Alan Cox alan at
Wed Apr 8 14:15:44 PDT 2009

> > if someone massively resizes a window with backing store (remembering it
> > can be mostly offscreen) your X server explodes.
> Hence the pleasure that is composite.

Compositing only has to deal with the visible resources. The design very
carefully keeps the rendering determined by the size of the bounding
heirarchy managed by the compositor. Different model and as a result a
rather more effective and controlled use of resources.

Note that your compositor doesn't have to do fancy 3D effects, its
pefectly valid to have a local compositor & wm using a compositor solely
to mask network latency and implement utterly boringly conventional 2D
window behaviour. What you can't do with it is backing store, because the
semantics are different and have to be to avoid the backing store
unbounded resource problem.

SaveUnder is a good deal more elegant, although horribly nasty stuff to
get right as you have to redirect drawing into a different offscreen
pixmap. Once you are trying to do anything interesting and modern the
delights of mixing direct rendering and saveunders are perhaps best left
to the horror movies.

Again compositing handles this cleanly and elegantly, and you can use
compositing solely for simple 2D resource buffering and management far
more effectively without breaking everything else interesting.

If you want to appreciate the joy of saveunder just scan bug databases of
all the vendors of X servers for saveunder bugs...


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