How is the data laid out in a 4 channel image matrix in OpenCV (CV_8UC4)?

Question

How is the data laid out in a 4 channel image matrix in OpencV (CV_8UC4)?

cv::Mat A = cv::Mat::zeros(height, width, CV_8UC4);

is it:

[R1,G1,B1,A1,R2,G2,B2,A2,...] 

or:

[B1,G1,R1,A1,B2,G2,R2,A2,...] 

or anything else?

Answer 1

It depends. If you are in BGRA space then it is:

[B1,G1,R1,A1,B2,G2,R2,A2,...]

If you are in RGBA space then it is:

[R1,G1,B1,A1,R2,G2,B2,A2,...]

And by default, OpenCV loads images as BGR (BGRA) sapce. So if you did not chage anything it should be:

[B1,G1,R1,A1,B2,G2,R2,A2,...]