satisfies low-delay condition or not.
m) Whether current block belongs to screen content or not.

Fig. 35 illustrates a flowchart of a method 3500 for video processing in accordance with embodiments of the present disclosure. The method 3500 is implemented during a conversion between a video unit of a video and a bitstream of the video.

At block 3510, for a conversion between a current video unit of a video and a bitstream of the video, a plurality of intra modes for the current video unit is determined. The current video unit is coded in a geometric partitioning mode (GPM) .

At block 3520, a plurality of transform kernel sets is determined based on the plurality of intra modes. A transform kernel set of the plurality of transform kernel sets includes at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set.

At block 3530, the conversion is performed based on the plurality of transform kernel sets.

The method 3500 enables determining the LFNST or NSPT kernel set based on the intra modes. The coding performance and coding efficiency can thus be improved.

In some embodiments, an inter LFNST or NSPT processing is applied to the current video unit based on the plurality of transform kernel sets. For example, the GPM mode may comprise: a regular GPM mode, a GPM inter-inter mode, a GPM inter-intra mode, a GPM merge mode with motion vector difference (MMVD) mode, a GPM template matching (TM) mode, a GPM intra block copy (IBC) mode, or a GPM intra template matching prediction (intraTMP) mode, etc.

In some embodiments, the plurality of intra modes are determined based on at least one of: a histogram of oriented gradients (HOG) of a prediction signal of the current video unit, or accumulated amplitudes of gradients of the prediction signal. For example, the plurality of intra modes comprises two intra modes yielding highest two HOG amplitudes among a list of candidate intra modes. For example, a determination of the HOG is same with that for DIMD mode.

In some embodiments, the plurality of intra modes are determined for a plurality of partitions of the current video unit. For example, for each candidate intra mode in a list of candidate intra modes, a first HOG is determined for a first prediction signal of a first partition of the current video unit and a second HOG is determined for a second prediction signal of a second partition of the current video unit. For example, the plurality of intra modes comprises a first intra mode yielding a highest HOG amplitude for the first partition and a second intra mode yielding a highest HOG amplitude for the second partition.

In some embodiments, at least one index indicates at least one of the plurality of intra modes. For example, an index indicating the intra mode index may be indicated in the bitstream for a GPM coded block.

In some embodiments, the conversion comprises encoding the current frame into the bitstream. Alternatively, or in addition, in some embodiments, the conversion comprises decoding the current frame from the bitstream.

According to further embodiments of the present disclosure, a non-transitory computer-readable recording medium is provided. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. In the method, a plurality of intra modes for a current video unit of the video is determined. The current video unit is coded in a geometric partitioning mode (GPM) . A plurality of transform kernel sets is determined based on the plurality of intra modes. A transform kernel set of the plurality of transform kernel sets comprises at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set. The bitstream is generated based on the plurality of transform kernel sets.

According to still further embodiments of the present disclosure, a method for storing bitstream of a video is provided. In the method, a plurality of intra modes for a current video unit of the video is determined. The current video unit is coded in a geometric partitioning mode (GPM) . A plurality of transform kernel sets is determined based on the plurality of intra modes. A transform kernel set of the plurality of transform kernel sets comprises at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set. The bitstream is generated based on the plurality of transform kernel sets. The bitstream is stored in a non-transitory computer-readable recording medium.

Fig. 36 illustrates a flowchart of a method 3600 for video processing in accordance with embodiments of the present disclosure. The method 3600 is implemented during a conversion between a video unit of a video and a bitstream of the video.

At block 3610, for a conversion between a current video unit of a video and a bitstream of the video, a target intra mode for the current video unit is determined.

At block 3620, at least one transform kernel set for inter coding is determined based on the target intra mode. The at least one transform kernel set includes at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set.

At block 3630, the conversion is performed based on the at least one transform kernel set.

The method 3600 enables determining LFNST or NSPT kernel set for inter coding based on the intra mode. The coding performance and coding efficiency can thus be improved.

In some embodiments, the target intra mode is determined based on at least one of: a template-based intra mode derivation (TIMD) mode of the current video unit, a decoder-side intra mode derivation (DIMD) mode of the current video unit, a DIMD mode of a neighboring reconstructed block, a coding mode of an adjacent intra coded block, a coding mode of a non-adjacent intra coded block, an intra propagation mode, a fixed intra mode, or an intra angular mode with an index offset relative to a further intra angular mode. The index offset may be set to±M, where M＞1, such as M=1.

In some embodiments, the TIMD mode may be the best or second best or N-th best intra mode yielding the least TM cost. The DIMD mode on the prediction block may be the best or second best or N-th best intra mode yielding the highest accumulated amplitudes of the gradient. The fixed intra mode comprises at least one of: a planar intra mode, a DC intra mode, or an intra mode with an angular index.

In some embodiments, the method 3600 further comprises: constructing an intra mode list for inter LFNST or NSPT; and determining the target intra mode from the intra mode list. For example, the intra mode list comprises the following intra modes in an order: a template-based intra mode derivation (TIMD) mode of the current video unit, a decoder-side intra mode derivation (DIMD) mode of the current video unit, a DIMD mode of a neighboring reconstructed block, a coding mode of an adjacent intra coded block, a coding mode of a non-adjacent intra coded block, an intra propagation mode, a fixed intra mode, or an intra angular mode with an index offset relative to a further intra angular mode. The fixed intra mode comprises at least one of: a planar intra mode, a DC intra mode, or an intra mode with an angular index.

In some embodiments, the number of intra modes in the intra mode list is less than or equal to a threshold number. For example, if the number of intra modes in the intra mode list reaches the threshold number, the construction of the intra mode list may terminate.

In some embodiments, the method 3600 further comprises: reordering the intra mode list based on a metric. The metric comprises a template matching cost.

In some embodiments, the construction of the intra mode list is performed with a pruning process. For example, if a first index of a first candidate intra mode is same with a second index of a second candidate intra mode in the intra mode list, the first candidate intra mode may be redundant.

In some embodiments, an index indicating a target candidate in the intra mode list is included in the bitstream. The index is coded with one of: a truncated Rice (TR) code, a truncated binary (TB) code, a k-th order Exponential-Golomb (EGk) code, k being an integer, or a fixed-length (FL) code.

In some embodiments, the target candidate indicated by the index is used to map corresponding LFNST or NSPT information, the LFNST or NSPT information comprising a LFNST or NSPT set (lfnstTrSetIdx) . For example, an index of a candidate LFNST or a candidate NSPT in the LFNST or NSPT set is included in the bitstream to indicate the candidate LFNST or the candidate NSPT.

In some embodiments, whether to and/or how to construct the intra mode list is based on coding information of at least one adjacent or non-adjacent block of the current video unit. The coding information indicates at least one of: whether a block is intra coded, or whether a block uses inter LFNST or NSPT or intra LFNST or NSPT.

In some embodiments, whether to and/or how to construct the intra mode list is based on coding information of the current video unit. For example, the coding information of the current video unit comprises at least one of: a coding mode of the current video unit, a width of the current video unit, a height of the current video unit, an area of the current video unit, a ratio of the width and the height of the current video unit, a smaller one of the width and the height, a larger one of the width and the height, a quantization parameter of the current video unit, a comparison of a temporal layer index of the current video unit with a threshold index, a value in the bitstream, a derived value, whether a current slice containing the current video unit satisfies a low-delay condition, or whether the current video unit belongs to a screen content.

