WO2019199093A1 - Intra prediction mode-based image processing method and device therefor - Google Patents

Abstract

A method for decoding a video signal and a device therefor are disclosed. Particularly, a method for decoding an image on the basis of an intra prediction mode can comprise the steps of: forming, on the basis of available reference samples near a current block, an intra prediction mode set that is to be used for a bi-directional intra prediction when the bi-directional intra prediction is applied to the current block; parsing a prediction mode index indicating the intra prediction mode to be applied to the current block in the intra prediction mode set; deriving, on the basis of the prediction direction of the intra prediction mode, a first reference sample and a second reference sample, from among the reference samples, that are to be used for the bi-directional intra prediction; and generating a prediction sample of the current block by weighted-summing the first reference sample and the second reference sample.

Description

Intra prediction mode based image processing method and apparatus therefor

The present invention relates to a still image or moving image processing method, and more particularly, to a method for encoding / decoding a still image or moving image based on an intra prediction mode and an apparatus supporting the same.

Compression coding refers to a series of signal processing techniques for transmitting digitized information through a communication line or for storing in a form suitable for a storage medium. Media such as an image, an image, an audio, and the like may be a target of compression encoding. In particular, a technique of performing compression encoding on an image is called video image compression.

Next-generation video content will be characterized by high spatial resolution, high frame rate and high dimensionality of scene representation. Processing such content would result in a tremendous increase in terms of memory storage, memory access rate, and processing power.

Accordingly, there is a need to design coding tools for more efficiently processing next generation video content.

An object of the present invention is to propose a method for performing intra prediction using bi-directional reference samples in order to reduce prediction error and improve compression performance.

It is also an object of the present invention to propose a bi-directional intra prediction method that depends on the available reference samples.

The technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned above will be clearly understood by those skilled in the art from the following description. Could be.

An aspect of the present invention is a method of decoding an image based on an intra prediction mode, wherein when bi-directional intra prediction is applied to a current block, based on available reference samples around the current block Configuring an intra prediction mode set used for the bidirectional intra prediction; Parsing a prediction mode index indicating an intra prediction mode applied to the current block within the intra prediction mode set; Deriving a first reference sample and a second reference sample used for the bidirectional intra prediction among the reference samples based on the prediction direction of the intra prediction mode; And weighting the first reference sample and the second reference sample to generate a prediction sample of the current block, wherein the set of intra prediction modes is within a specific angular range determined according to the available direction of the reference sample. It may be configured to include a predefined number of intra prediction modes in.

Advantageously, said intra prediction mode set is within an angle range greater than 90 degrees and less than or equal to 180 degrees and an angle range greater than or equal to 270 degrees and less than 360 degrees when left and upper reference samples around the current block are available. It may be configured to include intra prediction modes.

Advantageously, said intra prediction mode set is within an angle range greater than or equal to 0 degrees and less than 90 degrees and an angle range greater than or equal to 180 degrees and less than or equal to 270 degrees when right and upper reference samples around the current block are available. It may be configured to include intra prediction modes.

Advantageously, the intra prediction mode set may be configured to include intra prediction modes within the remaining angular range except for 90 degrees when left, right and top reference samples around the current block are available.

Preferably, a weight applied to the first reference sample and the second reference sample, respectively, is based on a ratio of the distance between the current sample and the first reference sample and the distance between the current sample and the second reference sample in the current block. Can be determined.

According to another aspect of the present invention, in an apparatus for decoding an image based on an intra prediction mode, when bi-directional intra prediction is applied to a current block, an available reference samples around the current block are applied. An intra prediction mode set configuration unit constituting an intra prediction mode set used for the bidirectional intra prediction based on the basis; A prediction mode index parser for parsing a prediction mode index indicating an intra prediction mode applied to the current block in the intra prediction mode set; A reference sample derivation unit for deriving a first reference sample and a second reference sample used for the bidirectional intra prediction among the reference samples based on a prediction direction of the intra prediction mode; And a prediction sample generator for weighting the first reference sample and the second reference sample to generate a prediction sample of the current block, wherein the intra prediction mode set is a specific angle determined according to an available direction of the reference sample. It may be configured to include a predefined number of intra prediction modes within a range.

Advantageously, said intra prediction mode set is within an angle range greater than 90 degrees and less than or equal to 180 degrees and an angle range greater than or equal to 270 degrees and less than 360 degrees when left and upper reference samples around the current block are available. And decode the intra prediction modes.

Advantageously, said intra prediction mode set is within an angle range greater than or equal to 0 degrees and less than 90 degrees and an angle range greater than or equal to 180 degrees and less than or equal to 270 degrees when right and upper reference samples around the current block are available. It may be configured to include intra prediction modes.

Advantageously, the intra prediction mode set may be configured to include intra prediction modes within the remaining angular range except for 90 degrees when left, right and top reference samples around the current block are available.

Preferably, a weight applied to the first reference sample and the second reference sample, respectively, is based on a ratio of the distance between the current sample and the first reference sample and the distance between the current sample and the second reference sample in the current block. Can be determined.

According to the embodiment of the present invention, by effectively using the intra prediction direction according to the reference sample, the prediction error can be reduced and the coding efficiency can be improved.

The effects obtainable in the present invention are not limited to the above-mentioned effects, and other effects not mentioned will be clearly understood by those skilled in the art from the following description. .

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, included as part of the detailed description in order to provide a thorough understanding of the present invention, provide embodiments of the present invention and together with the description, describe the technical features of the present invention.

1 is a schematic block diagram of an encoding apparatus in which an encoding of a video / image signal is performed, according to an embodiment to which the present invention is applied.

2 is a schematic block diagram of a decoding apparatus in which an embodiment of the present invention is applied and decoding of a video / image signal is performed.

3 is a diagram illustrating an example of a multi-type tree structure as an embodiment to which the present invention can be applied.

FIG. 4 is a diagram illustrating a signaling mechanism of partition partition information of a quadtree with nested multi-type tree structure according to an embodiment to which the present invention may be applied.

FIG. 5 is a diagram illustrating a method of dividing a CTU into multiple CUs based on a quadtree and accompanying multi-type tree structure as an embodiment to which the present invention may be applied.

FIG. 6 is a diagram illustrating a method of limiting ternary-tree splitting as an embodiment to which the present invention may be applied.

FIG. 7 is a diagram illustrating redundant division patterns that may occur in binary tree division and ternary tree division, as an embodiment to which the present invention may be applied.

8 and 9 are diagrams illustrating an intra prediction based video / image encoding method and an intra prediction unit in an encoding apparatus according to an embodiment of the present invention.

10 and 11 are diagrams illustrating an intra prediction based video / image decoding method and an intra prediction unit in a decoding apparatus according to an embodiment of the present invention.

12 and 13 illustrate a prediction direction of an intra prediction mode according to an embodiment to which the present invention may be applied.

14 is a diagram for describing a prediction angle of an intra prediction mode according to an embodiment to which the present invention is applied.

15 is a diagram illustrating a method of performing intra prediction using an available reference sample according to an embodiment to which the present invention is applied.

16 to 18 are diagrams illustrating the directionality of an intra prediction mode determined according to an available reference sample as an embodiment to which the present invention is applied.

19 is a diagram illustrating a method of generating a predictive sample through bidirectional intra prediction as an embodiment to which the present invention is applied.

20 is a flowchart illustrating a method of generating an intra prediction block according to an embodiment to which the present invention is applied.

21 is a diagram illustrating an intra prediction apparatus according to an embodiment to which the present invention is applied.

22 shows a video coding system to which the present invention is applied.

23 is a diagram illustrating a structure of a content streaming system according to an embodiment to which the present invention is applied.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The detailed description, which will be given below with reference to the accompanying drawings, is intended to explain exemplary embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. The following detailed description includes specific details in order to provide a thorough understanding of the present invention. However, one of ordinary skill in the art appreciates that the present invention may be practiced without these specific details.

In some instances, well-known structures and devices may be omitted or shown in block diagram form centering on the core functions of the structures and devices in order to avoid obscuring the concepts of the present invention.

In addition, the terminology used in the present invention was selected as a general term widely used as possible now, in a specific case will be described using terms arbitrarily selected by the applicant. In such a case, since the meaning is clearly described in the detailed description of the part, it should not be interpreted simply by the name of the term used in the description of the present invention, and it should be understood that the meaning of the term should be understood and interpreted. .

Specific terms used in the following description are provided to help the understanding of the present invention, and the use of such specific terms may be changed to other forms without departing from the technical spirit of the present invention. For example, signals, data, samples, pictures, frames, blocks, etc. may be appropriately replaced and interpreted in each coding process.

Hereinafter, in the present specification, the 'processing unit' refers to a unit in which a process of encoding / decoding such as prediction, transformation, and / or quantization is performed. Hereinafter, for convenience of description, the processing unit may be referred to as a 'processing block' or 'block'.

The processing unit may be interpreted to include a unit for the luma component and a unit for the chroma component. For example, the processing unit may correspond to a Coding Tree Unit (CTU), a Coding Unit (CU), a Prediction Unit (PU), or a Transform Unit (TU).

In addition, the processing unit may be interpreted as a unit for a luma component or a unit for a chroma component. For example, the processing unit may be a coding tree block (CTB), a coding block (CB), a prediction block (PU), or a transform block (TB) for a luma component. May correspond to. Or, it may correspond to a coding tree block (CTB), a coding block (CB), a prediction block (PU), or a transform block (TB) for a chroma component. In addition, the present invention is not limited thereto, and the processing unit may be interpreted to include a unit for a luma component and a unit for a chroma component.

