🚀 go-pugleaf

Hi Everybody!

Another quirky problem.  I need to mask an area of an image
by changing the alpha value within the area to 1 (for
example) and everything outside the area to (0).  The
catch is that I need to retain the original RGB data within
the whole image (not just the masked area! otherwise
clipping would work!).  I have a path to clip to, easy
enough, but how can I draw only to the alpha channel?  Or,
how do I use some strange combination of compositing
functions to do this?  The images are RGB tiffs, no alpha,
so compositing defaults to an opaque alpha (1, I
think)... 

In simpler terms, I need all of the image data, and the
alpha channel to mask a region of interest within
it...easy, right?  Any takers? 

Daniel Filiberti 

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Motto for a monitor, "It's better to burn out, than to fade
 away..."  [8?)}
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Daniel Filiberti
Oregon Health Sciences University
Biomedical Information Communication Center (BICC)
3181 Sam Jackson Park Road
Portland, Oregon  97201-3098
email:  filibert@ohsu.edu