| OpenCV 4.3.0 Open Source Computer Vision

Unwrap two-dimensional phase maps

## Goal

In this tutorial, you will learn how to use the phase unwrapping module to unwrap two-dimensional phase maps. The implementation is based on [131].

## Code

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#include <opencv2/imgproc.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/phase_unwrapping.hpp>
#include <iostream>
#include <fstream>
#include <stdio.h>
using namespace cv;
using namespace std;
static const char* keys =
{
"{@inputPath | | Path of the wrapped phase map saved in a yaml file }"
"{@outputUnwrappedName | | Path of the unwrapped phase map to be saved in a yaml file and as an 8 bit png}"
};
static void help()
{
cout << "\nThis example shows how to use the \"Phase unwrapping module\" to unwrap a phase map"
" saved in a yaml file (see extra_data\\phase_unwrapping\\data\\wrappedpeaks.yml)."
" The mat name in the file should be \"phaseValue\". The result is saved in a yaml file"
" too. Two images (wrapped.png and output_name.png) are also created"
" for visualization purpose."
"\nTo call: ./example_phase_unwrapping_unwrap <input_path> <output_unwrapped_name> \n"
<< endl;
}
int main(int argc, char **argv)
{
phase_unwrapping::HistogramPhaseUnwrapping::Params params;
CommandLineParser parser(argc, argv, keys);
String inputPath = parser.get<String>(0);
String outputUnwrappedName = parser.get<String>(1);
if( inputPath.empty() || outputUnwrappedName.empty() )
{
help();
return -1;
}
FileStorage fsInput(inputPath, FileStorage::READ);
FileStorage fsOutput(outputUnwrappedName + ".yml", FileStorage::WRITE);
Mat wPhaseMap;
Mat uPhaseMap;
Mat reliabilities;
fsInput["phaseValues"] >> wPhaseMap;
fsInput.release();
params.width = wPhaseMap.cols;
params.height = wPhaseMap.rows;
Ptr<phase_unwrapping::HistogramPhaseUnwrapping> phaseUnwrapping = phase_unwrapping::HistogramPhaseUnwrapping::create(params);
phaseUnwrapping->unwrapPhaseMap(wPhaseMap, uPhaseMap);
fsOutput << "phaseValues" << uPhaseMap;
fsOutput.release();
phaseUnwrapping->getInverseReliabilityMap(reliabilities);
Mat uPhaseMap8, wPhaseMap8, reliabilities8;
wPhaseMap.convertTo(wPhaseMap8, CV_8U, 255, 128);
uPhaseMap.convertTo(uPhaseMap8, CV_8U, 1, 128);
reliabilities.convertTo(reliabilities8, CV_8U, 255,128);
imshow("reliabilities", reliabilities);
imshow("wrapped phase map", wPhaseMap8);
imshow("unwrapped phase map", uPhaseMap8);
imwrite(outputUnwrappedName + ".png", uPhaseMap8);
imwrite("reliabilities.png", reliabilities8);
bool loop = true;
while( loop )
{
char key = (char)waitKey(0);
if( key == 27 )
{
loop = false;
}
}
return 0;
}

## Explanation

To use this example, wrapped phase map values should be stored in a yml file as CV_32FC1 Mat, under the name "phaseValues". Path to the data and a name to save the unwrapped phase map must be set in the command line. The results are saved with floating point precision in a yml file and as an 8-bit image for visualization purpose.

Some parameters can be chosen by the user:

- histThresh is a parameter used to divide the histogram in two parts. Bins before histThresh are smaller than the ones after histThresh. (Default value is 3*pi*pi).
- nbrOfSmallBins is the number of bins between 0 and histThresh. (Default value is 10).
- nbrOfLargeBins is the number of bins between histThresh and 32*pi*pi. (Default value is 5).

phase_unwrapping::HistogramPhaseUnwrapping::Params params;
CommandLineParser parser(argc, argv, keys);
String inputPath = parser.get<String>(0);
String outputUnwrappedName = parser.get<String>(1);
String outputWrappedName = parser.get<String>(2);
if( inputPath.empty() || outputUnwrappedName.empty() )
{
help();
return -1;
}
FileStorage fsInput(inputPath, FileStorage::READ);
FileStorage fsOutput(outputUnwrappedName + ".yml", FileStorage::WRITE);
Mat wPhaseMap;
Mat uPhaseMap;
Mat reliabilities;
fsInput["phaseValues"] >> wPhaseMap;
fsInput.release();
params.width = wPhaseMap.cols;
params.height = wPhaseMap.rows;
Ptr<phase_unwrapping::HistogramPhaseUnwrapping> phaseUnwrapping = phase_unwrapping::HistogramPhaseUnwrapping::create(params);

The wrapped phase map is unwrapped and the result is saved in a yml file. We can also get the reliabilities map for visualization purpose. The unwrapped phase map and the reliabilities map are converted to 8-bit images in order to be saved as png files.

phaseUnwrapping->unwrapPhaseMap(wPhaseMap, uPhaseMap);
fsOutput << "phaseValues" << uPhaseMap;
fsOutput.release();
phaseUnwrapping->getInverseReliabilityMap(reliabilities);
Mat uPhaseMap8, wPhaseMap8, reliabilities8;
wPhaseMap.convertTo(wPhaseMap8, CV_8U, 255, 128);
uPhaseMap.convertTo(uPhaseMap8, CV_8U, 1, 128);
reliabilities.convertTo(reliabilities8, CV_8U, 255,128);
imshow("reliabilities", reliabilities);
imshow("wrapped phase map", wPhaseMap8);
imshow("unwrapped phase map", uPhaseMap8);
imwrite(outputUnwrappedName + ".png", uPhaseMap8);
imwrite("reliabilities.png", reliabilities8);