In addition, the processing unit is not necessarily limited to square blocks, but may also be configured in a polygonal form having three or more vertices.

In the following specification, a pixel, a pixel, and the like are referred to collectively as samples. In addition, using a sample may mean using a pixel value or a pixel value.

1 is a schematic block diagram of an encoding apparatus in which an encoding of a video / image signal is performed, according to an embodiment to which the present invention is applied.

Referring to FIG. 1, the encoding apparatus 100 may include an image splitter 110, a subtractor 115, a transformer 120, a quantizer 130, an inverse quantizer 140, an inverse transformer 150, The adder 155, the filter 160, the memory 170, the inter predictor 180, the intra predictor 185, and the entropy encoder 190 may be configured. The inter predictor 180 and the intra predictor 185 may be collectively referred to as a predictor. In other words, the predictor may include an inter predictor 180 and an intra predictor 185. The transform unit 120, the quantization unit 130, the inverse quantization unit 140, and the inverse transform unit 150 may be included in the residual processing unit. The residual processing unit may further include a subtracting unit 115. As an example, the image divider 110, the subtractor 115, the transformer 120, the quantizer 130, the inverse quantizer 140, the inverse transformer 150, and the adder 155 may be described. The filtering unit 160, the inter prediction unit 180, the intra prediction unit 185, and the entropy encoding unit 190 may be configured by one hardware component (eg, an encoder or a processor). In addition, the memory 170 may include a decoded picture buffer (DPB) or may be configured by a digital storage medium.

The image divider 110 may divide the input image (or picture or frame) input to the encoding apparatus 100 into one or more processing units. For example, the processing unit may be called a coding unit (CU). In this case, the coding unit may be recursively divided according to a quad-tree binary-tree (QTBT) structure from a coding tree unit (CTU) or a largest coding unit (LCU). For example, one coding unit may be divided into a plurality of coding units of a deeper depth based on a quad tree structure and / or a binary tree structure. In this case, for example, the quad tree structure may be applied first and the binary tree structure may be applied later. Alternatively, the binary tree structure may be applied first. The coding procedure according to the present invention may be performed based on the final coding unit that is no longer split. In this case, the maximum coding unit may be used as the final coding unit immediately based on coding efficiency according to the image characteristic, or if necessary, the coding unit is recursively divided into coding units of lower depths and optimized. A coding unit of size may be used as the final coding unit. Here, the coding procedure may include a procedure of prediction, transform, and reconstruction, which will be described later. As another example, the processing unit may further include a prediction unit (PU) or a transform unit (TU). In this case, the prediction unit and the transform unit may be partitioned or partitioned from the aforementioned final coding unit, respectively. The prediction unit may be a unit of sample prediction, and the transformation unit may be a unit for deriving a transform coefficient and / or a unit for deriving a residual signal from the transform coefficient.

The unit may be used interchangeably with terms such as block or area in some cases. In a general case, an M × N block may represent a set of samples or transform coefficients composed of M columns and N rows. A sample may generally represent a pixel or a value of a pixel, and may only represent pixel / pixel values of the luma component, or only pixel / pixel values of the chroma component. A sample may be used as a term corresponding to one picture (or image) for a pixel or a pel.

The encoding apparatus 100 subtracts the prediction signal (predicted block, prediction sample array) output from the inter prediction unit 180 or the intra prediction unit 185 from the input image signal (original block, original sample array). A signal may be generated (residual signal, residual block, residual sample array), and the generated residual signal is transmitted to the converter 120. In this case, as shown, a unit that subtracts a prediction signal (prediction block, prediction sample array) from an input image signal (original block, original sample array) in the encoder 100 may be called a subtraction unit 115. The prediction unit may perform a prediction on a block to be processed (hereinafter, referred to as a current block) and generate a predicted block including prediction samples for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied on a current block or CU basis. As described later in the description of each prediction mode, the prediction unit may generate various information related to prediction, such as prediction mode information, and transmit the generated information to the entropy encoding unit 190. The information about the prediction may be encoded in the entropy encoding unit 190 and output in the form of a bitstream.

The intra predictor 185 may predict the current block by referring to the samples in the current picture. The referenced samples may be located in the neighborhood of the current block or may be located apart according to the prediction mode. In intra prediction, prediction modes may include a plurality of non-directional modes and a plurality of directional modes. Non-directional mode may include, for example, DC mode and planner mode (Planar mode). The directional mode may include, for example, 33 directional prediction modes or 65 directional prediction modes according to the degree of detail of the prediction direction. However, as an example, more or less directional prediction modes may be used depending on the setting. The intra predictor 185 may determine the prediction mode applied to the current block by using the prediction mode applied to the neighboring block.

The inter predictor 180 may derive the predicted block with respect to the current block based on the reference block (reference sample array) specified by the motion vector on the reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of blocks, subblocks, or samples based on the correlation of the motion information between the neighboring block and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring block may include a spatial neighboring block existing in the current picture and a temporal neighboring block present in the reference picture. The reference picture including the reference block and the reference picture including the temporal neighboring block may be the same or different. The temporal neighboring block may be referred to as a collocated reference block, a collocated CU (colCU), and the like, and a reference picture including the temporal neighboring block is called a collocated picture (colPic). It may be. For example, the inter prediction unit 180 constructs a motion information candidate list based on neighboring blocks and provides information indicating which candidates are used to derive a motion vector and / or a reference picture index of the current block. Can be generated. Inter prediction may be performed based on various prediction modes. For example, in case of a skip mode and a merge mode, the inter prediction unit 180 may use motion information of a neighboring block as motion information of a current block. In the skip mode, unlike the merge mode, the residual signal may not be transmitted. In the motion vector prediction (MVP) mode, the motion vector of the neighboring block is used as a motion vector predictor and the motion vector difference is signaled by signaling a motion vector difference. Can be directed.

The prediction signal generated by the inter predictor 180 or the intra predictor 185 may be used to generate a reconstruction signal or to generate a residual signal.

The transformer 120 may apply transform techniques to the residual signal to generate transform coefficients. For example, the transformation technique may include at least one of a discrete cosine transform (DCT), a discrete sine transform (DST), a karhunen-loeve transform (KLT), a graph-based transform (GBT), or a conditionally non-linear transform (CNT). It may include. Here, GBT means a conversion obtained from this graph when the relationship information between pixels is represented by a graph. CNT refers to a transform that is generated based on and generates a prediction signal using all previously reconstructed pixels. In addition, the conversion process may be applied to pixel blocks having the same size as the square, or may be applied to blocks of variable size rather than square.

The quantization unit 130 quantizes the transform coefficients and transmits them to the entropy encoding unit 190. The entropy encoding unit 190 encodes the quantized signal (information about the quantized transform coefficients) and outputs the bitstream. have. The information about the quantized transform coefficients may be referred to as residual information. The quantization unit 130 may rearrange block quantized transform coefficients into a one-dimensional vector form based on a coefficient scan order, and quantize the quantized transform coefficients based on the quantized transform coefficients in the one-dimensional vector form. Information about transform coefficients may be generated. The entropy encoding unit 190 may perform various encoding methods such as, for example, exponential Golomb, context-adaptive variable length coding (CAVLC), context-adaptive binary arithmetic coding (CABAC), and the like. The entropy encoding unit 190 may encode information necessary for video / image reconstruction other than quantized transform coefficients (for example, values of syntax elements) together or separately. Encoded information (eg, encoded video / image information) may be transmitted or stored in units of NALs (network abstraction layer) in the form of a bitstream. The bitstream may be transmitted over a network or may be stored in a digital storage medium. The network may include a broadcasting network and / or a communication network, and the digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and the like. The signal output from the entropy encoding unit 190 may include a transmitting unit (not shown) for transmitting and / or a storing unit (not shown) for storing as an internal / external element of the encoding apparatus 100, or the transmitting unit It may be a component of the entropy encoding unit 190.

The quantized transform coefficients output from the quantization unit 130 may be used to generate a prediction signal. For example, the quantized transform coefficients may be reconstructed in the residual signal by applying inverse quantization and inverse transform through inverse quantization unit 140 and inverse transform unit 150 in a loop. The adder 155 adds the reconstructed residual signal to the predicted signal output from the inter predictor 180 or the intra predictor 185 so that a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) is added. Can be generated. If there is no residual for the block to be processed, such as when the skip mode is applied, the predicted block may be used as the reconstructed block. The adder 155 may be called a restoration unit or a restoration block generation unit. The generated reconstruction signal may be used for intra prediction of a next processing target block in a current picture, and may be used for inter prediction of a next picture through filtering as described below.

The filtering unit 160 may improve subjective / objective image quality by applying filtering to the reconstruction signal. For example, the filtering unit 160 may generate a modified reconstructed picture by applying various filtering methods to the reconstructed picture, and the modified reconstructed picture is stored in the memory 170, specifically, the DPB of the memory 170. Can be stored in The various filtering methods may include, for example, deblocking filtering, a sample adaptive offset, an adaptive loop filter, a bilateral filter, and the like. As described below in the description of each filtering method, the filtering unit 160 may generate various information about the filtering and transmit the generated information to the entropy encoding unit 190. The filtering information may be encoded in the entropy encoding unit 190 and output in the form of a bitstream.

The modified reconstructed picture transmitted to the memory 170 may be used as the reference picture in the inter predictor 180. When the inter prediction is applied through the encoding apparatus, the encoding apparatus may avoid prediction mismatch between the encoding apparatus 100 and the decoding apparatus, and may improve encoding efficiency.

