Grab the contents on screen painted via OpenGL

I want to know if its possible to grab the contents of screen/surface painted via OpenGL? and how to do it?

I mean something like this (From Quarzt), If I have CGBitmapContextRef, I can do all the drawing 开发者_C百科activity on it and then create a CGBitmap out of it. Is there something similar? If not please suggest some different approach.

---

Yes, for this you have to use glReadPixels, but you should notice that heavy use of this function can be slow.