OpenCV for Android: how to copy a Mat object (image) to another Mat?

Question

Both Mat objects contain (different) images. I want to copy mSnapshot over mRgba. I tried these (separately), but none of them seem to change mRgba:

mSnapshot.assignTo(mRgba);
mSnapshot.copyTo(mRgba);
mRgba = mSnapshot;

This throws an exception:

mRgba.setTo(mSnapshot);

And this does work, and sets mRgba to be a completely black image:

mRgba.setTo(new Scalar(0,0,0,255));

What am I missing?

[Edit] The source files and exception can be found here.

Solution

To copy an image, you should use the clone() member function like this:

capture.retrieve(mSnapshot, Highgui.CV_CAP_ANDROID_COLOR_FRAME_RGBA);
mRgba = mSnapshot.clone();

Another note, OpenCV stores information in BGR order; therefore, your line:

mRgba.setTo(new Scalar(0,0,0,255));

Edited for clarity : This command is setting each pixel to (0, 0, 0, 255), so channels 1-3 are set to 0, and channel 4 (alpha) is set to 255. What if you tried this:

mRgba.setTo(new Scalar(0, 255, 0, 0)); // should be set to green.

Also, note that you can only use setTo once the matrix is allocated.

Hope that helps!