The memory 170 DPB may store the modified reconstructed picture for use as a reference picture in the inter predictor 180. The memory 170 may store the motion information of the block from which the motion information in the current picture is derived (or encoded) and / or the motion information of the blocks in the picture that have already been reconstructed. The stored motion information may be transmitted to the inter predictor 180 to use the motion information of the spatial neighboring block or the motion information of the temporal neighboring block. The memory 170 may store reconstructed samples of reconstructed blocks in the current picture, and transfer the reconstructed samples to the intra predictor 185.

2 is a schematic block diagram of a decoding apparatus in which an embodiment of the present invention is applied and decoding of a video / image signal is performed.

Referring to FIG. 2, the decoding apparatus 200 includes an entropy decoding unit 210, an inverse quantizer 220, an inverse transform unit 230, an adder 235, a filter 240, a memory 250, and an inter The prediction unit 260 and the intra prediction unit 265 may be configured. The inter predictor 260 and the intra predictor 265 may be collectively called a predictor. That is, the predictor may include an inter predictor 180 and an intra predictor 185. The inverse quantization unit 220 and the inverse transform unit 230 may be collectively called a residual processing unit. That is, the residual processing unit may include an inverse quantization unit 220 and an inverse transformation unit 230. The entropy decoder 210, the inverse quantizer 220, the inverse transformer 230, the adder 235, the filter 240, the inter predictor 260, and the intra predictor 265 are described in the embodiment. Can be configured by one hardware component (eg, decoder or processor). In addition, the memory 170 may include a decoded picture buffer (DPB) or may be configured by a digital storage medium.

When a bitstream including video / image information is input, the decoding apparatus 200 may reconstruct an image corresponding to a process in which video / image information is processed in the encoding apparatus of FIG. 1. For example, the decoding apparatus 200 may perform decoding using a processing unit applied in the encoding apparatus. Thus the processing unit of decoding may be a coding unit, for example, which may be split along a quad tree structure and / or a binary tree structure from a coding tree unit or a maximum coding unit. The reconstructed video signal decoded and output through the decoding apparatus 200 may be reproduced through the reproducing apparatus.

The decoding apparatus 200 may receive a signal output from the encoding apparatus of FIG. 1 in the form of a bitstream, and the received signal may be decoded through the entropy decoding unit 210. For example, the entropy decoding unit 210 may parse the bitstream to derive information (eg, video / image information) necessary for image reconstruction (or picture reconstruction). For example, the entropy decoding unit 210 decodes information in a bitstream based on a coding method such as exponential Golomb coding, CAVLC, or CABAC, quantized values of syntax elements required for image reconstruction, and transform coefficients for residuals. Can be output. More specifically, the CABAC entropy decoding method receives a bin corresponding to each syntax element in a bitstream, and decodes syntax element information and decoding information of neighboring and decoding target blocks or information of symbols / bins decoded in a previous step. The context model may be determined using the context model, the probability of occurrence of a bin may be predicted according to the determined context model, and arithmetic decoding of the bin may be performed to generate a symbol corresponding to the value of each syntax element. have. In this case, the CABAC entropy decoding method may update the context model by using the information of the decoded symbol / bin for the context model of the next symbol / bean after determining the context model. The information related to the prediction among the information decoded by the entropy decoding unit 2110 is provided to the prediction unit (the inter prediction unit 260 and the intra prediction unit 265), and the entropy decoding performed by the entropy decoding unit 210 is performed. Dual values, that is, quantized transform coefficients and related parameter information, may be input to the inverse quantizer 220. In addition, information on filtering among information decoded by the entropy decoding unit 210 may be provided to the filtering unit 240. Meanwhile, a receiver (not shown) that receives a signal output from the encoding apparatus may be further configured as an internal / external element of the decoding apparatus 200, or the receiver may be a component of the entropy decoding unit 210.

The inverse quantization unit 220 may dequantize the quantized transform coefficients and output the transform coefficients. The inverse quantization unit 220 may rearrange the quantized transform coefficients in the form of a two-dimensional block. In this case, the reordering may be performed based on the coefficient scan order performed by the encoding apparatus. The inverse quantization unit 220 may perform inverse quantization on quantized transform coefficients using a quantization parameter (for example, quantization step size information), and may obtain transform coefficients.

The inverse transformer 230 inversely transforms the transform coefficients to obtain a residual signal (residual block, residual sample array).

The prediction unit may perform prediction on the current block and generate a predicted block including prediction samples for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied to the current block based on the information about the prediction output from the entropy decoding unit 210, and may determine a specific intra / inter prediction mode.

The intra predictor 265 may predict the current block by referring to samples in the current picture. The referenced samples may be located in the neighborhood of the current block or may be located apart according to the prediction mode. In intra prediction, prediction modes may include a plurality of non-directional modes and a plurality of directional modes. The intra predictor 265 may determine the prediction mode applied to the current block by using the prediction mode applied to the neighboring block.

The inter prediction unit 260 may derive the predicted block for the current block based on the reference block (reference sample array) specified by the motion vector on the reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of blocks, subblocks, or samples based on the correlation of the motion information between the neighboring block and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring block may include a spatial neighboring block existing in the current picture and a temporal neighboring block present in the reference picture. For example, the inter prediction unit 260 may construct a motion information candidate list based on neighboring blocks and derive a motion vector and / or a reference picture index of the current block based on the received candidate selection information. Inter prediction may be performed based on various prediction modes, and the information about the prediction may include information indicating a mode of inter prediction for the current block.

The adder 235 adds the obtained residual signal to the predictive signal (predicted block, predictive sample array) output from the inter predictor 260 or the intra predictor 265 to restore the reconstructed signal (reconstructed picture, reconstructed block). , Restore sample array). If there is no residual for the block to be processed, such as when the skip mode is applied, the predicted block may be used as the reconstructed block.

The adder 235 may be called a restoration unit or a restoration block generation unit. The generated reconstruction signal may be used for intra prediction of a next processing target block in a current picture, and may be used for inter prediction of a next picture through filtering as described below.

The filtering unit 240 may improve subjective / objective image quality by applying filtering to the reconstruction signal. For example, the filtering unit 240 may generate a modified reconstructed picture by applying various filtering methods to the reconstructed picture, and the modified reconstructed picture may be stored in the memory 250, specifically, the DPB of the memory 250. Can be sent to. The various filtering methods may include, for example, deblocking filtering, a sample adaptive offset, an adaptive loop filter, a bilateral filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 250 may be used as the reference picture in the inter predictor 260. The memory 250 may store the motion information of the block from which the motion information in the current picture is derived (or decoded) and / or the motion information of the blocks in the picture that are already reconstructed. The stored motion information may be transmitted to the inter predictor 260 to use the motion information of the spatial neighboring block or the motion information of the temporal neighboring block. The memory 170 may store reconstructed samples of reconstructed blocks in the current picture, and transfer the reconstructed samples to the intra predictor 265.

In the present specification, the embodiments described by the filtering unit 160, the inter prediction unit 180, and the intra prediction unit 185 of the encoding apparatus 100 are respectively the filtering unit 240 and the inter prediction of the decoding apparatus 200. The same may also apply to the unit 260 and the intra predictor 265.

Block Partitioning

The video / image coding method according to this document may be performed based on various detailed techniques, and each detailed technique will be described as follows. Techniques described below include prediction, residual processing ((inverse) transformation, (inverse) quantization, etc.), syntax element coding, filtering, partitioning / division, etc. in the video / image encoding / decoding procedures described above and / or described below. It will be apparent to those skilled in the art that they may be involved in related procedures.

The block partitioning procedure according to this document may be performed by the image splitter 110 of the encoding apparatus described above, and the partitioning related information may be processed (encoded) by the entropy encoding unit 190 and transmitted to the decoding apparatus in the form of a bitstream. . The entropy decoding unit 210 of the decoding apparatus derives a block partitioning structure of the current picture based on the partitioning related information obtained from the bitstream, and based on this, a series of procedures (eg, prediction and residual) for image decoding. Processing, block reconstruction, in-loop filtering, etc.).

Partitioning of picture into CTUs

Pictures can be divided into a sequence of coding tree units (CTUs). The CTU may correspond to a coding tree block (CTB). Alternatively, the CTU may include a coding tree block of luma samples and two coding tree blocks of corresponding chroma samples. In other words, for a picture that includes three sample arrays, the CTU may include an N × N block of luma samples and two corresponding blocks of chroma samples.

The maximum allowable size of the CTU for coding and prediction may be different from the maximum allowable size of the CTU for transform. For example, the maximum allowable size of the luma block in the CTU may be 128x128.

Partitionig of the CTUs using a tree structure

The CTU may be divided into CUs based on a quad-tree (QT) structure. The quadtree structure may be referred to as a quaternary tree structure. This is to reflect various local characteristics. Meanwhile, in the present document, the CTU may be divided based on a multitype tree structure partition including a binary tree (BT) and a ternary tree (TT) as well as a quad tree. Hereinafter, the QTBT structure may include a quadtree and binary tree based partition structure, and the QTBTTT may include a quadtree, binary tree, and ternary tree based partition structure. Alternatively, the QTBT structure may include a quadtree, binary tree and ternary tree based partitioning structure. In a coding tree structure, a CU may have a square or rectangular shape. The CTU may first be divided into quadtree structures. After that, the leaf nodes of the quadtree structure may be further divided by the multitype tree structure.

