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Clearing alpha channel + depth buffer

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Hi can anyone tell me how to add geometry to the depth buffer but at the same time leave the framebuffer untouched?
Right now Im doing this:

glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_NEVER);
for(int K = 0; K < ConvexHull_list.size();K++){
ConvexHull * CH = ConvexHull_list[K];
CH->Render();
}
glDisable(GL_DEPTH_TEST);


Also,how do I reset the alpha channel(to 0) in the framebuffer without disturbing the rest of the values in the framebuffer?
I was going to render a quad with glColor4f(0,0,0,0);
But I dont think doing that by itself clears only the alpha values in the framebuffer.

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You are looking for glColorMask

Drawing to depth buffer only.
Your code should look like :

glColorMask (GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE); //disable write to color-buffer
glDepthMask(GL_TRUE);

glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LEQUAL); // or whatever

//render everything

glColorMask (GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE); //enable write to color buffer



For resetting the alpha channel only :

glColorMask (GL_FALSE, GL_FALSE, GL_FALSE, GL_TRUE); //only write to alpha
glDisable (GL_DEPTH_TEST);
glDepthMask(GL_FALSE);

//Render a full-screen quad with alpha=whatever you want

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Al alternative might be to put a glDrawPixels call into a display list; make sure that you use the same format and layout as your color buffer has and it should go quite fast.

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