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Whats the problem? You seem to know what you want to do!
See IM Examples on using CopyOpacity...
http://www.cit.gu.edu.au/~anthony/graph ... opyopacity
It is vital that the source image does NOT have a matte channel.
That is the input is a mask without any alpha channel.
If it has an alpha channal that is what is copied, resulting in something like
a 'Dst_In' operation (though not as good as transparent areas could become opaque!)
If you plan to use an image with an alpha channal mask, use a proper Duff-Porter Alpah Composition method like 'Dst_In' instead.
http://www.cit.gu.edu.au/~anthony/graph ... se/#dst_in
I'll highlight the quoted section in IM examples. as it is so important.