Xgl bottom top ?

[Jerome Glisse]

I may have miss something in the ogl specs :) But i truly don't see why 
bottom-top texture are faster than
top-bottom. Anyone could give me a pointer on that ?

 >>Why do you need bottom-to-top raster order?  I assume it has 
something to do with OpenGL's conventions.
 >
 >I'm glad you asked. Normally when using OpenGL you would just load all
 >texture data as it's stored by application and if this happens to be in
 >top-to-bottom raster order you would just take care of that when
 >assigning texture coordinates or when setting the texture matrix.
 >
 >With glitz, in general, and especially with Xgl, a lot of copying from
 >framebuffer/pbuffers to textures is done.

Moreover is there anywhere a pointer  for doc on the architecture of xgl 
or must i find this out
from the source ? I don't really understand why clip list or 
framebuffer,texture reading are needed.
Can't we draw the window using polygon, transparency using alpha 
transparency, and use z buffer
for window clipping ? Or do the X protocol absolutly need to access pixel ?

best

Jerome Glisse