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I posted this on StackOverflow here, but after only getting a few views, I've decided to crosspost here.

I'm a complete novice when it comes to OpenCV, so this is probably a dumb question.

I'm just trying to get something basic up and running- I want to draw the edges detected by the Canny algorithm directly on the image coming in. I currently have this:

I'm displaying the edge data from Canny directly, but now I want to get rid of the black and just show the white, on the image being processed.

I've tried googling things like "using binary image as alpha mask", but after a day of reading tutorials and trying everything I can find, I'm still not sure I know what's going on. OpenCV seems very powerful, so this is probably a pretty easy thing to do, so I'm hoping somebody can point me in the right direction.

Here's the code I'm using, most of which has been copied from the examples:

public Mat onCameraFrame(CvCameraViewFrame inputFrame) {

    Mat rgba = inputFrame.rgba();
    org.opencv.core.Size sizeRgba = rgba.size();

    Mat rgbaInnerWindow;


    int rows = (int) sizeRgba.height;
    int cols = (int) sizeRgba.width;

    int left = cols / 8;
    int top = rows / 8;

    int width = cols * 3 / 4;
    int height = rows * 3 / 4;

    //get sub-image
    rgbaInnerWindow = rgba.submat(top, top + height, left, left + width);

    //create edgesMat from sub-image
    Imgproc.Canny(rgbaInnerWindow, edgesMat, 100, 100);

    //copy the edgesMat back into the sub-image
   Imgproc.cvtColor(edgesMat, rgbaInnerWindow, Imgproc.COLOR_GRAY2BGRA, 4);

    rgbaInnerWindow.release();

    return rgba;
}

If anybody can point me to a basic example that does what I want, I'd be very grateful!