Alpha channel rendering

[Russell Shaw]

Hi,
The problem with transparent menus and other things is that now
there's no concept of obscured windows. So if there's 100 alpha channel
obscured windows, each on top of the next, then changing any one of
them means that all 100 of them get expose events and need to be redrawn.
If each window is double-buffered, then the server can redraw them without
expose events.

I have not done alpha channel stuff in X, so was wondering what extension
is used to do that? Does it modify the expose event semantics?