How do I change my command to only use the Blue channel?

[anthony]

A better method of adding extra masking in Alpha Compositin is to use the correct compose method.
As mask already has alpha, use it.

Code: Select all

  composite  Mask.rgb  Alpha.rgb -compose Dst_In  alpha_masked.rgb

the above finds that parts of Alpha that falls within the non-transparent areas of the overlay mask image. if you want the reverse use 'Dst_Out' instead.
see IM Examples, Alpha Composition for more details.
http://www.cit.gu.edu.au/~anthony/graph ... ose/#dstin

You do not need to extract and mathematically merge the masks.