Efficient way of reading depth values from depth buffer

Question

For an algorithm of mine I need to be able to access the depth buffer. I have no problem at all doing this using glReadPixels, but reading an 800x600 window is extremely slow (F

Answer 1

The fastest way of doing this is using asynchronous pixel buffer objects, there's a good explanation here:

http://www.songho.ca/opengl/gl_pbo.html

Answer 2

I would render to a FBO and read its depth buffer after the frame has been rendered. PBOs are outdated technology.