IplImage Pixel Access JavaCV

Question

I\'m trying to access Pixel by Pixel of an IplImage. Im using Java and Processing, and sometimes I need to access pixel by pixel. I\'ve done this so far, but I don\'t know what\

Answer 1

The following code loads an image from the disk and performs a grayscale conversion by iterating over the pixels of the image:

    IplImage image = cvLoadImage("pipeline.png", CV_LOAD_IMAGE_COLOR);
    if (image == null) 
    {
        System.out.println("Erro ao carregar imagem!");
        return;
    }

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

    CvMat mtx = CvMat.createHeader(image.height(), image.width(), CV_32FC1);  
    cvGetMat(image, mtx, null, 0); 

    System.out.println(mtx.rows() + "x" + mtx.cols());

    for (int i = 0; i < mtx.rows(); i++)
    {
        for (int j = 0; j < mtx.cols(); j++)
        {
            CvScalar rgb = cvGet2D(mtx, i, j);
            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(mtx, i, j, scalar);
        }
    }        

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

    cvReleaseImage(image); 

Not sure if creating a new CvMat is the best approach, but it's the only way I know to work in javacv.

EDIT:

Unfortunately you use data types that are not from OpenCV, like PImage, but I did my best to simulate what you are doing.

    // Create a black image
    IplImage imageDst = IplImage.create(imageSrc.width(), imageSrc.height(), IPL_DEPTH_8U, 3); 

    // Create a temporary mat to iterate over it's pixels
    CvMat imagePixels = CvMat.createHeader(imageDst.height(), imageDst.width(), CV_32FC1); 

    // Copy black image to temporary mat
    cvGetMat(imageDst, imagePixels, null, 0);

    int x, y;
    for(y=0; y<imageSrc.height(); y++)
       for(x=0; x<imageSrc.width(); x++)
       {
            // At this point you tried to do: locPImage = x + y * width;
            // but I think you might have mistaken for: locPImage = y + x * width;

            //...
       }
       imageDst = new IplImage(imagePixels); 

Answer 2

The fastest way to iterate over each pixel in JavaCV is:

ByteBuffer buffer = image.getByteBuffer();

for(int y = 0; y < image.height(); y++) {
    for(int x = 0; x < image.width(); x++) {
        int index = y * image.widthStep() + x * image.nChannels();

        // Used to read the pixel value - the 0xFF is needed to cast from
        // an unsigned byte to an int.
        int value = buffer.get(index) & 0xFF;

        // Sets the pixel to a value (greyscale).
        buffer.put(index, value);

        // Sets the pixel to a value (RGB, stored in BGR order).
        buffer.put(index, blue);
        buffer.put(index + 1, green);
        buffer.put(index + 2, red);
    }
}