3 is a diagram illustrating an example of a multi-type tree structure as an embodiment to which the present invention can be applied.

In one embodiment of the present invention, the multitype tree structure may include four partition types as shown in FIG. The four types of split include vertical binary splitting (SPLIT_BT_VER), horizontal binary splitting (SPLIT_BT_HOR), vertical ternary splitting (SPLIT_TT_VER), and horizontal ternary splitting (SPLIT_TT_HOR). ) May be included. Leaf nodes of the multitype tree structure may be called CUs. These CUs can be used for prediction and transform procedures. In general, CU, PU, and TU may have the same block size in this document. However, when the maximum supported transform length is smaller than the width or height of the color component of the CU, the CU and the TU may have different block sizes.

FIG. 4 is a diagram illustrating a signaling mechanism of partition partition information of a quadtree with nested multi-type tree structure according to an embodiment to which the present invention may be applied.

Here, the CTU is treated as the root of the quadtree, and is partitioned for the first time into a quadtree structure. Each quadtree leaf node may then be further partitioned into a multitype tree structure. In the multitype tree structure, a first flag (ex. Mtt_split_cu_flag) is signaled to indicate whether the node is additionally partitioned. If the node is additionally partitioned, a second flag (ex. Mtt_split_cu_verticla_flag) may be signaled to indicate the splitting direction. Thereafter, a third flag (ex. Mtt_split_cu_binary_flag) may be signaled to indicate whether the partition type is binary partition or ternary partition. For example, based on the mtt_split_cu_vertical_flag and the mtt_split_cu_binary_flag, a multi-type tree splitting mode (MttSplitMode) of a CU may be derived as shown in Table 1 below.

FIG. 5 is a diagram illustrating a method of dividing a CTU into multiple CUs based on a quadtree and accompanying multi-type tree structure as an embodiment to which the present invention may be applied.

Here, bold block edges represent quadtree partitioning and the remaining edges represent multitype tree partitioning. Quadtree partitions involving a multitype tree can provide a content-adapted coding tree structure. The CU may correspond to a coding block (CB). Alternatively, the CU may include a coding block of luma samples and two coding blocks of corresponding chroma samples. The size of a CU may be as large as CTU, or may be cut by 4 × 4 in luma sample units. For example, in the 4: 2: 0 color format (or chroma format), the maximum chroma CB size may be 64x64 and the minimum chroma CB size may be 2x2.

For example, in this document, the maximum allowable luma TB size may be 64x64 and the maximum allowable chroma TB size may be 32x32. If the width or height of the CB divided according to the tree structure is larger than the maximum transform width or height, the CB may be automatically (or implicitly) split until the TB size limit in the horizontal and vertical directions is satisfied.

Meanwhile, for a quadtree coding tree scheme involving a multitype tree, the following parameters may be defined and identified as SPS syntax elements.

CTU size: the root node size of a quaternary tree

MinQTSize: the minimum allowed quaternary tree leaf node size

MaxBtSize: the maximum allowed binary tree root node size

MaxTtSize: the maximum allowed ternary tree root node size

MaxMttDepth: the maximum allowed hierarchy depth of multi-type tree splitting from a quadtree leaf

MinBtSize: the minimum allowed binary tree leaf node size

MinTtSize: the minimum allowed ternary tree leaf node size

As an example of a quadtree coding tree structure involving a multitype tree, the CTU size may be set to 64x64 blocks of 128x128 luma samples and two corresponding chroma samples (in 4: 2: 0 chroma format). In this case, MinOTSize can be set to 16x16, MaxBtSize to 128x128, MaxTtSzie to 64x64, MinBtSize and MinTtSize (for both width and height) to 4x4, and MaxMttDepth to 4. Quarttree partitioning may be applied to the CTU to generate quadtree leaf nodes. The quadtree leaf node may be called a leaf QT node. Quadtree leaf nodes may have a 128x128 size (i.e. the CTU size) from a 16x16 size (i.e. the MinOTSize). If the leaf QT node is 128x128, it may not be additionally divided into a binary tree / a ternary tree. This is because in this case, even if split, it exceeds MaxBtsize and MaxTtszie (i.e. 64x64). In other cases, leaf QT nodes may be further partitioned into a multitype tree. Therefore, the leaf QT node is the root node for the multitype tree, and the leaf QT node may have a multitype tree depth (mttDepth) 0 value. If the multitype tree depth reaches MaxMttdepth (ex. 4), further splitting may not be considered further. If the width of the multitype tree node is equal to MinBtSize and less than or equal to 2xMinTtSize, then no further horizontal split may be considered. If the height of the multitype tree node is equal to MinBtSize and less than or equal to 2xMinTtSize, no further vertical split may be considered.

FIG. 6 is a diagram illustrating a method of limiting ternary-tree splitting as an embodiment to which the present invention may be applied.

With reference to FIG. 6, to allow for 64x64 luma blocks and 32x32 chroma pipeline designs in a hardware decoder, TT partitioning may be limited in certain cases. For example, when the width or height of the luma coding block is greater than a predetermined specific value (eg, 32 and 64), TT partitioning may be limited as shown in FIG. 6.

In this document, the coding tree scheme may support that the luma and chroma blocks have separate block tree structures. For P and B slices, luma and chroma CTBs in one CTU may be limited to have the same coding tree structure. However, for I slices, luma and chroma blocks may have a separate block tree structure from each other. If an individual block tree mode is applied, the luma CTB may be split into CUs based on a particular coding tree structure, and the chroma CTB may be split into chroma CUs based on another coding tree structure. This may mean that a CU in an I slice may consist of a coding block of a luma component or coding blocks of two chroma components, and a CU of a P or B slice may be composed of blocks of three color components.

In the above-described "Partitionig of the CTUs using a tree structure", a quadtree coding tree structure involving a multitype tree has been described, but a structure in which a CU is divided is not limited thereto. For example, the BT structure and the TT structure may be interpreted as a concept included in a multiple partitioning tree (MPT) structure, and the CU may be interpreted to be divided through the QT structure and the MPT structure. In one example where a CU is split through a QT structure and an MPT structure, a syntax element (eg, MPT_split_type) that contains information about how many blocks the leaf node of the QT structure is divided into and the leaf node of the QT structure are vertical The partition structure may be determined by signaling a syntax element (eg, MPT_split_mode) including information about which direction is divided into and horizontally.

In another example, the CU may be partitioned in a different way than the QT structure, BT structure or TT structure. That is, according to the QT structure, the CU of the lower depth is divided into 1/4 size of the CU of the upper depth, or the CU of the lower depth is divided into 1/2 size of the CU of the upper depth according to the BT structure, or according to the TT structure. Unlike the CU of the lower depth is divided into 1/4 or 1/2 size of the CU of the upper depth, the CU of the lower depth is sometimes 1/5, 1/3, 3/8, 3 of the CU of the upper depth. It can be divided into / 5, 2/3 or 5/8 size, the way in which the CU is divided is not limited to this.

If a portion of a tree node block exceeds the bottom or right picture boundary, the tree node block is placed so that all samples of all coded CUs are located within the picture boundaries. May be limited. In this case, for example, the following division rule may be applied.

-If a portion of a tree node block exceeds both the bottom and the right picture boundaries,

-If the block is a QT node and the size of the block is larger than the minimum QT size, the block is forced to be split with QT split mode.

-Otherwise, the block is forced to be split with SPLIT_BT_HOR mode

-Otherwise if a portion of a tree node block exceeds the bottom picture boundaries,

-If the block is a QT node, and the size of the block is larger than the minimum QT size, and the size of the block is larger than the maximum BT size, the block is forced to be split with QT split mode.

-Otherwise, if the block is a QT node, and the size of the block is larger than the minimum QT size and the size of the block is smaller than or equal to the maximum BT size, the block is forced to be split with QT split mode or SPLIT_BT_HOR mode.

-Otherwise (the block is a BTT node or the size of the block is smaller than or equal to the minimum QT size), the block is forced to be split with SPLIT_BT_HOR mode.

-Otherwise if a portion of a tree node block exceeds the right picture boundaries,

-If the block is a QT node, and the size of the block is larger than the minimum QT size, and the size of the block is larger than the maximum BT size, the block is forced to be split with QT split mode.

-Otherwise, if the block is a QT node, and the size of the block is larger than the minimum QT size and the size of the block is smaller than or equal to the maximum BT size, the block is forced to be split with QT split mode or SPLIT_BT_VER mode.

-Otherwise (the block is a BTT node or the size of the block is smaller than or equal to the minimum QT size), the block is forced to be split with SPLIT_BT_VER mode.

On the other hand, the quadtree coded block structure with the multi-type tree described above can provide a very flexible block partitioning structure. Because of the partition types supported in a multitype tree, different partition patterns can sometimes lead to potentially identical coding block structure results. By limiting the occurrence of such redundant partition patterns, the data amount of partitioning information can be reduced. It demonstrates with reference to the following drawings.

FIG. 7 is a diagram illustrating redundant division patterns that may occur in binary tree division and ternary tree division, as an embodiment to which the present invention may be applied.

As shown in FIG. 7, two levels of consecutive binary splits in one direction have the same coding block structure as the binary split for the center partition after the ternary split. . In this case, the binary tree split in the given direction for the center partition of the ternary tree split may be limited. This restriction can be applied for CUs of all pictures. If this particular partitioning is restricted, the signaling of the corresponding syntax elements can be modified to reflect this limited case, thereby reducing the number of bits signaled for partitioning. For example, as shown in FIG. 7, when the binary tree split for the center partition of the CU is restricted, the mtt_split_cu_binary_flag syntax element indicating whether the split is a binary split or a tenary split is not signaled, and its value is Can be inferred by the decoder to zero.

