[PD] GEM: multiply with movie

[lorenz kirnbauer]

IOhannes m zmoelnig wrote:
> b) use something like thoralf proposed: cover the parts of the image
> that you don't want to see (think of a landscape behind a wall with a
> moving window; since Gem (openGL) uses positive (additive) geometry, you
> (unfotunately) cannot just cut out the window, so you have to build the
> changing walls)

thank you very much for your quite informative answer !

after all, it's kind of sad for me, that there is no subtractive 
geometry in pd..
i know a little bit about openGL from an university course.. and know, 
that in openGL you are able to work easily with subtractive geometry..
you are able to just subtract one object from the other.
and that was the functionality i was searching for in gem, in the 
beginning.

would be glad, if some further version of gem would have this 
possiblity.. would save me quite a lot of workarounds ;)

thanks so far..

lorenz aka orjo