GLX_EXT_include_inferiors (was Re: compiz on aiglx)

[Kristian Høgsberg]

Ian Romanick wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> Kristian Høgsberg wrote:
> 
>> Overview
>>
>>     This extension introduces a new FBConfig attribute,
>>     GLX_INCLUDE_INFERIORS_EXT.  Creating a context with an FBConfig that has
>>     GLX_INCLUDE_INFERIORS_EXT set will prevent inferiors (child windows) from
>>     clipping output to the drawable currently associated with the context.
> 
> Would it be possible to elaborate on this a bit?  I'm not really sure
> what this is even intended to do or why anyone would want to use it.
> This isn't to say that I don't think it's useful, more to say that I
> don't get it.
> 
> If I have window A and a child window B, what happens if I draw to
> window A?  Does this just provide a guarantee that if B obscures pixel
> x,y of A that I can later read back (e.g., using glReadPixels) x,y and
> get the result that I drew?  Does this mean that result of drawing x,y
> of A gets "broadcast" somehow to B?

The wording and the basic idea is taken from the equivalent X GC 
subwindow_mode value.  The default behaviour (ClipByChildren) for X 
rendering is that output is clipped to the child windows of the output 
window.  So if you're drawing to window A and the primitive you're 
drawing overlaps with the child window B, the overlapping part will not 
be drawn and the contents of window B remains untouched.

If you're instead using a GC that has the subwindow_mode value set to 
IncludeInferiors, drawing to the parent window will not get clipped to 
the child windows, in effect rendering over the child windows as if they 
weren't there.  There is no way you can get this with behavior with GL, 
it always operates in ClipByChildren mode.

glReadPixels will return whatever pixels are in the window, wether they 
belong to subwindows or not and wether they were drawn with 
IncludeInferiors or not.  I think.

The motivation for the GLX_EXT_include_inferiors extension is that when 
rendering to the root window, even if the child windows (i.e. the 
top-level windows) have been redirected, they still clip output to the 
root window.  For a compositing manager to be able to use the root 
window to draw the desktop, we need a way to specify the equivalent of 
IncludeInferiors for GL rendering.  GLX_EXT_include_inferiors provides 
this mechanism.

The first draft has the IncludeInferiors as a context attribute, because 
  it a X GC attribute, and I just copied that at first.  But as David 
mentions this is probably better implemented as a GlxWindow attribute. 
Which we don't have atm in Xorg glx, but that's one part of glx 1.3 
that's pretty easy to add support for.

Kristian