[PD] chroma_key to alpha with Gem?

[Hans-Christoph Steiner]

I was wondering whether it is possible convert a specific color, like  
green-screen chroma key, to alpha information using Gem.  They idea is  
to overlap many green-screen videos.  I can see the pieces, but I  
can't see the whole.

.hc

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Man has survived hitherto because he was too ignorant to know how to  
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change them, or perish.    -William Carlos Williams