Why is X11 Composite extension incompatible with Stereo visuals?

Question

In the NVIDIA README for the Quadro card X driver, there is this comment:

Workstation overlays, stereo visuals, and the unified back buffer (UBB) are incompatible with Composite. These features will be automatically disabled when Composite is detected

Is there some fundamental X reason why this is so? Why are quadro cards on Windows able to do translucent windows and have active stereo visuals at the same time?

Is someone working on fixing this? It seems like the future is compositing desktops, and Stereo visuals are also becoming more popular for some kinds of apps.

Answer 1

Composite prevents direct drawing to the viewport and that's where things get complicated. Basically all hardware overlay operations struggle with composite. The clue is that the hardware overlay shouldn't happen on the screen itself but on a off-screen pixmap which then for example can be mapped to a texture by the compositor.

For example, xv did not work with early compositing (I don't know if it works now). Direct GL rendering also didn't work with early compositing but this seems to be resolved now. I assume nvidia developers don't have the time/priority to make their other proprietary overlay methods work with indirect rendering. In theory it should work if it is not bad design.