Prediction

The decoded portion of the current picture or other pictures in which the current processing unit is included may be used to reconstruct the current processing unit in which decoding is performed.

Intra picture or I picture (slice), which uses only the current picture for reconstruction, i.e. performs only intra picture prediction, predicts a picture (slice) using at most one motion vector and reference index to predict each unit A picture using a predictive picture or P picture (slice), up to two motion vectors, and a reference index (slice) may be referred to as a bi-predictive picture or a B picture (slice).

Inter prediction means a prediction method of deriving a current processing block based on data elements (eg, sample values or motion vectors, etc.) of pictures other than the current picture. That is, a method of predicting pixel values of the current processing block by referring to reconstructed regions in other reconstructed pictures other than the current picture.

Hereinafter, intra prediction (or intra prediction) will be described in more detail.

Intra prediction (or intra prediction)

Intra prediction means a prediction method that derives the current processing block from data elements (eg, sample values, etc.) of the same decoded picture (or slice). That is, a method of predicting pixel values of the current processing block by referring to reconstructed regions in the current picture.

Intra prediction may indicate prediction for generating a prediction sample for a current block based on reference samples outside the current block in a picture to which the current block belongs (hereinafter, referred to as a current picture).

The present invention describes the detailed description of the intra prediction method described above with reference to FIGS. 1 and 2, and the decoder may be represented by the intra prediction-based video / image decoding method of FIG. 10 described later and the intra prediction unit in the decoding apparatus of FIG. 11. . In addition, the encoder may be represented by the intra prediction-based video / video encoding method of FIG. 8 and the intra prediction unit in the encoding apparatus of FIG. 9. In addition, the data encoded by FIGS. 8 and 9 may be stored in the form of a bitstream.

When intra prediction is applied to the current block, peripheral reference samples to be used for intra prediction of the current block may be derived. The peripheral reference samples of the current block are samples adjacent to the left boundary of the current block of size nWxnH and a total of 2xnH samples neighboring the bottom-left, and samples adjacent to the top boundary of the current block. And a total of 2xnW samples neighboring the top-right and one sample neighboring the top-left of the current block. Alternatively, the peripheral reference samples of the current block may include a plurality of upper peripheral samples and a plurality of left peripheral samples. In addition, the peripheral reference samples of the current block are a total of nH samples adjacent to the right boundary of the current block of size nWxnH, a total of nW samples adjacent to the bottom boundary of the current block and the lower right side of the current block. It may include one sample neighboring (bottom-right).

However, some of the peripheral reference samples of the current block may not be decoded yet or available. In this case, the decoder may construct the surrounding reference samples to use for prediction by substituting the samples that are not available with the available samples. Alternatively, peripheral reference samples to be used for prediction may be configured through interpolation of the available samples.

When the neighbor reference samples are derived, the prediction sample can be derived based on the average or interpolation of neighboring reference samples of the current block, and (ii) the prediction among the neighbor reference samples of the current block. The prediction sample may be derived based on a reference sample present in a specific (prediction) direction with respect to the sample. In case of (i), it may be called non-directional mode or non-angle mode, and in case of (ii), it may be called directional mode or angular mode. The interpolation between the second neighboring sample and the first neighboring sample located in a direction opposite to the prediction direction of the intra prediction mode of the current block based on the prediction sample of the current block among the neighboring reference samples may be performed. Prediction samples may be generated. The above case may be referred to as linear interpolation intra prediction (LIP). In addition, a temporary prediction sample of the current block is derived based on filtered neighbor reference samples, and at least one of the existing neighbor reference samples, that is, unfiltered neighbor reference samples, derived according to the intra prediction mode. A weighted sum of a reference sample and the temporary prediction sample may be used to derive the prediction sample of the current block. The above case may be referred to as position dependent intra prediction (PDPC). Meanwhile, post-processing filtering may be performed on the predicted sample derived as needed.

In detail, the intra prediction procedure may include an intra prediction mode determination step, a peripheral reference sample derivation step, and an intra prediction mode based prediction sample derivation step. In addition, a post-filtering step may be performed on the predicted sample derived as needed.

A video / image encoding procedure based on intra prediction and an intra prediction unit in the encoding apparatus may roughly include, for example, the following.

8 and 9 are diagrams illustrating an intra prediction based video / image encoding method and an intra prediction unit in an encoding apparatus according to an embodiment of the present invention.

8 and 9, S801 may be performed by the intra predictor 185 of the encoding apparatus, and S802 may be performed by the residual processor of the encoding apparatus. In detail, S802 may be performed by the subtraction unit 115 of the encoding apparatus. In S803, the prediction information may be derived by the intra prediction unit 185 and encoded by the entropy encoding unit 190. In S803, the residual information may be derived by the residual processor and encoded by the entropy encoding unit 190. The residual information is information about the residual samples. The residual information may include information about quantized transform coefficients for the residual samples.

As described above, the residual samples may be derived as transform coefficients through the transform unit 120 of the encoding apparatus, and the transform coefficients may be derived as transform coefficients quantized through the quantization unit 130. Information about the quantized transform coefficients may be encoded by the entropy encoding unit 190 through a residual coding procedure.

The encoding apparatus performs intra prediction on the current block (S801). The encoding apparatus may derive an intra prediction mode for the current block, derive the peripheral reference samples of the current block, and generate the prediction samples in the current block based on the intra prediction mode and the peripheral reference samples. Here, the intra prediction mode determination, the peripheral reference samples (the procedure of generating the prediction and the prediction samples may be performed simultaneously or one procedure may be performed before the other procedure. For example, the intra prediction unit of the encoding apparatus ( 185 may include a prediction mode determiner 186, a reference sample derivator 187, and a prediction sample derivator 188, and the prediction mode determiner 186 determines an intra prediction mode for the current block. The reference sample derivator 187 may derive peripheral reference samples of the current block, and the predictive sample derivator 188 may derive the motion samples of the current block. When the predictive sample filtering procedure is performed, the intra predictor 185 may further include a predictive sample filter unit (not shown) The encoding apparatus may further include the current block among a plurality of intra prediction modes. The encoding apparatus may compare an RD cost for the intra prediction modes and determine an optimal intra prediction mode for the current block.

Meanwhile, the encoding apparatus may perform a prediction sample filtering procedure. Predictive sample filtering may be referred to as post filtering. Some or all of the prediction samples may be filtered by the prediction sample filtering procedure. In some cases, the prediction sample filtering procedure may be omitted.

The encoding apparatus generates residual samples for the current block based on the (filtered) prediction sample (S802). The encoding apparatus may encode image information including prediction mode information indicating the intra prediction mode and residual information regarding the residual samples (S803). The encoded image information may be output in the form of a bitstream. The output bitstream may be delivered to the decoding apparatus via a storage medium or a network.

Meanwhile, as described above, the encoding apparatus may generate a reconstructed picture (including the reconstructed samples and the reconstructed block) based on the reference samples and the residual samples. This is because the encoding apparatus derives the same prediction result as that performed in the decoding apparatus, and thus the coding efficiency can be increased. As described above, an in-loop filtering procedure may be further applied to the reconstructed picture.

10 and 11 are diagrams illustrating an intra prediction based video / image decoding method and an intra prediction unit in a decoding apparatus according to an embodiment of the present invention.

10 and 11, the decoding apparatus may perform an operation corresponding to the operation performed by the encoding apparatus. The decoding apparatus may perform prediction on the current block and derive prediction samples based on the received prediction information.

In detail, the decoding apparatus may derive the intra prediction mode for the current block based on the received prediction mode information (S1001). The decoding apparatus may derive peripheral reference samples of the current block (S1002). The decoding apparatus generates prediction samples in the current block based on the intra prediction mode and the peripheral reference samples (S1003). In this case, the decoding apparatus may perform a prediction sample filtering procedure. Predictive sample filtering may be referred to as post filtering. Some or all of the prediction samples may be filtered by the prediction sample filtering procedure. In some cases, the prediction sample filtering procedure may be omitted.

The decoding apparatus generates residual samples for the current block based on the received residual information (S1004). The decoding apparatus may generate reconstructed samples for the current block based on the (filtered) prediction samples and the residual samples, and generate a reconstructed picture based on the (S1005).

Here, the intra prediction unit 265 of the decoding apparatus may include a prediction mode determiner 266, a reference sample derivator 267, and a prediction sample derivator 268, and the prediction mode determiner 266 may be encoded. The intra prediction mode for the current block is determined based on the prediction mode information received by the prediction mode determiner 186 of the apparatus, and the reference sample derivator 266 derives the neighbor reference samples of the current block and predicts the prediction mode. The sample derivator 267 may derive the predictive samples of the current block. Although not shown, when the above-described prediction sample filtering procedure is performed, the intra prediction unit 265 may further include a prediction sample filter (not shown).

The prediction mode information may include flag information (ex. Prev_intra_luma_pred_flag) indicating whether a most probable mode (MPM) is applied to the current block or a remaining mode is applied, and the MPM is the current When applied to a block, the prediction mode information may further include index information (ex. Mpm_idx) indicating one of the intra prediction mode candidates (MPM candidates). The intra prediction mode candidates (MPM candidates) may consist of an MPM candidate list or an MPM list. In addition, when the MPM is not applied to the current block, the prediction mode information further includes remaining mode information (ex. Rem_inra_luma_pred_mode) indicating one of the intra prediction modes except for the intra prediction mode candidates (MPM candidates). It may include. The decoding apparatus may determine the intra prediction mode of the current block based on the prediction mode information. The prediction mode information may be encoded / decoded through a coding method described below. For example, the prediction mode information may be encoded / decoded through encoding coding (ex. CABAC, CAVLC) based on truncated (rice) binary code.

