I'm curious how I'd go about causing pixels being drawn in a quad to be blended by alpha values already in the color buffer.

To clarify, say I've written out a mask of sorts into the alpha channel of the color buffer. Then I'm drawing a billboarded quad over the scene and I want the alpha value of each fragment of that quad to be multiplied by the alpha of the corresponding fragment in the color buffer.

Is this possible through the fixed function pipeline, or barring that, can a fragment program read from the color buffer ( specifically from the corresponding value in the color buffer )?

Thanks,

glBlendFunc(..... GL_DST_ALPHA .....);

I'll be damned. I've wondered what GL_DST_ALPHA actually referred to, now I know. Thanks,