In some embodiments, a first syntax element in the bitstream indicates whether an inter LFNST or NSPT is applied or not for the current video unit. For example, the first syntax element indicates to apply the inter LFNST or NSPT, and a second syntax element in the bitstream indicates an index of an intra mode associated with the inter LFNST or NSPT.

In some embodiments, if a value of the first syntax element satisfies a condition, the second syntax element may be included in the bitstream. For example, the condition is the value of the first syntax is larger than 0. The first syntax element indicates an index of a LFNST or NSPT candidate, and the second syntax element indicates an intra mode for deriving an index of a LFNST or NSPT set.

According to further embodiments of the present disclosure, a non-transitory computer-readable recording medium is provided. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. In the method, a target intra mode for a current video unit of the video is determined. At least one transform kernel set for inter coding is determined based on the target intra mode. The at least one transform kernel set comprises at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set. The bitstream is generated based on the at least one transform kernel set.

According to still further embodiments of the present disclosure, a method for storing bitstream of a video is provided. In the method, a target intra mode for a current video unit of the video is determined. At least one transform kernel set for inter coding is determined based on the target intra mode. The at least one transform kernel set comprises at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set. The bitstream is generated based on the at least one transform kernel set. The bitstream is stored in a non-transitory computer-readable recording medium.

Fig. 37 illustrates a flowchart of a method 3700 for video processing in accordance with embodiments of the present disclosure. The method 3700 is implemented during a conversion between a video unit of a video and a bitstream of the video.

At block 3710, for a conversion between a current video unit of a video and a bitstream of the video, at least one merge list for the current video unit for an inter transform is determined. The inter transform includes at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) .

At block 3720, the conversion is performed based on the at least one merge list.

The method 3700 enables constructing the merge list for inter LFNST or NSPT. The coding performance can thus be improved.

In some embodiments, a merge candidate in the at least one merge list comprises at least partial of information related to the inter transform. For example, the information related to the inter transform comprises at least one of: an index of an LFNST or NSPT set, or a candidate index of an LFNST or NSPT candidate in the LFNST or NSPT set.

In some embodiments, a merge candidate for an inter LFNST or NSPT may be obtained from adjacent locations of the current video unit. In some other embodiments, a merge candidate for an inter LFNST or NSPT may be obtained from non-adjacent locations of the current video unit.

In some embodiments, a merge candidate for an inter LFNST or NSPT is obtained from at least one location. The at least one location may be determined based on at least one of a motion vector or a block vector of the current video unit. In some other embodiments, a merge candidate for an inter LFNST or NSPT is obtained from a history table comprising inter LFNST or NSPT information associated with previously coded blocks. In some other embodiments, a merge candidate for an inter LFNST or NSPT is obtained from at least one temporal location.

In some embodiments, a plurality of merge candidates for an inter LFNST or NSPT are checked and included in the at least one merge list in a pre-defined order. For example, at least one merge candidate specified by a motion vector or a block vector of the current video unit is checked before at least one merge candidate obtained from adjacent blocks. For example, an adjacent merge candidate is checked before a non-adjacent merge candidate. For example, a checking order of the plurality of merge candidates is based on distances of the plurality of merge candidates to a current location of the current video unit. For example, a first priority of a first merge candidate of a first location is higher than a second priority of a second merge candidate of a second location, a first distance from the first location to a current location of the current video unit being less than a second location from the second location to the current location. An order of the first and second merge candidates to be checked is determined based on the first priority and second priority.

In some embodiments, the method 3700 further comprises: determining an availability of an inter LFNST or NSPT merge candidate in a location based on at least one of: whether a coding block associated with the location is intra-coded, whether a candidate index of the LNFST or NSPT merge candidate is equal to a first value, or whether the coding block associated with the location uses an inter or intra LFNST or NSPT. The first value may be set to 0. For example, if the coding information of the location satisfies a condition, corresponding inter or intra LFNST or NSPT information may be included in the at least one merge list.

In some embodiments, a checking process of merge candidates terminates when the number of available candidates in the at least one merge candidate list reaches a maximum allowed number.

In some embodiments, the method 3700 further comprises: storing a history table comprising inter LFNST or NSPT information associated with at least one previously coded block. The history table is maintained or updated during the conversion.

In some embodiments, a construction of the at least one merge list for inter LFNST or NSPT is along with a pruning process. For example, if a first index of a first candidate is same with at least one of a second index of a LFNST set or a third index of an LFNST candidate in the at least one merge list, the first candidate may be determined as redundant.

In some embodiments, an index indicating a candidate in the at last one merge list is indicated in the bitstream. For example, the index is coded with one of: a truncated Rice (TR) code, a truncated binary (BR) code, a k-th order Exponential Golomb (EGk) code, k being an integer, or a fixed-length (FL) code. For example, inherited inter LFNST or NSPT information indicated by the index is used to conduct an inter LFNST or NSPT.

In some embodiments, a first syntax element in the bitstream indicates whether an inter LFNST or NSPT is used or not for the current video unit. For example, the first syntax element indicates to use the inter LFNST or NSPT, and a second syntax element in the bitstream indicates a candidate index in an inter LFNST or NSPT merge list. For example, at least one of an LFNST or NSPT set or an LFNST or NSPT candidate indicated by the second syntax element is used for the inter LFNST or NSPT.

In some embodiments, the first syntax element indicates to use the inter LFNST or NSPT, and a second syntax element in the bitstream indicates whether a merge mode is used or not for the current video unit. The second syntax element indicates to use an inter LFNST or NSPT merge mode, a third syntax element in the bitstream indicates a candidate index in an inter LFNST or NSPT merge list. The second syntax element indicates an inter LFNST or NSPT merge mode being not used, a fourth syntax element in the bitstream indicates a candidate index in an LFNST or NSPT set.

In some embodiments, if the inter LFNST or NSPT merge mode being not used, an intra mode may be determined for an LFNST or NSPT set. The intra mode is mapped to an index for the LFNST or NSPT set. For example, the fourth syntax element indicates a kernel set of the inter LFNST or NSPT set.

In some embodiments, the first syntax element indicates to use the inter LFNST or NSPT and the first syntax element satisfies a condition, and a second syntax element in the bitstream indicates a candidate index in an LFNST or NSPT set. For example, the condition comprises a value of the first syntax element being a predefined value. For example, the first syntax element indicates to use the inter LFNST or NSPT but the first syntax element fails to satisfy the condition, a value of the first syntax element or a transformed value of the first syntax element is used as a candidate index in an inter LFNST or NSPT merge list. For example, no further syntax element regarding LFNST or NSPT is included in the bitstream, and inherited inter LFNST or NSPT information is used for the inter LFNST or NSPT.

In some embodiments, if the first syntax element failes to satisfy the condition, an intra mode may be determined for an LFNST or NSPT set. The intra mode is mapped to an index for the LFNST or NSPT set. For example, the second syntax element indicates a kernel set of the inter LFNST or NSPT set.

According to further embodiments of the present disclosure, a non-transitory computer-readable recording medium is provided. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. In the method, at least one merge list for a current video unit of the video for an inter transform is determined. The inter transform comprises at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) . The bitstream is generated based on the at least one merge list.