Determine intra prediction mode

When intra prediction is applied, the intra prediction mode applied to the current block may be determined using the intra prediction mode of the neighboring block. For example, the decoding apparatus may select one of the most probable mode (mpm) candidates derived based on the intra prediction mode of the left block of the current block and the intra prediction mode of the upper block based on the received mpm index, or One of the remaining intra prediction modes not included in the mpm candidates may be selected based on the remaining intra prediction mode information. The mpm index may be signaled in the form of an mpm_idx syntax element, and the remaining intra prediction mode information may be signaled in the form of a rem_intra_luma_pred_mode syntax element. For example, the remaining intra prediction mode information may index remaining intra prediction modes not included in the mpm candidates among all intra prediction modes in order of prediction mode number to indicate one of them.

12 and 13 illustrate a prediction direction of an intra prediction mode according to an embodiment to which the present invention may be applied.

Referring to FIG. 12, the intra prediction mode may include two non-directional intra prediction modes and 33 directional intra prediction modes. The non-directional intra prediction modes may include a planar intra prediction mode and a DC intra prediction mode, and the directional intra prediction modes may include 2 to 34 intra prediction modes. The planner intra prediction mode may be called a planner mode, and the DC intra prediction mode may be called a DC mode.

Meanwhile, in order to capture an arbitrary edge direction presented in a natural video, the directional intra prediction mode may be extended from 33 to 65 as shown in FIG. 13. In this case, the intra prediction mode may include two non-directional intra prediction modes and 65 directional intra prediction modes. The non-directional intra prediction modes may include a planar intra prediction mode and a DC intra prediction mode, and the directional intra prediction modes may include 2 to 66 intra prediction modes. Extended Directional Intra Prediction It can be applied to blocks of all sizes and to both luma and chroma components.

Alternatively, the intra prediction mode may include two non-directional intra prediction modes and 129 directional intra prediction modes. The non-directional intra prediction modes may include a planar intra prediction mode and a DC intra prediction mode, and the directional intra prediction modes may include 2 to 130 intra prediction modes.

The prediction unit of the encoding apparatus / decoding apparatus may derive a reference sample according to the intra prediction mode of the current block among neighbor reference samples of the current block, and generate a prediction sample of the current block based on the reference sample. .

For example, the prediction sample may be derived based on the average or interpolation of neighboring reference samples of the current block, and (ii) specific to the prediction sample among the neighboring reference samples of the current block. The prediction sample may be derived based on a reference sample present in the (prediction) direction. In case of (i), it may be called non-directional mode or non-angle mode, and in case of (ii), it may be called directional mode or angular mode. Further, in one embodiment, multi-reference sample lines may be used that utilize one or more reference sample lines for intra prediction for more accurate prediction.

As described above, intra prediction uses correlation between samples within a block. However, as the distance between the predicted sample and the reference sample increases, the correlation may decrease, and a prediction error may occur.

Accordingly, the present invention proposes a method of performing intra prediction using bi-directional reference samples in order to reduce such prediction error and improve compression performance. In particular, the present invention proposes a bidirectional intra prediction method that depends on the available reference samples.

The encoding / decoding of still pictures and videos can be applied for bit streams or other types of file formats. Bit streams and files may be stored in various types of storage devices within the device, or streamed in various network environments such as cellular networks, Internet protocols, and the like. When streamed to an end user device with a display, the bit stream or file can be decoded and played. The invention can be applied to any device having an encoder and / or a decoder regardless of the display of the bit stream.

According to the embodiment of the present invention, by effectively using the intra prediction direction according to the reference sample, the prediction error can be reduced and the coding efficiency can be improved.

In the sample prediction step, the encoder / decoder may perform intra prediction along the prediction direction to effectively capture edges in the image.

14 is a diagram for describing a prediction angle of an intra prediction mode according to an embodiment to which the present invention is applied.

The encoder / decoder may perform directional prediction using an intra prediction mode having an angle as shown in FIG. 14. For example, when the angle of the prediction mode is 180 degrees, the prediction sample of the current block may be generated using the horizontal left sample. Or, for example, when the angle of the prediction mode is 270 degrees, the prediction sample of the current block may be generated using the vertical upper sample.

Hereinafter, the prediction direction of the intra prediction mode will be described based on the angle shown in FIG. 14.

15 is a diagram illustrating a method of performing intra prediction using an available reference sample according to an embodiment to which the present invention is applied.

In FIG. 15, it is assumed that an intra prediction mode having an angle of 315 degrees based on the angle illustrated in FIG. 14 described above is applied to a current block. The shaded blocks represent reference samples around the current block. The encoder / decoder may generate a prediction block (or prediction sample) of the current block by using reference samples around the current block according to the prediction direction of the intra prediction mode of the current block.

Application of the intra prediction mode as shown in FIG. 15 means that an edge exists in the current block along the prediction direction of the intra prediction mode from the upper reference sample. Meanwhile, when an intra prediction mode of 135 degrees, that is, an opposite direction of 315 degrees is applied, it means that an edge exists from the left reference sample to the upper reference sample along the prediction direction of the intra prediction mode.

Hereinafter, embodiments of the present invention propose a directional intra mode depending on the reference sample availability.

16 to 18 are diagrams illustrating the directionality of an intra prediction mode determined according to an available reference sample as an embodiment to which the present invention is applied.

In Figures 16-18, the double arrow indicates that bi-directional intra prediction is available, and the shaded area indicates the available prediction angle (or range of prediction angles). As an embodiment, the encoder / decoder may construct (or generate) an intra prediction mode set that includes a predetermined number of prediction modes in the shaded region.

In other words, the encoder / decoder may construct (or generate) an intra prediction mode set within a prediction angle range defined according to the available reference samples. In the present invention, the intra prediction mode set is not limited to its name, and the intra mode set, the prediction mode set, the intra prediction mode group, the prediction mode group, the intra mode group, the intra prediction mode list, the prediction mode list, the intra mode list Or the like.

Specifically, if an upper reference sample and a left reference sample are available as shown in FIG. 16 (a), between 90 degrees and 180 degrees and / or 270 degrees and 360 degrees as shown in FIG. 16 (b). An directional intra prediction mode with angle may be used for bidirectional intra prediction. In one embodiment, when upper and left reference samples are available, an angle range greater than 90 degrees and less than or equal to 180 degrees (ie, 90 <angle <= 180) and / or greater than or equal to 270 degrees and less than 360 degrees Directional intra mode with angles within an angular range (ie, 270 <= angle <360) may be used for bidirectional intra prediction.

In other words, the encoder / decoder has an angular range between 90 degrees and 180 degrees and / or 270 degrees and 360 degrees (or more than 90 degrees) for bidirectional intra prediction if the upper and left reference samples are available for intra prediction as reconstructed samples. Larger than less than or equal to 180 degrees and / or greater than or equal to 270 degrees and less than 360 degrees) to configure a set of intra prediction modes including a certain number of intra prediction modes.

The encoder / decoder may determine the intra prediction mode based on the set of intra prediction modes determined according to the available reference samples, and perform bidirectional intra prediction using the determined intra prediction mode. At this time, the encoder / decoder derives the first reference sample from the left reference sample, derives the second reference sample from the upper reference sample, and derives the derived first reference sample and the second reference sample based on the determined intra prediction mode. Can be weighted to produce the final predicted sample. The method of generating the final prediction sample is described in detail below.

In addition, in one embodiment, intra prediction modes within an angular range determined according to reference samples available to the predefined prediction mode table may be mapped.

In addition, in one embodiment, the number of intra prediction modes within a specific angle range may be determined differently according to the division of the angle. For example, angles between 0 degrees and 90 degrees may be divided every 3 degrees, and in this case, the number of intra prediction modes may be determined as 29 except for 0 degrees and 90 degrees angles. As another example, nonlinear splitting for a particular angular range may be applied. For example, the first intra prediction mode and the second intra prediction mode may have angles of 2 degrees and 5 degrees, respectively.

Referring to FIG. 17, it is assumed that right and upper reference samples are available, as shown in FIG. 17A. If right and top reference samples are available, as shown in Fig. 17 (b), a directional intra prediction mode with an angle between 0 degrees and 90 degrees and / or 180 degrees and 270 degrees may be used for bidirectional intra prediction. Can be. In one embodiment, when right and top reference samples are available, an angle range greater than or equal to 0 degrees and less than 90 degrees (ie, 0 = <angle <90) and / or greater than 180 degrees and less than or equal to 270 degrees Directional intra mode with angles within an angular range (ie, 180 <angle <= 270) may be used for bidirectional intra prediction.

In other words, the encoder / decoder has an angular range between 0 degrees and 90 degrees and / or 180 degrees and 270 degrees (or more than 0 degrees) for bidirectional intra prediction if right and upper reference samples are available for intra prediction as reconstructed samples. Greater than or equal to less than 90 degrees and / or greater than 180 degrees and less than or equal to 270 degrees) to configure a set of intra prediction modes comprising a certain number of intra prediction modes. For example, if an arbitrary coding order or coding order (or scan order) is applied from right to left, the right and top samples of the current block may be available for prediction as reconstructed samples.

