How to convert Color of a Rectangle (ROI) in a matrix with OpenCV Java?

Question

I'm trying to convert a color in a ROI of an matrix with OpenCV Java with following Code

public Mat detect(Mat image){
    Rect roi = new Rect(new Point(50, 50), new Point(image.width() - 50, image.height() - 50));
    Mat mask = image.submat(roi);
    Mat temp = new Mat();
    Imgproc.cvtColor(mask, temp, Imgproc.COLOR_BGRA2GRAY,0);
    temp.copyTo(mask);
    return image;
}

But the result is the same as the given image. If I change the Imgproc.cvtColor() call with a Imgproc.medianBlur() effect for example:

    ...
    Imgproc.medianBlur(mask, temp, 11);
    ... 

There is a blurred rectangle visible. I think while the process of Imgproc.cvtColor() the references to the original image are replaced by some newly created. So how else can I convert the color of a ROI in a matrix without loosing the references?

I appreciate any help, thanks!

P.S: I wanted to add some sample images but my reputation is not high enough. I'm sorry for this and hope you can imagine my problem even without samples.

Answer 1

you can't have an image, that is part 1 channel, and 4 channel otherwise.

the blur example worked fine, because it did not change the number of channels.

you'll have to convert your gray (sub)img to rgba again, before copying

Mat temp = new Mat();
Imgproc.cvtColor(mask, temp, Imgproc.COLOR_BGRA2GRAY,0);
Mat temp_rgba = new Mat();
Imgproc.cvtColor(temp, temp_rgba, Imgproc.COLOR_GRAY2BGRA,0);
temp_rgba.copyTo(mask);