I have after a few passes a screen consisting of shades of gray. Each pixel has an rgba color of k, k, k, k. Now, I want to draw on the screen another texture, but only on the parts of the screen of color (1, 1, 1, 1), and not on the rest. Is there any way to do this in (preferably unextended) openGL? Thanks

Try alpha test.

I''ve a little program on my site for using alpha test.

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Leyder Dylan
http://ibelgique.ifrance.com/Slug-Production/

Yes, but alpha test tests the INCOMING value, not the value already on screen. That''s the problem.