The encoder / decoder may determine the intra prediction mode based on the set of intra prediction modes determined according to the available reference samples, and perform bidirectional intra prediction using the determined intra prediction mode. At this time, the encoder / decoder derives the first reference sample from the left reference sample, derives the second reference sample from the upper reference sample, and derives the derived first reference sample and the second reference sample based on the determined intra prediction mode. Can be weighted to produce the final predicted sample.

In FIG. 17A, the dotted upper left reference sample represents a case where more reference samples are required for intra prediction due to the angle of the prediction direction. In one embodiment, the reference sample at that location (ie, the dotted reference sample) may be padded using the reference sample at the nearest location. For example, two upper left reference samples may be padded using adjacent reference samples for the prediction mode in the right downward direction.

In addition, in one embodiment, intra prediction modes within an angular range determined according to reference samples available to the predefined prediction mode table may be mapped.

In addition, in one embodiment, the number of intra prediction modes within a specific angle range may be determined differently according to the division of the angle. For example, angles between 0 degrees and 90 degrees may be divided every 3 degrees, and in this case, the number of intra prediction modes may be determined as 29 except for 0 degrees and 90 degrees angles. As another example, nonlinear splitting for a particular angular range may be applied. For example, the first intra prediction mode and the second intra prediction mode may have angles of 2 degrees and 5 degrees, respectively.

Referring to FIG. 18, it is assumed that left, top and right reference samples are available, as shown in FIG. 18A. If left, top and right reference samples are available, as shown in FIG. 18B, the directional intra prediction mode in all directions except 90 degrees may be used for bidirectional intra prediction.

In other words, the encoder / decoder includes an intra prediction mode that includes a certain number of intra prediction modes within all angular ranges except 90 degrees for bidirectional intra prediction, if left, top and right reference samples are available for intra prediction as reconstructed samples. You can configure a set. For example, if an arbitrary coding order or coding order (or scan order) is applied from right to left, the left, top and right samples of the current block may be available for prediction as reconstructed samples.

The encoder / decoder may determine the intra prediction mode based on the set of intra prediction modes determined according to the available reference samples, and perform bidirectional intra prediction using the determined intra prediction mode. At this time, the encoder / decoder derives the first reference sample from the left reference sample, derives the second reference sample from the upper reference sample, and derives the derived first reference sample and the second reference sample based on the determined intra prediction mode. Can be weighted to produce the final predicted sample.

In addition, in one embodiment, intra prediction modes within an angular range determined according to reference samples available to the predefined prediction mode table may be mapped.

In addition, in one embodiment, the number of intra prediction modes within a specific angle range may be determined differently according to the division of the angle. For example, angles between 0 degrees and 90 degrees may be divided every 3 degrees, and in this case, the number of intra prediction modes may be determined as 29 except for 0 degrees and 90 degrees angles. As another example, nonlinear splitting for a particular angular range may be applied. For example, the first intra prediction mode and the second intra prediction mode may have angles of 2 degrees and 5 degrees, respectively.

Hereinafter, in an embodiment of the present invention, a method of performing bidirectional intra prediction using the reference sample dependent intra prediction mode described in Embodiment 2 is proposed.

19 is a diagram illustrating a method of generating a predictive sample through bidirectional intra prediction as an embodiment to which the present invention is applied.

Referring to FIG. 19, it is assumed that a reference sample dependent intra prediction mode is applied as in FIG. 16 and the upper and left reference samples are available. In addition, it is assumed that the angle of the intra prediction mode is 300 degrees.

In an embodiment of the present invention, the encoder / decoder predicts a prediction sample by weighting reference samples based on a distance ratio between a bidirectional reference sample (first reference sample and a second reference sample) and a current sample determined according to the prediction direction of the prediction mode. Can be generated (or derived).

In this case, the A reference sample and the C reference sample may be used (or selected or determined) as reference samples for the prediction sample P1 at the position [2, 2] according to the direction of the intra prediction mode. In this case, the prediction sample P1 may be generated (or derived, calculated) by the weighted average of the A reference sample and the C reference sample.

In Equation 1, d_top represents the distance between the A reference sample located in the upper reference sample array and P1, and d_left represents the distance between the C reference sample located in the left reference sample array and P1.

In one embodiment, for 160 degrees with an opposite angle of 300 degrees, d_top and d_left may be switched with each other because the directional intra mode starts from the left reference sample to the upper reference sample.

Likewise, in the case of FIGS. 17 and 18, the above-described bidirectional intra prediction method may be applied.

Also, in one embodiment, the positions of A and C may be fractional positions rather than integer positions. In this case, the sample value of the corresponding fractional position may be derived by performing interpolation based on the reference sample of the integer position. Also, for convenience of implementation, the ratio of floating point position 1 / (d_left + d_top) may be rounded to the ratio of integer positions.

Embodiments of the present invention described above may be implemented independently, or one or more embodiments may be implemented in combination.

20 is a flowchart illustrating a method of generating an intra prediction block according to an embodiment to which the present invention is applied.

Referring to FIG. 20, a decoder is mainly described for convenience of description, but the present invention is not limited thereto, and the method of generating an intra prediction block according to the embodiment of the present invention may be performed in the same manner in the encoder and the decoder.

The decoder is configured to use an intra prediction mode set used for the bidirectional intra prediction based on available reference samples around the current block when bi-directional intra prediction is applied to the current block. (S2001).

The decoder parses a prediction mode index indicating an intra prediction mode applied to the current block in the intra prediction mode set (S2002).

The decoder derives a first reference sample and a second reference sample used for the bidirectional intra prediction among the reference samples based on the prediction direction of the intra prediction mode (S2003).

The decoder weights the first reference sample and the second reference sample to generate a prediction sample of the current block (S2004).

In an embodiment, as described above, the intra prediction mode set may be configured to include a predefined number of intra prediction modes within a specific angular range determined according to the available direction of the reference sample.

As described above with reference to FIG. 16, the intra prediction mode set is intra within an angle range of 90 degrees to 180 degrees and an angle range of 270 degrees to 360 degrees when left and upper reference samples around the current block are available. It may be configured to include prediction modes.

As described above with reference to FIG. 17, the intra prediction mode set is intra within an angle range of 0 degrees to 90 degrees and an angle range of 180 degrees to 270 degrees when right and upper reference samples around the current block are available. It may be configured to include prediction modes.

As described above with reference to FIG. 18, the intra prediction mode set may be configured to include intra prediction modes within the remaining angular range except for 90 degrees and 270 degrees when left, right and top reference samples around the current block are available. Can be.

In addition, as described above, the weights applied to the first reference sample and the second reference sample, respectively, are the distance between the current sample and the first reference sample in the current block and the distance between the current sample and the second reference sample. It can be determined based on the ratio of. As an example, the method described above with reference to FIG. 19 and Equation 1 may be applied.

21 is a diagram illustrating an intra prediction apparatus according to an embodiment to which the present invention is applied.

In FIG. 21, for convenience of description, the intra predictor is illustrated as one block, but the intra predictor may be implemented in a configuration included in the encoder and / or the decoder.

Referring to FIG. 21, the intra predictor implements the functions, processes, and / or methods proposed in FIGS. 8 to 20. In detail, the intra prediction unit may include an intra prediction mode set configuration unit 2101, a prediction mode index parsing unit 2102, a reference sample derivation unit 2103, and a prediction sample generation unit 2104.

The intra prediction mode set configuration unit 2101 performs intra prediction used for the bidirectional intra prediction based on the available reference samples around the current block when bi-directional intra prediction is applied to the current block. Configure an intra prediction mode set.

A prediction mode index parser 2102 parses a prediction mode index indicating an intra prediction mode applied to the current block in the intra prediction mode set.

The reference sample derivator 2103 derives a first reference sample and a second reference sample used for the bidirectional intra prediction among the reference samples based on the prediction direction of the intra prediction mode.

The prediction sample generator 2104 weights the first reference sample and the second reference sample to generate a prediction sample of the current block.

In an embodiment, as described above, the intra prediction mode set may be configured to include a predefined number of intra prediction modes within a specific angular range determined according to the available direction of the reference sample.

As described above with reference to FIG. 16, the intra prediction mode set is intra within an angle range of 90 degrees to 180 degrees and an angle range of 270 degrees to 360 degrees when left and upper reference samples around the current block are available. It may be configured to include prediction modes.

As described above with reference to FIG. 17, the intra prediction mode set is intra within an angle range of 0 degrees to 90 degrees and an angle range of 180 degrees to 270 degrees when right and upper reference samples around the current block are available. It may be configured to include prediction modes.

As described above with reference to FIG. 18, the intra prediction mode set may be configured to include intra prediction modes within the remaining angular range except for 90 degrees and 270 degrees when left, right and top reference samples around the current block are available. Can be.

In addition, as described above, the weights applied to the first reference sample and the second reference sample, respectively, are the distance between the current sample and the first reference sample in the current block and the distance between the current sample and the second reference sample. It can be determined based on the ratio of. As an example, the method described above with reference to FIG. 19 and Equation 1 may be applied.

22 shows a video coding system to which the present invention is applied.

The video coding system can include a source device and a receiving device. The source device may deliver the encoded video / image information or data to a receiving device through a digital storage medium or network in a file or streaming form.

The source device may include a video source, an encoding apparatus, and a transmitter. The receiving device may include a receiver, a decoding apparatus, and a renderer. The encoding device may be called a video / image encoding device, and the decoding device may be called a video / image decoding device. The transmitter may be included in the encoding device. The receiver may be included in the decoding device. The renderer may include a display unit, and the display unit may be configured as a separate device or an external component.