According to still further embodiments of the present disclosure, a method for storing bitstream of a video is provided. In the method, at least one merge list for a current video unit of the video for an inter transform is determined. The inter transform comprises at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) . The bitstream is generated based on the at least one merge list. The bitstream is stored in a non-transitory computer-readable recording medium.

In some embodiments, the current video unit or a video unit comprises at least one of: a colour component, a sub-picture, a slice, a tile, a coding tree unit (CTU) , a CTU row, a groups of CTU, a coding unit (CU) , a prediction unit (PU) , a transform unit (TU) , a coding tree block (CTB) , a coding block (CB) , a prediction block (PB) , a transform block (TB) , a block, sub-block of a block, sub-region within a block, or a region that contains more than one sample or pixel.

In some embodiments, an indication of whether to and/or how to apply the method 3500 and/or the method 3600 and/or the method 3700 is indicated at one of: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level.

In some embodiments, an indication of whether to and/or how to apply the method 3500 and/or the method 3600 and/or the method 3700 is indicated in one of: a sequence header, a picture header, a sequence parameter set (SPS) , a video parameter set (VPS) , a dependency parameter set (DPS) , a decoding capability information (DCI) , a picture parameter set (PPS) , an adaptation parameter sets (APS) , a slice header, or a tile group header.

In some embodiments, whether to and/or how to apply the method 3500 and/or the method 3600 and/or the method 3700 is based on at least one of: a message included in one of: a dependency parameter set (DPS) , a sequence parameter set (SPS) , a video parameter set (VPS) , a picture parameter set (PPS) , an adaptation parameter sets (APS) , a picture header, a slice header, a tile group header, a largest coding unit (LCU) , a coding unit (CU) , a LCU row, a group of LCUs, a transform unit (TU) , a prediction unit (PU) block, or a video coding unit, a position of CU, PU, TU, block, or the video coding unit, a block dimension of the current video unit and/or neighboring blocks of the current video unit, a block shape of the current video unit and/or neighboring blocks of the current video unit, a coded mode of a block, an indication of a color format, a coding tree structure, a slice, a tile group type, a picture type, a colour component, a temporal layer identifier (ID) , profiles, levels, or tiers of a standard, whether a current slice comprising the current video unit satisfies a low-delay condition, or whether the current video unit belongs to screen content. For example, the color component may be only applied on chroma components or luma component.

In some embodiments, the coded mode comprises one of: an intra block copy (IBC) , a non-IBC inter mode, or a non-IBC subblock mode. The color format comprises one of: 4: 2: 0, or 4: 4: 4.

The methods 3500, 3600 and/or 3700 may be applied separately, or in any combination. With these methods, the coding performance can be enhanced.

Implementations of the present disclosure can be described in view of the following clauses, the features of which can be combined in any reasonable manner.

Clause 1. A method for video processing, comprising: determining, for a conversion between a current video unit of a video and a bitstream of the video, a plurality of intra modes for the current video unit, the current video unit being coded in a geometric partitioning mode (GPM) ; determine a plurality of transform kernel sets based on the plurality of intra modes, a transform kernel set of the plurality of transform kernel sets comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and perform the conversion based on the plurality of transform kernel sets.

Clause 2. The method of clause 1, wherein an inter LFNST or NSPT processing is applied to the current video unit based on the plurality of transform kernel sets.

Clause 3. The method of clause 1 or 2, wherein the GPM mode comprises at least one of: a regular GPM mode, a GPM inter-inter mode, a GPM inter-intra mode, a GPM merge mode with motion vector difference (MMVD) mode, a GPM template matching (TM) mode, a GPM intra block copy (IBC) mode, or a GPM intra template matching prediction (intraTMP) mode.

Clause 4. The method of any of clauses 1-3, wherein the plurality of intra modes are determined based on at least one of: a histogram of oriented gradients (HOG) of a prediction signal of the current video unit, or accumulated amplitudes of gradients of the prediction signal.

Clause 5. The method of clause 4, wherein the plurality of intra modes comprises two intra modes yielding highest two HOG amplitudes among a list of candidate intra modes.

Clause 6. The method of clause 4, wherein a determination of the HOG is same with that for DIMD mode.

Clause 7. The method of any of clauses 1-3, wherein the plurality of intra modes are determined for a plurality of partitions of the current video unit.

Clause 8. The method of clause 7, wherein for each candidate intra mode in a list of candidate intra modes, a first HOG is determined for a first prediction signal of a first partition of the current video unit and a second HOG is determined for a second prediction signal of a second partition of the current video unit.

Clause 9. The method of clause 8, wherein the plurality of intra modes comprises a first intra mode yielding a highest HOG amplitude for the first partition and a second intra mode yielding a highest HOG amplitude for the second partition.

Clause 10. The method of any of clauses 1-9, wherein at least one index indicating at least one of the plurality of intra modes is indicated in the bitstream for a GPM coded block.

Clause 11. A method for video processing, comprising: determining, for a conversion between a current video unit of a video and a bitstream of the video, a target intra mode for the current video unit; determine at least one transform kernel set for inter coding based on the target intra mode, the at least one transform kernel set comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and perform the conversion based on the at least one transform kernel set.

Clause 12. The method of clause 11, wherein the target intra mode is determined based on at least one of: a template-based intra mode derivation (TIMD) mode of the current video unit, a decoder-side intra mode derivation (DIMD) mode of the current video unit, a DIMD mode of a neighboring reconstructed block, a coding mode of an adjacent intra coded block, a coding mode of a non-adjacent intra coded block, an intra propagation mode, a fixed intra mode, or an intra angular mode with an index offset relative to a further intra angular mode.

Clause 13. The method of clause 12, wherein the fixed intra mode comprises at least one of: a planar intra mode, a DC intra mode, or an intra mode with an angular index.

Clause 14. The method of any of clauses 11-13, further comprising: constructing an intra mode list for inter LFNST or NSPT; and determining the target intra mode from the intra mode list.

Clause 15. The method of clause 14, wherein the intra mode list comprises the following intra modes in an order: a template-based intra mode derivation (TIMD) mode of the current video unit, a decoder-side intra mode derivation (DIMD) mode of the current video unit, a DIMD mode of a neighboring reconstructed block, a coding mode of an adjacent intra coded block, a coding mode of a non-adjacent intra coded block, an intra propagation mode, a fixed intra mode, or an intra angular mode with an index offset relative to a further intra angular mode.

Clause 16. The method of clause 15, wherein the fixed intra mode comprises at least one of: a planar intra mode, a DC intra mode, or an intra mode with an angular index.

Clause 17. The method of any of clauses 14-16, wherein the number of intra modes in the intra mode list is less than or equal to a threshold number.

Clause 18. The method of clause 17, wherein in response to the number of intra modes in the intra mode list reaches the threshold number, the construction of the intra mode list terminates.

Clause 19. The method of any of clauses 14-18, further comprising: reordering the intra mode list based on a metric, the metric comprising a template matching cost.

Clause 20. The method of any of clauses 14-19, wherein the construction of the intra mode list is performed with a pruning process.

Clause 21. The method of clause 20, wherein in response to a first index of a first candidate intra mode being same with a second index of a second candidate intra mode in the intra mode list, the first candidate intra mode is redundant.

