[compiz] free from GLX_EXT_texture_from_pixmap

[David Reveman]

On Wed, 2006-11-15 at 01:22 +0000, Ioannis Nousias wrote: 
> You might have hear this before. I just came across it today.
> 
> from http://blog.beryl-project.org/
> 
> "I am running your svn and have noticed this non-tfp Beryl, what is this?
> 
> It is exactly what it sounds like, an early attempt (with some success I 
> might add) to make Beryl run without the need for texture_from_pixmap. 
> No more black window bug from nvidia, no more Xgl, no more AIGLX. Oh 
> yeah… and its faster than Xgl/AIGLX (nvidia beta performance looks to be 
> about the same). The code is all thanks to Onestone who is a true bling 
> hero, and the idea itself originates from the Project Looking Glass 
> people. Kudos to them and their great work, we hope to incorporate more 
> of their experience to Beryl’s code base in the form of optimizations 
> and improvements."
> 
> it seems to be still in it's infancy though. Even if it's not as 
> good/fast as the GLX extension, it's still nice to have an alternative 
> path in case the extension is not available.

I'm assuming that they CopyTexSubImage pixmap content into textures.
This is nothing new. We knew that this works when writing the
GLX_EXT_tfp spec, of course. We actually agreed that CopyTexSubImage is
the appropriate way to handle the case where you have a window that is
larger than the maximum texture size. However, that very rarely happens
so there hasn't been a lot of motivation for adding this fall-back path
to compiz yet.

Using CopyTexSubImage might give similar performance to GLX_EXT_tfp
implementations right now as they are not very optimized yet. However,
CopyTexSubImage will always require a copy operation to get the pixmap
content into a texture and as both a texture and a pixmap need to exist,
the memory consumption is twice the amount that is really necessary.
CopyTexSubImage also requires you to do glXMakeCurrent each time you
need to update a texture, which can be expensive. GLX_EXT_tfp solves all
these issues.

-David