The video source may acquire the video / image through a process of capturing, synthesizing, or generating the video / image. The video source may comprise a video / image capture device and / or a video / image generation device. The video / image capture device may include, for example, one or more cameras, video / image archives including previously captured video / images, and the like. Video / image generation devices may include, for example, computers, tablets and smartphones, and may (electronically) generate video / images. For example, a virtual video / image may be generated through a computer or the like. In this case, the video / image capturing process may be replaced by a process of generating related data.

The encoding device may encode the input video / image. The encoding apparatus may perform a series of procedures such as prediction, transform, and quantization for compression and coding efficiency. The encoded data (encoded video / image information) may be output in the form of a bitstream.

The transmitter may transmit the encoded video / video information or data output in the form of a bitstream to the receiver of the receiving device through a digital storage medium or a network in the form of a file or streaming. The digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and the like. The transmission unit may include an element for generating a media file through a predetermined file format, and may include an element for transmission through a broadcast / communication network. The receiver may extract the bitstream and transmit the extracted bitstream to the decoding apparatus.

The decoding apparatus may decode the video / image by performing a series of procedures such as inverse quantization, inverse transformation, and prediction corresponding to the operation of the encoding apparatus.

The renderer may render the decoded video / image. The rendered video / image may be displayed through the display unit.

23 is a diagram illustrating a structure of a content streaming system according to an embodiment to which the present invention is applied.

Referring to FIG. 23, a content streaming system to which the present invention is applied may largely include an encoding server, a streaming server, a web server, a media storage, a user device, and a multimedia input device.

The encoding server compresses content input from multimedia input devices such as a smart phone, a camera, a camcorder, etc. into digital data to generate a bitstream and transmit the bitstream to the streaming server. As another example, when multimedia input devices such as smart phones, cameras, camcorders, etc. directly generate a bitstream, the encoding server may be omitted.

The bitstream may be generated by an encoding method or a bitstream generation method to which the present invention is applied, and the streaming server may temporarily store the bitstream in the process of transmitting or receiving the bitstream.

The streaming server transmits the multimedia data to the user device based on the user's request through the web server, and the web server serves as a medium for informing the user of what service. When a user requests a desired service from the web server, the web server delivers it to a streaming server, and the streaming server transmits multimedia data to the user. In this case, the content streaming system may include a separate control server. In this case, the control server plays a role of controlling a command / response between devices in the content streaming system.

The streaming server may receive content from a media store and / or an encoding server. For example, when the content is received from the encoding server, the content may be received in real time. In this case, in order to provide a smooth streaming service, the streaming server may store the bitstream for a predetermined time.

Examples of the user device include a mobile phone, a smart phone, a laptop computer, a digital broadcasting terminal, a personal digital assistant (PDA), a portable multimedia player (PMP), navigation, a slate PC, Tablet PCs, ultrabooks, wearable devices (e.g., smartwatches, glass glasses, head mounted displays), digital TVs, desktops Computer, digital signage, and the like.

Each server in the content streaming system may be operated as a distributed server, in which case data received from each server may be distributed.

As described above, the embodiments described herein may be implemented and performed on a processor, microprocessor, controller, or chip. For example, the functional units shown in each drawing may be implemented and performed on a computer, processor, microprocessor, controller, or chip.

In addition, the decoder and encoder to which the present invention is applied include a multimedia broadcasting transmitting and receiving device, a mobile communication terminal, a home cinema video device, a digital cinema video device, a surveillance camera, a video chat device, a real time communication device such as video communication, a mobile streaming device, Storage media, camcorders, video on demand (VoD) service providing devices, OTT video (Over the top video) devices, Internet streaming service providing devices, three-dimensional (3D) video devices, video telephony video devices, and medical video devices. It can be used to process video signals or data signals. For example, the OTT video device may include a game console, a Blu-ray player, an internet access TV, a home theater system, a smartphone, a tablet PC, a digital video recorder (DVR), and the like.

In addition, the processing method to which the present invention is applied can be produced in the form of a program executed by a computer, and stored in a computer-readable recording medium. Multimedia data having a data structure according to the present invention can also be stored in a computer-readable recording medium. The computer readable recording medium includes all kinds of storage devices and distributed storage devices in which computer readable data is stored. The computer-readable recording medium may be, for example, a Blu-ray disc (BD), a universal serial bus (USB), a ROM, a PROM, an EPROM, an EEPROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, and an optical disc. It may include a data storage device. The computer-readable recording medium also includes media embodied in the form of a carrier wave (eg, transmission over the Internet). In addition, the bitstream generated by the encoding method may be stored in a computer-readable recording medium or transmitted through a wired or wireless communication network.

In addition, an embodiment of the present invention may be implemented as a computer program product by program code, which may be performed on a computer by an embodiment of the present invention. The program code may be stored on a carrier readable by a computer.

The embodiments described above are the components and features of the present invention are combined in a predetermined form. Each component or feature should be considered optional unless stated otherwise. Each component or feature may be embodied in a form that is not combined with other components or features. It is also possible to combine some of the components and / or features to form an embodiment of the invention. The order of the operations described in the embodiments of the present invention may be changed. Some components or features of one embodiment may be included in another embodiment or may be replaced with corresponding components or features of another embodiment. It is obvious that the claims may be combined to form an embodiment by combining claims that do not have an explicit citation relationship in the claims or as new claims by post-application correction.

Embodiments according to the present invention may be implemented by various means, for example, hardware, firmware, software, or a combination thereof. In the case of a hardware implementation, an embodiment of the present invention may include one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), FPGAs ( field programmable gate arrays), processors, controllers, microcontrollers, microprocessors, and the like.

In the case of implementation by firmware or software, an embodiment of the present invention may be implemented in the form of a module, procedure, function, etc. that performs the functions or operations described above. The software code may be stored in memory and driven by the processor. The memory may be located inside or outside the processor, and may exchange data with the processor by various known means.

It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential features of the present invention. Accordingly, the above detailed description should not be construed as limiting in all aspects and should be considered as illustrative. The scope of the invention should be determined by reasonable interpretation of the appended claims, and all changes within the equivalent scope of the invention are included in the scope of the invention.

As mentioned above, preferred embodiments of the present invention are disclosed for the purpose of illustration, and those skilled in the art can improve and change various other embodiments within the spirit and technical scope of the present invention disclosed in the appended claims below. , Replacement or addition would be possible.

Claims (10)

In the method of decoding an image based on the intra prediction mode, When bi-directional intra prediction is applied to the current block, configure an intra prediction mode set used for the bidirectional intra prediction based on available reference samples around the current block. Doing; Parsing a prediction mode index indicating an intra prediction mode applied to the current block within the intra prediction mode set; Deriving a first reference sample and a second reference sample used for the bidirectional intra prediction among the reference samples based on the prediction direction of the intra prediction mode; And Generating a prediction sample of the current block by weighting the first reference sample and the second reference sample, The intra prediction mode set is configured to include a predefined number of intra prediction modes within a particular angular range determined according to the available direction of the reference sample. The method of claim 1, The intra prediction mode set is an intra prediction mode within an angle range greater than 90 degrees and less than or equal to 180 degrees and an angle range greater than or equal to 270 degrees and less than 360 degrees when left and upper reference samples around the current block are available. And a decoding method. The method of claim 1, The intra prediction mode set is an intra prediction mode within an angle range greater than or equal to 0 degrees and less than 90 degrees and an angle range greater than 180 degrees and less than or equal to 270 degrees when right and upper reference samples around the current block are available. And a decoding method. The method of claim 1, The intra prediction mode set is configured to include intra prediction modes within the remaining angular range except 90 degrees when left, right and top reference samples around the current block are available. The method of claim 1, The weights applied to the first reference sample and the second reference sample, respectively, are determined based on a ratio between the distance between the current sample and the first reference sample and the distance between the current sample and the second reference sample in the current block. , Decoding method. An apparatus for decoding an image based on an intra prediction mode, When bi-directional intra prediction is applied to the current block, configure an intra prediction mode set used for the bidirectional intra prediction based on available reference samples around the current block. An intra prediction mode set configuration unit; A prediction mode index parser for parsing a prediction mode index indicating an intra prediction mode applied to the current block in the intra prediction mode set; A reference sample derivation unit for deriving a first reference sample and a second reference sample used for the bidirectional intra prediction among the reference samples based on a prediction direction of the intra prediction mode; And A prediction sample generator configured to weight the first reference sample and the second reference sample to generate a prediction sample of the current block, The intra prediction mode set is configured to include a predefined number of intra prediction modes within a particular angular range determined according to the available direction of the reference sample. The method of claim 6, The intra prediction mode set is an intra prediction mode within an angle range greater than 90 degrees and less than or equal to 180 degrees and an angle range greater than or equal to 270 degrees and less than 360 degrees when left and upper reference samples around the current block are available. And a decoding device. The method of claim 6, The intra prediction mode set is an intra prediction mode within an angular range greater than or equal to 0 degrees and less than 90 degrees and an angular range greater than 180 degrees and less than or equal to 270 degrees when right and upper reference samples around the current block are available. And a decoding device. The method of claim 6, And the intra prediction mode set is configured to include intra prediction modes within the remaining angular range except 90 degrees when left, right and top reference samples around the current block are available. The method of claim 6, The weights applied to the first reference sample and the second reference sample, respectively, are determined based on a ratio between the distance between the current sample and the first reference sample and the distance between the current sample and the second reference sample in the current block. Decoding device.

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