copy depth values from a buffer to another

Hi !

Im trying to save the depth values in a rectangular zone into the video ram, to restore it later. I currently do it with readpixels/drawpixels, but as it copies to/from main ram its very slow. What I would like is to copy/draw from video ram to another buffer of video ram.

If I understood the glspec correctly, there are several color buffers (front, back, left, right…), but only one depth or stencil buffer. Is that right ?

So my first question is : is there a way to do it ? (copy a rectangular zone of depth values from one place to another, not used, and viceversa)

If not, is there an extention to do this ?