Not only is our point transformation code is quite slow (well at least
compared to a real GPU), but by deriving the texture coordinates from
the vertex position we can elide the multiple arrays that we need to
construct and pass to GL - improving performance by eliminating CPU
overhead from needless transforms and data shovelling.
However, not all vertex emission is suitable. For instance, for glyphs
we need to emit discontiguous texture coordinates for each glyph, but
span generation is suitable - which fortuitously also has the largest
vertex density and so benefits the most.
The only real concern is for hardware without true vertex shader support
(e.g. i915) but there we are already invoking the VS to transform the
vertex into the viewport. We would need to eliminate that transform as
well as manually compute the texture coordinates in order to eliminate
the vertex recomputation pass.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Not 100% improvement, there are still a variety of failures with
GLXWindows, but getting there. At least it fixes more things than its
breaks...
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
