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authorPaul Berry <[email protected]>2013-09-20 09:04:31 -0700
committerPaul Berry <[email protected]>2013-10-03 13:49:15 -0700
commit800610f9eb6ad24b5fefc9206fb700c7ae2f0ec8 (patch)
tree6e62eed31079f763378240c0cd6c7863686e8e3e /docs/relnotes/6.5.3.html
parent9267565ee4248f7bc8efebd8c994a93ff1e0683d (diff)
i965/fs: Improve accuracy of dFdy() to match dFdx().
Previously, we computed dFdy() using the following instruction: add(8) dst<1>F src<4,4,0)F -src.2<4,4,0>F { align1 1Q } That had the disadvantage that it computed the same value for all 4 pixels of a 2x2 subspan, which meant that it was less accurate than dFdx(). This patch changes it to the following instruction when c->key.high_quality_derivatives is set: add(8) dst<1>F src<4,4,1>.xyxyF -src<4,4,1>.zwzwF { align16 1Q } This gives it comparable accuracy to dFdx(). Unfortunately, align16 instructions can't be compressed, so in SIMD16 shaders, instead of emitting this instruction: add(16) dst<1>F src<4,4,1>.xyxyF -src<4,4,1>.zwzwF { align16 1H } We need to unroll to two instructions: add(8) dst<1>F src<4,4,1>.xyxyF -src<4,4,1>.zwzwF { align16 1Q } add(8) (dst+1)<1>F (src+1)<4,4,1>.xyxyF -(src+1)<4,4,1>.zwzwF { align16 2Q } Fixes piglit test spec/glsl-1.10/execution/fs-dfdy-accuracy. Acked-by: Chris Forbes <[email protected]> Reviewed-by: Eric Anholt <[email protected]>
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