Clause 22. The method of any of clauses 14-21, wherein an index indicating a target candidate in the intra mode list is included in the bitstream.

Clause 23. The method of clause 22, wherein the index is coded with one of: a truncated Rice (TR) code, a truncated binary (TB) code, a k-th order Exponential-Golomb (EGk) code, k being an integer, or a fixed-length (FL) code.

Clause 24. The method of clause 22 or 23, wherein the target candidate indicated by the index is used to map corresponding LFNST or NSPT information, the LFNST or NSPT information comprising a LFNST or NSPT set.

Clause 25. The method of clause 24, wherein an index of a candidate LFNST or a candidate NSPT in the LFNST or NSPT set is included in the bitstream to indicate the candidate LFNST or the candidate NSPT.

Clause 26. The method of any of clauses 11-25, wherein whether to and/or how to construct the intra mode list is based on coding information of at least one adjacent or non-adjacent block of the current video unit, wherein the coding information indicates at least one of: whether a block is intra coded, or whether a block uses inter LFNST or NSPT or intra LFNST or NSPT.

Clause 27. The method of any of clauses 11-25, wherein whether to and/or how to construct the intra mode list is based on coding information of the current video unit.

Clause 28. The method of clause 27, wherein the coding information of the current video unit comprises at least one of: a coding mode of the current video unit, a width of the current video unit, a height of the current video unit, an area of the current video unit, a ratio of the width and the height of the current video unit, a smaller one of the width and the height, a larger one of the width and the height, a quantization parameter of the current video unit, a comparison of a temporal layer index of the current video unit with a threshold index, a value in the bitstream, a derived value, whether a current slice containing the current video unit satisfies a low-delay condition, or whether the current video unit belongs to a screen content.

Clause 29. The method of any of clauses 11-28, wherein a first syntax element in the bitstream indicates whether an inter LFNST or NSPT is applied or not for the current video unit.

Clause 30. The method of clause 29, wherein the first syntax element indicates to apply the inter LFNST or NSPT, and a second syntax element in the bitstream indicates an index of an intra mode associated with the inter LFNST or NSPT.

Clause 31. The method of clause 30, wherein in response to a value of the first syntax element satisfying a condition, the second syntax element is included in the bitstream.

Clause 32. The method of clause 30 or 31, wherein the first syntax element indicates an index of a LFNST or NSPT candidate, and the second syntax element indicates an intra mode for deriving an index of a LFNST or NSPT set.

Clause 33. A method for video processing, comprising: determining, for a conversion between a current video unit of a video and a bitstream of the video, at least one merge list for the current video unit for an inter transform, the inter transform comprising at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) ; and perform the conversion based on the at least one merge list.

Clause 34. The method of clause 33, wherein a merge candidate in the at least one merge list comprises at least partial of information related to the inter transform.

Clause 35. The method of clause 33 or 34, wherein the information related to the inter transform comprises at least one of: an index of an LFNST or NSPT set, or a candidate index of an LFNST or NSPT candidate in the LFNST or NSPT set.

Clause 36. The method of any of clauses 33-35, wherein a merge candidate for an inter LFNST or NSPT is obtained from at least one of: adjacent locations of the current video unit, or non-adjacent locations of the current video unit.

Clause 37. The method of any of clauses 33-36, wherein a merge candidate for an inter LFNST or NSPT is obtained from at least one location, the at least one location being determined based on at least one of a motion vector or a block vector of the current video unit.

Clause 38. The method of any of clauses 33-37, wherein a merge candidate for an inter LFNST or NSPT is obtained from a history table comprising inter LFNST or NSPT information associated with previously coded blocks.

Clause 39. The method of any of clauses 33-38, wherein a merge candidate for an inter LFNST or NSPT is obtained from at least one temporal location.

Clause 40. The method of any of clauses 33-39, wherein a plurality of merge candidates for an inter LFNST or NSPT are checked and included in the at least one merge list in a pre-defined order.

Clause 41. The method of clause 40, wherein at least one merge candidate specified by a motion vector or a block vector of the current video unit is checked before at least one merge candidate obtained from adjacent blocks.

Clause 42. The method of clause 40 or 41, wherein an adjacent merge candidate is checked before a non-adjacent merge candidate.

Clause 43. The method of any of clauses 40-42, wherein a checking order of the plurality of merge candidates is based on distances of the plurality of merge candidates to a current location of the current video unit.

Clause 44. The method of any of clauses 40-43, wherein a first priority of a first merge candidate of a first location is higher than a second priority of a second merge candidate of a second location, a first distance from the first location to a current location of the current video unit being less than a second location from the second location to the current location, and wherein an order of the first and second merge candidates to be checked is determined based on the first priority and second priority.

Clause 45. The method of any of clauses 33-44, further comprising: determining an availability of an inter LFNST or NSPT merge candidate in a location based on at least one of: whether a coding block associated with the location is intra-coded, whether a candidate index of the LNFST or NSPT merge candidate is equal to a first value, or whether the coding block associated with the location uses an inter or intra LFNST or NSPT.

Clause 46. The method of clause 45, wherein in response to the coding information of the location satisfying a condition, corresponding inter or intra LFNST or NSPT information is included in the at least one merge list.

Clause 47. The method of any of clauses 33-46, wherein a checking process of merge candidates terminates in response to the number of available candidates in the at least one merge candidate list reaching a maximum allowed number.

Clause 48. The method of any of clauses 33-47, further comprising: storing a history table comprising inter LFNST or NSPT information associated with at least one previously coded block.

Clause 49. The method of clause 48, wherein the history table is maintained or updated during the conversion.

Clause 50. The method of any of clauses 33-47, wherein a construction of the at least one merge list for inter LFNST or NSPT is along with a pruning process.

Clause 51. The method of clause 50, wherein in response to a first index of a first candidate is same with at least one of a second index of a LFNST set or a third index of an LFNST candidate in the at least one merge list, the first candidate is determined as redundant.

Clause 52. The method of any of clauses 33-51, wherein an index indicating a candidate in the at last one merge list is indicated in the bitstream.

Clause 53. The method of clause 52, wherein the index is coded with one of: a truncated Rice (TR) code, a truncated binary (BR) code, a k-th order Exponential Golomb (EGk) code, k being an integer, or a fixed-length (FL) code.

Clause 54. The method of clause 52, wherein inherited inter LFNST or NSPT information indicated by the index is used to conduct an inter LFNST or NSPT.

Clause 55. The method of any of clauses 33-54, wherein a first syntax element in the bitstream indicates whether an inter LFNST or NSPT is used or not for the current video unit.

Clause 56. The method of clause 55, wherein the first syntax element indicates to use the inter LFNST or NSPT, and a second syntax element in the bitstream indicates a candidate index in an inter LFNST or NSPT merge list.

Clause 57. The method of clause 56, wherein at least one of an LFNST or NSPT set or an LFNST or NSPT candidate indicated by the second syntax element is used for the inter LFNST or NSPT.

Clause 58. The method of clause 55, wherein the first syntax element indicates to use the inter LFNST or NSPT, and a second syntax element in the bitstream indicates whether a merge mode is used or not for the current video unit.

Clause 59. The method of clause 58, wherein the second syntax element indicates to use an inter LFNST or NSPT merge mode, a third syntax element in the bitstream indicates a candidate index in an inter LFNST or NSPT merge list.

