Convert RGB/A Mat to YCbCr Y Plane and CbCr Plane

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I'm aware of the function cv::cvtColorTwoPlane (InputArray src1, InputArray src2, OutputArray dst, int code) for going from the separate Y and CbCr Mat's to an RGB Mat, but what if I have an RGB Mat and want to write to the Y and CbCr Mat's. Is there a function for this? I couldn't find anything in the opencv source code and documention. If there isn't a function is there another efficient way to do this?