Compiz by default ?

[Michael Vogt]

Dear Friends,


one of the goals for this cycle was to enable compiz by default for
all systems where this is feasible. We added a detection from Tribe-2
on to see if the system is able to run compiz and turn it on then.
This detection works well and we are happy with the results.

We still need to discuss if we should enable compiz by default for the
final release or not. This discussion was started yesterday by
Matthew Garrett at the techboard meeting and it was decided that this
needs to be discussed in a wider forum. 

The remaining issues with enabling compiz by default are that there
are the following regressions compared to a non-composited system:

Video playback - basic Xv playback should work on all video drivers we
support [1] but there is no composited video playback. That means, that
all effects will just give a black window for the video player. Moving
the window around results in a black window too until the window is
settled, then the playing comes back.

Windowed 3d applications - currently the 3d hardware renders directly
into the framebuffer. This means that no transformation effects are
possible [2], but it also means that no damage events are generated
when a window is moved around. This results in artifacts on the screen
where the 3d application was. You can try this on a compiz system by
runing glxgrears (intel, ati) and move it around on the screen or move
another window over it. Work is being done in this area, but it will
not be ready for gutsy.

Performance regression on windowed 3d applications - some video
drivers drop significantly in 3d performance when compiz is
running. For fullscreen apps this can be worked around with a compiz
setting, but not for windowed 3d applications.

General performance regressions - people with older hardware will see
a slow system when compiz is enabled because of the additional
requirements it imposes on the hardware (especially on systems with
CPU < 1Ghz, older GPUs).


The above regressions make composite-by-default something we really
need to discuss. The question is if we should tradeoff the usability
improvments and blink improvements for the above regressions. 

I think the video playback will just look like a cosmetic issue
(unprofessional at worst), but if users run into the windowed 3d
rendering artifacts that will look like we ship something that is
broken.

What we need to do in any case is to raise awareness to the users
about this feature. If we decide to enable it by default we should
popup something that explains that certain things may not work quite
well. If we decide that we should not enable it, we should have a
check that tests if it is likely that the system is able to run compiz
and if so, popup a notification that tell them what to do to enable
desktop-effects. The effects tab in the apperance capplet should also
get some additional text that explains that desktop effects may cause
issues with windowed 3d applications. 


Thanks, 
 Michael


[1] see https://wiki.ubuntu.com/CompizTeam "Status of Xv video"
    section
[2] http://hoegsberg.blogspot.com/2007/08/redirected-direct-rendering.html