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24 == CV_8UC4, 16 == CV_8UC3.

so, to convert ImageMat to the format of rgba (misnomer?) you would use:

Mat dst = new Mat();
Imgproc.cvtColor( ImageMat, dst, Imgproc.COLOR_BGRA2BGR );

or, into the other direction:

Imgproc.cvtColor( rgba, dst, Imgproc.COLOR_BGR2BGRA );

also see docs, and remember, that opencv's Mat''s are in BGR order, while native android has it RGB(A)