Clause 60. The method of clause 58, wherein the second syntax element indicates an inter LFNST or NSPT merge mode being not used, a fourth syntax element in the bitstream indicates a candidate index in an LFNST or NSPT set.

Clause 61. The method of clause 60, wherein in response to the inter LFNST or NSPT merge mode being not used, an intra mode is determined for an LFNST or NSPT set.

Clause 62. The method of clause 61, wherein the intra mode is mapped to an index for the LFNST or NSPT set.

Clause 63. The method of clause 61, wherein the fourth syntax element indicates a kernel set of the inter LFNST or NSPT set.

Clause 64. The method of clause 55, wherein the first syntax element indicates to use the inter LFNST or NSPT and the first syntax element satisfies a condition, and a second syntax element in the bitstream indicates a candidate index in an LFNST or NSPT set.

Clause 65. The method of clause 64, wherein the condition comprises a value of the first syntax element being a predefined value.

Clause 66. The method of clause 64 or 65, wherein the first syntax element indicates to use the inter LFNST or NSPT but the first syntax element fails to satisfy the condition, a value of the first syntax element or a transformed value of the first syntax element is used as a candidate index in an inter LFNST or NSPT merge list.

Clause 67. The method of clause 66, wherein no further syntax element regarding LFNST or NSPT is included in the bitstream, and inherited inter LFNST or NSPT information is used for the inter LFNST or NSPT.

Clause 68. The method of clause 66, wherein in response to the first syntax element failing to satisfy the condition, an intra mode is determined for an LFNST or NSPT set.

Clause 69. The method of clause 68, wherein the intra mode is mapped to an index for the LFNST or NSPT set.

Clause 70. The method of clause 69, wherein the second syntax element indicates a kernel set of the inter LFNST or NSPT set.

Clause 71. The method of any of clauses 1-70, wherein the current video unit or a video unit comprises at least one of: a colour component, a sub-picture, a slice, a tile, a coding tree unit (CTU) , a CTU row, a groups of CTU, a coding unit (CU) , a prediction unit (PU) , a transform unit (TU) , a coding tree block (CTB) , a coding block (CB) , a prediction block (PB) , a transform block (TB) , a block, sub-block of a block, sub-region within a block, or a region that contains more than one sample or pixel.

Clause 72. The method of any of clauses 1-71, wherein an indication of whether to and/or how to apply the method is indicated at one of: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level.

Clause 73. The method of any of clauses 1-71, wherein an indication of whether to and/or how to apply the method is indicated in one of: a sequence header, a picture header, a sequence parameter set (SPS) , a video parameter set (VPS) , a dependency parameter set (DPS) , a decoding capability information (DCI) , a picture parameter set (PPS) , an adaptation parameter sets (APS) , a slice header, or a tile group header.

Clause 74. The method of any of clauses 1-71, wherein whether to and/or how to apply the method is based on at least one of: a message included in one of: a dependency parameter set (DPS) , a sequence parameter set (SPS) , a video parameter set (VPS) , a picture parameter set (PPS) , an adaptation parameter sets (APS) , a picture header, a slice header, a tile group header, a largest coding unit (LCU) , a coding unit (CU) , a LCU row, a group of LCUs, a transform unit (TU) , a prediction unit (PU) block, or a video coding unit, a position of CU, PU, TU, block, or the video coding unit, a block dimension of the current video unit and/or neighboring blocks of the current video unit, a block shape of the current video unit and/or neighboring blocks of the current video unit, a coded mode of a block, an indication of a color format, a coding tree structure, a slice, a tile group type, a picture type, a colour component, a temporal layer identifier (ID) , profiles, levels, or tiers of a standard, whether a current slice comprising the current video unit satisfies a low-delay condition, or whether the current video unit belongs to screen content.

Clause 75. The method of clause 74, wherein the coded mode comprises one of: an intra block copy (IBC) , a non-IBC inter mode, or a non-IBC subblock mode, or wherein the color format comprises one of: 4: 2: 0, or 4: 4: 4.

Clause 76. The method of any of clauses 1-75, wherein the conversion comprises encoding the current video unit into the bitstream.

Clause 77. The method of any of clauses 1-75, wherein the conversion comprises decoding the current video unit from the bitstream.

Clause 78. An apparatus for video processing comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to perform a method in accordance with any of clauses 1-77.

Clause 79. A non-transitory computer-readable storage medium storing instructions that cause a processor to perform a method in accordance with any of clauses 1-77.

Clause 80. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises: determining a plurality of intra modes for a current video unit of the video, the current video unit being coded in a geometric partitioning mode (GPM) ; determining a plurality of transform kernel sets based on the plurality of intra modes, a transform kernel set of the plurality of transform kernel sets comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and generating the bitstream based on the plurality of transform kernel sets.

Clause 81. A method for storing a bitstream of a video, comprising: determining a plurality of intra modes for a current video unit of the video, the current video unit being coded in a geometric partitioning mode (GPM) ; determining a plurality of transform kernel sets based on the plurality of intra modes, a transform kernel set of the plurality of transform kernel sets comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; generating the bitstream based on the plurality of transform kernel sets; and storing the bitstream in a non-transitory computer-readable recording medium.

Clause 82. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises: determining a target intra mode for a current video unit of the video; determine at least one transform kernel set for inter coding based on the target intra mode, the at least one transform kernel set comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and generating the bitstream based on the at least one transform kernel set.

Clause 83. A method for storing a bitstream of a video, comprising: determining a target intra mode for a current video unit of the video; determine at least one transform kernel set for inter coding based on the target intra mode, the at least one transform kernel set comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; generating the bitstream based on the at least one transform kernel set; and storing the bitstream in a non-transitory computer-readable recording medium.

Clause 84. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises: determining at least one merge list for a current video unit of the video for an inter transform, the inter transform comprising at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) ; and generating the bitstream based on the at least one merge list.

Clause 85. A method for storing a bitstream of a video, comprising: determining at least one merge list for a current video unit of the video for an inter transform, the inter transform comprising at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) ; generating the bitstream based on the at least one merge list; and storing the bitstream in a non-transitory computer-readable recording medium.
Example Device

Fig. 38 illustrates a block diagram of a computing device 3800 in which various embodiments of the present disclosure can be implemented. The computing device 3800 may be implemented as or included in the source device 110 (or the video encoder 114 or 200) or the destination device 120 (or the video decoder 124 or 300) .

It would be appreciated that the computing device 3800 shown in Fig. 38 is merely for purpose of illustration, without suggesting any limitation to the functions and scopes of the embodiments of the present disclosure in any manner.

As shown in Fig. 38, the computing device 3800 includes a general-purpose computing device 3800. The computing device 3800 may at least comprise one or more processors or processing units 3810, a memory 3820, a storage unit 3830, one or more communication units 3840, one or more input devices 3850, and one or more output devices 3860.

