Looping through OpenCV Mat in Java bindings

c++, image-processing, java, javacv, opencv

Solution

If the `Mat` is truly gray, it's probably of type `CV_8U`:

for (int i = 0; i < gray.rows; i++)
{
    for (int j = 0; j < gray.cols; j++)
    {
        byte pixel = gray.get(i, j);
    }
}

I don't have my environment setup to test this code. For more info check this thread.

If you are using JavaCV, some time ago I wrote a code to iterate on the pixels of a colored image and convert it to grayscale like this:

    IplImage imageSrc = cvLoadImage("pipeline.png", CV_LOAD_IMAGE_COLOR);
    if (imageSrc == null) 
    {
        System.out.println("!!! Failed loading image");
        return;
    }

    double r, g, b;
    r = g = b = 0.0;               

    IplImage imageDst = IplImage.create(imageSrc.width(), imageSrc.height(), IPL_DEPTH_8U, 3);

    CvMat imagePixels = CvMat.createHeader(imageDst.height(), imageDst.width(), CV_32FC1);  
    cvGetMat(imageDst, imagePixels, null, 0); 
    int x, y;
    for(y=0; y<imageSrc.height(); y++)
       for(x=0; x<imageSrc.width(); x++)
       {
            CvScalar rgb = cvGet2D(imagePixels, y, x);
            r = rgb.val(0);
            g = rgb.val(2);
            b = rgb.val(1);

            double gray = (r + g + b) / 3;

            CvScalar scalar = new CvScalar();
            scalar.setVal(0, gray);
            scalar.setVal(1, gray);
            scalar.setVal(2, gray);
           cvSet2D(imagePixels, y, x, scalar);
       }
       imageDst = new IplImage(imagePixels);  

       IplImage result = new IplImage(imagePixels); 
       cvSaveImage("manual_gray.png", result);

       cvReleaseImage(imageSrc);

Problem

I am trying to convert a C++ method from a previous answer I received using OpenCV to Java using OpenCV Java bindings C++ code: ``` cv::Mat gray; cv::Mat element = cv::getStructuringElement(cv::MORPH_CROSS, cv::Size(2 * erosion_size + 1, 2 * erosion_size + 1), cv::Point(erosion_size, erosion_size) ); cv::erode(gray, gray, element); // Scan the image searching for points and store them in a vector std::vector<cv::Point> points; cv::Mat_<uchar>::iterator it = gray.begin<uchar>(); cv::Mat_<uchar>::iterator end = gray.end<uchar>(); for (; it != end; it++) { if (*it) points.push_back(it.pos()); } ``` I can't figure out how to loop through the `gray` variable as they are in C code. Below is what I've got so far: Java Code: ``` Mat gray = new Mat(); Mat element = Imgproc.getStructuringElement( Imgproc.MORPH_CROSS, new Size(2 * erosion_size + 1, 2 * erosion_size + 1), new Point(erosion_size, erosion_size) ); Imgproc.erode(img, gray, element); List <Point> p = new ArrayList<Point>(); //How to loop through the gray variable? ``` I went through the API's and it seems that `Mat` does have methods for getting `rows` and `cols` but I can't call `Iterator` on anything..

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