glCopyTexImage2D as alternative to glReadPixels (OpenGL ES)

Question

I'm currently grabbing a screenshot of an OpenGL ES game using glReadPixels. The screenshots are great however the call to glReadPixels causes a small stutter in the game.

glCopyTexImage2D has been suggested as a more efficient replacement for glReadPixels. How does glCopyTexImage2D work? For some context I'm using this Apple method.

I'm relatively new to OpenGL so any help is much appreciated :-)

Answer 1

The time delay is caused by transferring a large amount of memory between GPU and CPU. This can be solved by transferring in chunks, a little bit per frame. But if you read from the framebuffer over a series of frame, the image would be changing meanwhile.

So you make a copy, video RAM into video RAM (very fast), and then it won't change as you transfer it piecemeal.