In some embodiments, the computing device 3800 may be implemented as any user terminal or server terminal having the computing capability. The server terminal may be a server, a large-scale computing device or the like that is provided by a service provider. The user terminal may for example be any type of mobile terminal, fixed terminal, or portable terminal, including a mobile phone, station, unit, device, multimedia computer, multimedia tablet, Internet node, communicator, desktop computer, laptop computer, notebook computer, netbook computer, tablet computer, personal communication system (PCS) device, personal navigation device, personal digital assistant (PDA) , audio/video player, digital camera/video camera, positioning device, television receiver, radio broadcast receiver, E-book device, gaming device, or any combination thereof, including the accessories and peripherals of these devices, or any combination thereof. It would be contemplated that the computing device 3800 can support any type of interface to a user (such as “wearable” circuitry and the like) .

The processing unit 3810 may be a physical or virtual processor and can implement various processes based on programs stored in the memory 3820. In a multi-processor system, multiple processing units execute computer executable instructions in parallel so as to improve the parallel processing capability of the computing device 3800. The processing unit 3810 may also be referred to as a central processing unit (CPU) , a microprocessor, a controller or a microcontroller.

The computing device 3800 typically includes various computer storage medium. Such medium can be any medium accessible by the computing device 3800, including, but not limited to, volatile and non-volatile medium, or detachable and non-detachable medium. The memory 3820 can be a volatile memory (for example, a register, cache, Random Access Memory (RAM) ) , a non-volatile memory (such as a Read-Only Memory (ROM) , Electrically Erasable Programmable Read-Only Memory (EEPROM) , or a flash memory) , or any combination thereof. The storage unit 3830 may be any detachable or non-detachable medium and may include a machine-readable medium such as a memory, flash memory drive, magnetic disk or another other media, which can be used for storing information and/or data and can be accessed in the computing device 3800.

The computing device 3800 may further include additional detachable/non-detachable, volatile/non-volatile memory medium. Although not shown in Fig. 38, it is possible to provide a magnetic disk drive for reading from and/or writing into a detachable and non-volatile magnetic disk and an optical disk drive for reading from and/or writing into a detachable non-volatile optical disk. In such cases, each drive may be connected to a bus (not shown) via one or more data medium interfaces.

The communication unit 3840 communicates with a further computing device via the communication medium. In addition, the functions of the components in the computing device 3800 can be implemented by a single computing cluster or multiple computing machines that can communicate via communication connections. Therefore, the computing device 3800 can operate in a networked environment using a logical connection with one or more other servers, networked personal computers (PCs) or further general network nodes.

The input device 3850 may be one or more of a variety of input devices, such as a mouse, keyboard, tracking ball, voice-input device, and the like. The output device 3860 may be one or more of a variety of output devices, such as a display, loudspeaker, printer, and the like. By means of the communication unit 3840, the computing device 3800 can further communicate with one or more external devices (not shown) such as the storage devices and display device, with one or more devices enabling the user to interact with the computing device 3800, or any devices (such as a network card, a modem and the like) enabling the computing device 3800 to communicate with one or more other computing devices, if required. Such communication can be performed via input/output (I/O) interfaces (not shown) .

In some embodiments, instead of being integrated in a single device, some or all components of the computing device 3800 may also be arranged in cloud computing architecture. In the cloud computing architecture, the components may be provided remotely and work together to implement the functionalities described in the present disclosure. In some embodiments, cloud computing provides computing, software, data access and storage service, which will not require end users to be aware of the physical locations or configurations of the systems or hardware providing these services. In various embodiments, the cloud computing provides the services via a wide area network (such as Internet) using suitable protocols. For example, a cloud computing provider provides applications over the wide area network, which can be accessed through a web browser or any other computing components. The software or components of the cloud computing architecture and corresponding data may be stored on a server at a remote position. The computing resources in the cloud computing environment may be merged or distributed at locations in a remote data center. Cloud computing infrastructures may provide the services through a shared data center, though they behave as a single access point for the users. Therefore, the cloud computing architectures may be used to provide the components and functionalities described herein from a service provider at a remote location. Alternatively, they may be provided from a conventional server or installed directly or otherwise on a client device.

The computing device 3800 may be used to implement video encoding/decoding in embodiments of the present disclosure. The memory 3820 may include one or more video coding modules 3825 having one or more program instructions. These modules are accessible and executable by the processing unit 3810 to perform the functionalities of the various embodiments described herein.

In the example embodiments of performing video encoding, the input device 3850 may receive video data as an input 3870 to be encoded. The video data may be processed, for example, by the video coding module 3825, to generate an encoded bitstream. The encoded bitstream may be provided via the output device 3860 as an output 3880.

In the example embodiments of performing video decoding, the input device 3850 may receive an encoded bitstream as the input 3870. The encoded bitstream may be processed, for example, by the video coding module 3825, to generate decoded video data. The decoded video data may be provided via the output device 3860 as the output 3880.

While this disclosure has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present application as defined by the appended claims. Such variations are intended to be covered by the scope of this present application. As such, the foregoing description of embodiments of the present application is not intended to be limiting.

Claims

A method for video processing, comprising:

determining, for a conversion between a current video unit of a video and a bitstream of the video, a plurality of intra modes for the current video unit, the current video unit being coded in a geometric partitioning mode (GPM) ;

determine a plurality of transform kernel sets based on the plurality of intra modes, a transform kernel set of the plurality of transform kernel sets comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and

perform the conversion based on the plurality of transform kernel sets.
The method of claim 1, wherein an inter LFNST or NSPT processing is applied to the current video unit based on the plurality of transform kernel sets.
The method of claim 1 or 2, wherein the GPM mode comprises at least one of: a regular GPM mode, a GPM inter-inter mode, a GPM inter-intra mode, a GPM merge mode with motion vector difference (MMVD) mode, a GPM template matching (TM) mode, a GPM intra block copy (IBC) mode, or a GPM intra template matching prediction (intraTMP) mode.
The method of any of claims 1-3, wherein the plurality of intra modes are determined based on at least one of: a histogram of oriented gradients (HOG) of a prediction signal of the current video unit, or accumulated amplitudes of gradients of the prediction signal.
The method of claim 4, wherein the plurality of intra modes comprises two intra modes yielding highest two HOG amplitudes among a list of candidate intra modes.
The method of claim 4, wherein a determination of the HOG is same with that for DIMD mode.
The method of any of claims 1-3, wherein the plurality of intra modes are determined for a plurality of partitions of the current video unit.
The method of claim 7, wherein for each candidate intra mode in a list of candidate intra modes, a first HOG is determined for a first prediction signal of a first partition of the current video unit and a second HOG is determined for a second prediction signal of a second partition of the current video unit.
The method of claim 8, wherein the plurality of intra modes comprises a first intra mode yielding a highest HOG amplitude for the first partition and a second intra mode yielding a highest HOG amplitude for the second partition.
The method of any of claims 1-9, wherein at least one index indicating at least one of the plurality of intra modes is indicated in the bitstream for a GPM coded block.
A method for video processing, comprising:

determining, for a conversion between a current video unit of a video and a bitstream of the video, a target intra mode for the current video unit;

determine at least one transform kernel set for inter coding based on the target intra mode, the at least one transform kernel set comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and

perform the conversion based on the at least one transform kernel set.
The method of claim 11, wherein the target intra mode is determined based on at least one of:

