Convert image in android to DCT image using OpenCV and JavaCV

android_open_cv.java

// Author: Rheza Satria (c) 2015
// Please mention this gist if you inspired/used it!
public static Bitmap applyDCT(Bitmap image, int blockSize) {
    int _w = image.getWidth();
    int _h = image.getHeight();

    // Preload the opencv_objdetect module to work around a known bug.
    Loader.load(opencv_objdetect.class);
    AndroidFrameConverter afc = new AndroidFrameConverter();
    Frame frame = afc.convert(image);

    OpenCVFrameConverter.ToMat converterToMat = new OpenCVFrameConverter.ToMat();
    Mat img = converterToMat.convert(frame);

    Mat img2 = new Mat(_h, _w, CV_32F);
    cvtColor(img, img2, CV_RGB2GRAY);

    Mat img3 = new Mat(_h, _w, CV_32F);
    img2.convertTo(img3, CV_32F);

    Mat freq = new Mat(_h, _w, CV_32F);
    dct(img3, freq);

    Mat img4 = new Mat(_h, _w, img.type());
    freq.convertTo(img4, img.type());

    Frame freqFrame = converterToMat.convert(img4);
    return afc.convert(freqFrame);
}