[PD] GEM: openGL and CPU usage

[Martin Dupras]

I'm working on some VJ-type pd patches at the moment, and am a bit
puzzled by how GEM uses openGL, and what is processed on the CPU.

Hewlett-packard is lending us a twin 2.8GHz Xeon machine with 1gig of
DDR ram, which is fitted at the moment with a matrox G400 (don't ask,
we've got a GeForce 4 Ti4600 card coming our way imminently.)

I was doing some testing using gem and pd, playing with a large avi divx
file (ca 500MB). 

If I use pix_movie and use it to display on a rectangle geo, the movie
plays fine. If however I apply any transformations, the whole thing
slows down considerably. I understand that it must be because of the
graphics card.

Can anyone elaborate a little on how GEM exploits openGL, and what
objects use the CPU for processing? I would in particular know what
kinds of approaches would help get good video processing performance.

Here's another question. If I apply two videos to two rectangles, put
them in front of one another, and make the frontmost one half
transparent, the black pixels are not transparent (in other words, they
are grey and half opaque). I suppose that must be to do with how openGL
works, but is there a way to make it behave in such a way that a pixel
that has no color is actually transparent? 

- martin