a template-based intra mode derivation (TIMD) mode of the current video unit,

a decoder-side intra mode derivation (DIMD) mode of the current video unit,

a DIMD mode of a neighboring reconstructed block,

a coding mode of an adjacent intra coded block,

a coding mode of a non-adjacent intra coded block,

an intra propagation mode,

a fixed intra mode, or

an intra angular mode with an index offset relative to a further intra angular mode.
The method of claim 12, wherein the fixed intra mode comprises at least one of: a planar intra mode, a DC intra mode, or an intra mode with an angular index.
The method of any of claims 11-13, further comprising:

constructing an intra mode list for inter LFNST or NSPT; and

determining the target intra mode from the intra mode list.
The method of claim 14, wherein the intra mode list comprises the following intra modes in an order:

a template-based intra mode derivation (TIMD) mode of the current video unit,

a decoder-side intra mode derivation (DIMD) mode of the current video unit,

a DIMD mode of a neighboring reconstructed block,

a coding mode of an adjacent intra coded block,

a coding mode of a non-adjacent intra coded block,

an intra propagation mode,

a fixed intra mode, or

an intra angular mode with an index offset relative to a further intra angular mode.
The method of claim 15, wherein the fixed intra mode comprises at least one of: a planar intra mode, a DC intra mode, or an intra mode with an angular index.
The method of any of claims 14-16, wherein the number of intra modes in the intra mode list is less than or equal to a threshold number.
The method of claim 17, wherein in response to the number of intra modes in the intra mode list reaches the threshold number, the construction of the intra mode list terminates.
The method of any of claims 14-18, further comprising:

reordering the intra mode list based on a metric, the metric comprising a template matching cost.
The method of any of claims 14-19, wherein the construction of the intra mode list is performed with a pruning process.
The method of claim 20, wherein in response to a first index of a first candidate intra mode being same with a second index of a second candidate intra mode in the intra mode list, the first candidate intra mode is redundant.
The method of any of claims 14-21, wherein an index indicating a target candidate in the intra mode list is included in the bitstream.
The method of claim 22, wherein the index is coded with one of: a truncated Rice (TR) code, a truncated binary (TB) code, a k-th order Exponential-Golomb (EGk) code, k being an integer, or a fixed-length (FL) code.
The method of claim 22 or 23, wherein the target candidate indicated by the index is used to map corresponding LFNST or NSPT information, the LFNST or NSPT information comprising a LFNST or NSPT set.
The method of claim 24, wherein an index of a candidate LFNST or a candidate NSPT in the LFNST or NSPT set is included in the bitstream to indicate the candidate LFNST or the candidate NSPT.
The method of any of claims 11-25, wherein whether to and/or how to construct the intra mode list is based on coding information of at least one adjacent or non-adjacent block of the current video unit,

wherein the coding information indicates at least one of: whether a block is intra coded, or whether a block uses inter LFNST or NSPT or intra LFNST or NSPT.
The method of any of claims 11-25, wherein whether to and/or how to construct the intra mode list is based on coding information of the current video unit.
The method of claim 27, wherein the coding information of the current video unit comprises at least one of:

a coding mode of the current video unit,

a width of the current video unit,

a height of the current video unit,

an area of the current video unit,

a ratio of the width and the height of the current video unit,

a smaller one of the width and the height,

a larger one of the width and the height,

a quantization parameter of the current video unit,

a comparison of a temporal layer index of the current video unit with a threshold index,

a value in the bitstream,

a derived value,

whether a current slice containing the current video unit satisfies a low-delay condition, or

whether the current video unit belongs to a screen content.
The method of any of claims 11-28, wherein a first syntax element in the bitstream indicates whether an inter LFNST or NSPT is applied or not for the current video unit.
The method of claim 29, wherein the first syntax element indicates to apply the inter LFNST or NSPT, and a second syntax element in the bitstream indicates an index of an intra mode associated with the inter LFNST or NSPT.
The method of claim 30, wherein in response to a value of the first syntax element satisfying a condition, the second syntax element is included in the bitstream.
The method of claim 30 or 31, wherein the first syntax element indicates an index of a LFNST or NSPT candidate, and the second syntax element indicates an intra mode for deriving an index of a LFNST or NSPT set.
A method for video processing, comprising:

determining, for a conversion between a current video unit of a video and a bitstream of the video, at least one merge list for the current video unit for an inter transform, the inter transform comprising at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) ; and

perform the conversion based on the at least one merge list.
The method of claim 33, wherein a merge candidate in the at least one merge list comprises at least partial of information related to the inter transform.
The method of claim 33 or 34, wherein the information related to the inter transform comprises at least one of:

an index of an LFNST or NSPT set, or

a candidate index of an LFNST or NSPT candidate in the LFNST or NSPT set.
The method of any of claims 33-35, wherein a merge candidate for an inter LFNST or NSPT is obtained from at least one of: adjacent locations of the current video unit, or non-adjacent locations of the current video unit.
The method of any of claims 33-36, wherein a merge candidate for an inter LFNST or NSPT is obtained from at least one location, the at least one location being determined based on at least one of a motion vector or a block vector of the current video unit.
The method of any of claims 33-37, wherein a merge candidate for an inter LFNST or NSPT is obtained from a history table comprising inter LFNST or NSPT information associated with previously coded blocks.
The method of any of claims 33-38, wherein a merge candidate for an inter LFNST or NSPT is obtained from at least one temporal location.
The method of any of claims 33-39, wherein a plurality of merge candidates for an inter LFNST or NSPT are checked and included in the at least one merge list in a pre-defined order.
The method of claim 40, wherein at least one merge candidate specified by a motion vector or a block vector of the current video unit is checked before at least one merge candidate obtained from adjacent blocks.
The method of claim 40 or 41, wherein an adjacent merge candidate is checked before a non-adjacent merge candidate.
The method of any of claims 40-42, wherein a checking order of the plurality of merge candidates is based on distances of the plurality of merge candidates to a current location of the current video unit.
The method of any of claims 40-43, wherein a first priority of a first merge candidate of a first location is higher than a second priority of a second merge candidate of a second location, a first distance from the first location to a current location of the current video unit being less than a second location from the second location to the current location, and

wherein an order of the first and second merge candidates to be checked is determined based on the first priority and second priority.
The method of any of claims 33-44, further comprising:

determining an availability of an inter LFNST or NSPT merge candidate in a location based on at least one of:

whether a coding block associated with the location is intra-coded,

whether a candidate index of the LNFST or NSPT merge candidate is equal to a first value, or

whether the coding block associated with the location uses an inter or intra LFNST or NSPT.
The method of claim 45, wherein in response to the coding information of the location satisfying a condition, corresponding inter or intra LFNST or NSPT information is included in the at least one merge list.
The method of any of claims 33-46, wherein a checking process of merge candidates terminates in response to the number of available candidates in the at least one merge candidate list reaching a maximum allowed number.
The method of any of claims 33-47, further comprising:

