Copy smaller mat to part of bigger mat

edit

So far I have this:

@Override
    public void onCameraViewStarted(int height, int width) {
        mat1 = new Mat(width, height, CvType.CV_8UC3);
        mat2 = new Mat(width, 175, CvType.CV_8UC3);
        mat3 = new Mat(width, height, CvType.CV_8UC3, new Scalar(255,0,0));
    }

and this:

@Override
    public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {
        mat1 = inputFrame.rgba();
        Rect roi = new Rect(75, 0, 175, 320);
        mat2 = mat1.submat(roi).clone();
        mat2.copyTo(mat3.submat(new Rect(75,0,175,320)));
        return mat3;
}

But all I get is a red image ...

Mat(height, width, -- please.

you also overwrite most of your preallocated Mat's, so that's kinda useless (and misleading)

also, Rect(x,y,w,h)

also: mat1 will likely be CV_8UC4, not CV_8UC3

All my Mat's are rotated because the preview in opencv4android does not have the right rotation.

How do I get mat2 displayed on mat3?

Ok now I have this but it is still not working..

@Override
    public void onCameraViewStarted(int width, int height) {
        mat1 = new Mat(height, width, CvType.CV_8UC4);
        mat2 = new Mat(175, width, CvType.CV_8UC3);
        mat3 = new Mat(height, width, CvType.CV_8UC3, new Scalar(255,0,0));
    }

@Override
    public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {
        mat1 = inputFrame.rgba();

        Rect roi = new Rect(75, 0, 175, 320);
        mat2 = mat1.submat(roi).clone();
        mat2.copyTo(mat3.submat(roi));

        return mat3;
    }