Capturing from textures

I am working on a engine which must be able to render a simple scene and then capture it back to system memory and encode it with DivX at a high rate (at least 10 fps). Yes, I realize this takes a pretty good computer to do at all. I have it working well with Direct3D, because Direct3D 9 offers a GetRenderTargetData() API which is already hardware accelerated. but the OpenGL renderer is lacking in capture code at this point. I tried glReadPixels() and a couple other ways of doing it, but they are very slow. I found an example on the internet last week that said this method should be fast. It used a combination of several extensions, PBuffers, Render To Texture, and then a texture capture extension. They would setup a PBuffer and make a texture out of it. Then render to that texture, and then use the final extension to read that texture out of VRAM very quickly. I have been able to implement the Render to Texture, but now that I'm ready to capture the texture, I can't find the example again. Does anybody know the name of the extension that example might have been using? Thanks