storing a history table comprising inter LFNST or NSPT information associated with at least one previously coded block.
The method of claim 48, wherein the history table is maintained or updated during the conversion.
The method of any of claims 33-47, wherein a construction of the at least one merge list for inter LFNST or NSPT is along with a pruning process.
The method of claim 50, wherein in response to a first index of a first candidate is same with at least one of a second index of a LFNST set or a third index of an LFNST candidate in the at least one merge list, the first candidate is determined as redundant.
The method of any of claims 33-51, wherein an index indicating a candidate in the at last one merge list is indicated in the bitstream.
The method of claim 52, wherein the index is coded with one of: a truncated Rice (TR) code, a truncated binary (BR) code, a k-th order Exponential Golomb (EGk) code, k being an integer, or a fixed-length (FL) code.
The method of claim 52, wherein inherited inter LFNST or NSPT information indicated by the index is used to conduct an inter LFNST or NSPT.
The method of any of claims 33-54, wherein a first syntax element in the bitstream indicates whether an inter LFNST or NSPT is used or not for the current video unit.
The method of claim 55, wherein the first syntax element indicates to use the inter LFNST or NSPT, and a second syntax element in the bitstream indicates a candidate index in an inter LFNST or NSPT merge list.
The method of claim 56, wherein at least one of an LFNST or NSPT set or an LFNST or NSPT candidate indicated by the second syntax element is used for the inter LFNST or NSPT.
The method of claim 55, wherein the first syntax element indicates to use the inter LFNST or NSPT, and a second syntax element in the bitstream indicates whether a merge mode is used or not for the current video unit.
The method of claim 58, wherein the second syntax element indicates to use an inter LFNST or NSPT merge mode, a third syntax element in the bitstream indicates a candidate index in an inter LFNST or NSPT merge list.
The method of claim 58, wherein the second syntax element indicates an inter LFNST or NSPT merge mode being not used, a fourth syntax element in the bitstream indicates a candidate index in an LFNST or NSPT set.
The method of claim 60, wherein in response to the inter LFNST or NSPT merge mode being not used, an intra mode is determined for an LFNST or NSPT set.
The method of claim 61, wherein the intra mode is mapped to an index for the LFNST or NSPT set.
The method of claim 61, wherein the fourth syntax element indicates a kernel set of the inter LFNST or NSPT set.
The method of claim 55, wherein the first syntax element indicates to use the inter LFNST or NSPT and the first syntax element satisfies a condition, and a second syntax element in the bitstream indicates a candidate index in an LFNST or NSPT set.
The method of claim 64, wherein the condition comprises a value of the first syntax element being a predefined value.
The method of claim 64 or 65, wherein the first syntax element indicates to use the inter LFNST or NSPT but the first syntax element fails to satisfy the condition, a value of the first syntax element or a transformed value of the first syntax element is used as a candidate index in an inter LFNST or NSPT merge list.
The method of claim 66, wherein no further syntax element regarding LFNST or NSPT is included in the bitstream, and inherited inter LFNST or NSPT information is used for the inter LFNST or NSPT.
The method of claim 66, wherein in response to the first syntax element failing to satisfy the condition, an intra mode is determined for an LFNST or NSPT set.
The method of claim 68, wherein the intra mode is mapped to an index for the LFNST or NSPT set.
The method of claim 69, wherein the second syntax element indicates a kernel set of the inter LFNST or NSPT set.
The method of any of claims 1-70, wherein the current video unit or a video unit comprises at least one of:

a colour component, a sub-picture, a slice, a tile, a coding tree unit (CTU) , a CTU row, a groups of CTU, a coding unit (CU) , a prediction unit (PU) , a transform unit (TU) , a coding tree block (CTB) , a coding block (CB) , a prediction block (PB) , a transform block (TB) , a block, sub-block of a block, sub-region within a block, or a region that contains more than one sample or pixel.
The method of any of claims 1-71, wherein an indication of whether to and/or how to apply the method is indicated at one of: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level.
The method of any of claims 1-71, wherein an indication of whether to and/or how to apply the method is indicated in one of:

a sequence header,

a picture header,

a sequence parameter set (SPS) ,

a video parameter set (VPS) ,

a dependency parameter set (DPS) ,

a decoding capability information (DCI) ,

a picture parameter set (PPS) ,

an adaptation parameter sets (APS) ,

a slice header, or

a tile group header.
The method of any of claims 1-71, wherein whether to and/or how to apply the method is based on at least one of:

a message included in one of: a dependency parameter set (DPS) , a sequence parameter set (SPS) , a video parameter set (VPS) , a picture parameter set (PPS) , an adaptation parameter sets (APS) , a picture header, a slice header, a tile group header, a largest coding unit (LCU) , a coding unit (CU) , a LCU row, a group of LCUs, a transform unit (TU) , a prediction unit (PU) block, or a video coding unit,

a position of CU, PU, TU, block, or the video coding unit,

a block dimension of the current video unit and/or neighboring blocks of the current video unit,

a block shape of the current video unit and/or neighboring blocks of the current video unit,

a coded mode of a block,

an indication of a color format,

a coding tree structure,

a slice,

a tile group type,

a picture type,

a colour component,

a temporal layer identifier (ID) ,

profiles, levels, or tiers of a standard,

whether a current slice comprising the current video unit satisfies a low-delay condition, or

whether the current video unit belongs to screen content.
The method of claim 74, wherein the coded mode comprises one of: an intra block copy (IBC) , a non-IBC inter mode, or a non-IBC subblock mode, or

wherein the color format comprises one of: 4: 2: 0, or 4: 4: 4.
The method of any of claims 1-75, wherein the conversion comprises encoding the current video unit into the bitstream.
The method of any of claims 1-75, wherein the conversion comprises decoding the current video unit from the bitstream.
An apparatus for video processing comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to perform a method in accordance with any of claims 1-77.
A non-transitory computer-readable storage medium storing instructions that cause a processor to perform a method in accordance with any of claims 1-77.
A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises:

determining a plurality of intra modes for a current video unit of the video, the current video unit being coded in a geometric partitioning mode (GPM) ;

determining a plurality of transform kernel sets based on the plurality of intra modes, a transform kernel set of the plurality of transform kernel sets comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and

generating the bitstream based on the plurality of transform kernel sets.
A method for storing a bitstream of a video, comprising:

determining a plurality of intra modes for a current video unit of the video, the current video unit being coded in a geometric partitioning mode (GPM) ;

determining a plurality of transform kernel sets based on the plurality of intra modes, a transform kernel set of the plurality of transform kernel sets comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set;

generating the bitstream based on the plurality of transform kernel sets; and

storing the bitstream in a non-transitory computer-readable recording medium.
A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises:

determining a target intra mode for a current video unit of the video;

determining at least one transform kernel set for inter coding based on the target intra mode, the at least one transform kernel set comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set; and

generating the bitstream based on the at least one transform kernel set.
A method for storing a bitstream of a video, comprising:

determining a target intra mode for a current video unit of the video;

determining at least one transform kernel set for inter coding based on the target intra mode, the at least one transform kernel set comprising at least one of: a low-frequency non-separable transform (LFNST) kernel set or a non-separable primary transform (NSPT) kernel set;

generating the bitstream based on the at least one transform kernel set; and

storing the bitstream in a non-transitory computer-readable recording medium.
A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises:

determining at least one merge list for a current video unit of the video for an inter transform, the inter transform comprising at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) ; and

generating the bitstream based on the at least one merge list.
A method for storing a bitstream of a video, comprising:

determining at least one merge list for a current video unit of the video for an inter transform, the inter transform comprising at least one of: an inter low-frequency non-separable transform (LFNST) or an inter non-separable primary transform (NSPT) ;

generating the bitstream based on the at least one merge list; and

storing the bitstream in a non-transitory computer-readable recording medium.