[cairo] [PATCH 2/2] gl: Remove GL fixed-function matrix usage.

[Eric Anholt]

On Tue, 1 Feb 2011 11:38:15 +0200, Alexandros Frantzis <alexandros.frantzis at linaro.org> wrote:
> On Mon, Jan 31, 2011 at 08:20:00PM -0800, Eric Anholt wrote:
> > No significant performance difference (though a simpler diff showed a
> > 2% hit)
> > +    double dest_x1 = x1, dest_y1 = y1, dest_x2 = x2, dest_y2 = y2;
> > +
> > +    cairo_matrix_transform_point (&ctx->dest_matrix, &dest_x1, &dest_y1);
> > +    cairo_matrix_transform_point (&ctx->dest_matrix, &dest_x2, &dest_y2);
> > +
> 
> Hi Eric,
> 
> my alternative take on this can be found in the WIP branch
> 'gl-replace-builtin-shader-variables':
> 
> http://git.linaro.org/gitweb?p=people/afrantzis/cairo.git;a=shortlog;h=refs/heads/gl-replace-builtin-shader-variables
> 
> I am using a ModelViewProjectionMatrix custom uniform to pass the
> transformation to the shader.
> 
> I am tempted to say that my approach may be more efficient, as the per
> vertex tranformation is happening in the GPU instead of the CPU.
> However, I haven't benchmarked this yet, so I 'll wait until I have more
> information before I make any bold statements.

Uploading constants is relatively expensive (a compare to see if we need
to upload, an extra WC copy to the constant buffer which is "slow", then
the constant change actually getting pipelined through the hw), and we
change source and destination a lot.  This also cuts the VS from 5
instructions to 3 on 965, and it should be 2 if we did better VS
optimization.  What I'm benchmarking is CPU bound, so I think I captured
that, and for GPU-bound workloads this patch would be better than doing
the math in the VS.

I was actually surprised I didn't find this patch to be a win -- it
looks like it mostly to had to do with the double precision math being
expensive.  Given that we're operating on limited-size surfaces, I think
we could justify storing floating point matrices here and doing the math
in floats.
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