Some ideas how XComposite could be improved

Keith Packard keithp at
Mon Nov 19 21:19:00 PST 2007

On Tue, 2007-11-20 at 06:01 +0100, Dennis Kasprzyk wrote:
> Hi,
> I thought a while about the current XComposite functionality and found some 
> points that could be improved.
> ARGB windows currently have to be drawn with premultiplied alpha. This leads 
> to dark (black) areas of the window, if no composite manager is used. Most of 
> the ARGB windows would look better in a non composited environment if non 
> premultiplied alpha could be used. The composite extension could provide 2 
> argb visuals (one for premultiplied alpha and one for non premultiplied 
> alpha) to inform the composite manager how to draw the window correctly. An 
> application developer could then choose the best visual for his application.

The Composite extension doesn't actually enforce any interpretation of
the extra bits in the pixel. It's only a convention that we use
premultiplied alpha. Premultiplied alpha is also a lot easier to compute
as it doesn't suffer from divide-by-zero issues with transparent pixels.

> Another problem is the XShape extension. XShape currently also clippes drawing 
> operations to the redirected pixmap. If this could be disabled, an ARGB 
> application could use XShape to have a shape when no composite manager is 
> running, but also inform the composite manager (with a window hint) to ignore 
> the XShape informations to paint the whole window pixmap. With something like 
> this an application could be correctly shaped when no composite manager is 
> running, but provide nice antialiased edges (and maybe shadows) if a 
> composite manager is running.

We've already started seeing conventions that indicate when a
compositing manager is running; having applications not shape their
windows in this case would be more efficient than having the computed
shape be ignored.

> All current ideas to implement window previews in a taskbar rely on some kind 
> of communication of the taskbar with the composite manager. My idea is to 
> provide a more flexible system directly in the composite extension that could 
> also work without a running composite manager. 

Automatic redirection does precisely this already.

keith.packard at
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