compiz - Mar 2007 - [PATCH] Rendering: swap buffers and copy only needed parts

Hi,

I've been experiencing serious tearing in compiz when screen is painted only
partly. Either "Sync To Vblank" option in compiz doesn't work or
copying
repainted parts from backbuffer to frontbuffer is too slow. And since many 
people rely on driver's implementation of Sync to Vblank, it would be
optimal
to always use glXSwapBuffers. This can be accomplished via keeping up-to-date 
copy of frontbuffer in backbuffer and painting updates over it.

My patch (swap-buffers-and-copy-only-needed.patch) keeps copy of last painted 
region and in case of partial screen repaint uses it to determine which parts 
should be copied from frontbuffer to backbuffer. Since region going to be 
painted this time is known, it can be substracted from last painted region. 
Thus only parts that were painted in previous pass and aren't going to be 
repainted, are copied saving expensive pixel copy operations.

Since glxCopySubBufferMESA does only implement copy-from-backbuffer (correct 
me if I'm wrong) it is no longer needed (remove-copySubBuffer.patch).

I've been using this a quite long time and it works generally well. Problem 
arises only when a plugin paints outside of the given region. For example 
screenshot (quick-n-dirty patch provided) and scale (shadows don't belong to
damaged area, causing slowly darkening and finally solid black border around 
scaled windows) do that.

Pyry
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On Sat, 2007-03-17 at 08:20 +0200, Pyry Haulos wrote:
> Hi,
> 
> I've been experiencing serious tearing in compiz when screen is painted
only
> partly. Either "Sync To Vblank" option in compiz doesn't work
or copying
> repainted parts from backbuffer to frontbuffer is too slow. And since many 
> people rely on driver's implementation of Sync to Vblank, it would be
optimal
> to always use glXSwapBuffers. This can be accomplished via keeping
up-to-date
> copy of frontbuffer in backbuffer and painting updates over it.
"Sync To Vblank" works perfectly for me last time I tried it. Your
driver need to support it. Direct rendering is required and nvidia's
driver is the only way to get this right now.

Relying on the driver to force "Sync To Vblank" when glXSwapBuffers is
called is usually a very bad idea as if used with indirect rendering it
most likely causes the X server to block when swapping buffers and this
has horrible performance affects on the X server.
> 
> My patch (swap-buffers-and-copy-only-needed.patch) keeps copy of last
painted
> region and in case of partial screen repaint uses it to determine which
parts
> should be copied from frontbuffer to backbuffer. Since region going to be 
> painted this time is known, it can be substracted from last painted region.
> Thus only parts that were painted in previous pass and aren't going to
be
> repainted, are copied saving expensive pixel copy operations.
> 
> Since glxCopySubBufferMESA does only implement copy-from-backbuffer
(correct
> me if I'm wrong) it is no longer needed (remove-copySubBuffer.patch).
> 
> I've been using this a quite long time and it works generally well.
Problem
> arises only when a plugin paints outside of the given region. For example 
> screenshot (quick-n-dirty patch provided) and scale (shadows don't
belong to
> damaged area, causing slowly darkening and finally solid black border
around
> scaled windows) do that.
There's a few cases where this is very hard to prevent. The current
design relies on it being allowed to draw things outside the screen
region we're updating. It's especially important for the blur plugin.

- David