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 [90].

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);