Overlap Image in a specific way?_C++

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How can following overlapping be performed?

Note: Mask can be created with any of R G B layer, because black areas are completely black, i.e. '0' for each R G B's. In this way efficiency can be improved.

In MATLAB this can be done with:

mask = (Im2> 0);           % masking 3 channels (computationally expensive)
Im1(mask) = Im2(mask);     % Im1 will become the final overlapped image