Re: rendering-cleanup-next

[Alexander Larsson <alexl redhat com>]

On Sat, 2010-09-11 at 11:16 -0400, Havoc Pennington wrote:

> *
> 
> I'm skeptical of removing ability to set window background to None.
> This feature can be important to avoid visual artifacts on X11. The
> API should maybe change to not involve a "NULL" pixmap; conceptually,
> what this means is "window system should not auto-clear or auto-paint
> the background on exposes" - I don't know if any other platform has
> the concept. A new API could be
> gdk_window_set_paint_background(gboolean) rather than set_pixmap(NULL)
> 
> I think d3802ca "Remove calls that try to set GDK_NO_BG on their
> windows" probably results in some things being more ugly.
> (flashing/flicker)
> 
> This almost goes away as an issue if dropping all subwindows, but I
> think setting None on toplevels will still be important sometimes. I
> guess in future-composited-desktop-world this also fades in importance
> but I'm not sure on when/whether/details.

I'm personally a tiny bit uneasy about dropping bg None, as in some
cases its really required to do flicker-free stuff in X. However, with a
modern Gtk+ these situations are quite rare, and I don't think any of
these changes really cause any flicker, since:

1) In practice almost all non-toplevel GdkWindows in a typical Gtk+ app
are client-side (i.e. have no corresponding native window). This means
that mapping them does not make X clear the area to the background, and
there is no flicker.

2) Even for windows with native windows there is often no flicker, as
the X11 gdk backend aggressively set+unsets background None around
things like moves, resizes, maps and reparents in order to not render
anything to the screen until we do the expose.

-- 
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 Alexander Larsson                                            Red Hat, Inc 
       alexl redhat com            alexander larsson gmail com 
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