skia2/resources/sksl/intrinsics/DFdy.sksl
John Stiles 9078a89b26 Avoid mix-up between MAIN_COORDS and FRAGCOORD.
When compiling test shaders, we were setting SK_FRAGCOORD_BUILTIN on the
`coords` parameter to main() instead of SK_MAIN_COORDS_BUILTIN. These
two built-ins don't have the same type (float2 vs. float4) and don't
mean quite the same thing.

The SPIR-V code generator saw a variable with the SK_FRAGCOORD_BUILTIN
builtin value and assumed the presence of a global variable named
`sk_FragCoord`, which didn't exist (because it was never referenced in
the code, so it was never cloned in from the sksl_frag module).

This is only a concern when compiling test shaders with skslc; real
shaders don't hit these code paths. The generated code here is still
imperfect; if you look closely, you'll see the GLSL and Metal code is
referencing the `coords` variable but it's never declared anywhere.

Change-Id: I3ad249469927ff35eb1e75d6536f95317502708f
Bug: skia:12340
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/440520
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2021-08-18 20:11:19 +00:00

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uniform half4 testInputs; // equals (-1.25, 0, 0.75, 2.25)
uniform half4 colorGreen, colorRed;
half4 main(float2 coords) {
half4 expected = half4(0); // derivative of a constant is zero
return (dFdx(testInputs.x) == expected.x &&
dFdx(testInputs.xy) == expected.xy &&
dFdx(testInputs.xyz) == expected.xyz &&
dFdx(testInputs.xyzw) == expected.xyzw &&
sign(dFdy(coords.xx)) == half2(0, 0) &&
sign(dFdy(coords.yy)) == half2(1, 1) &&
sign(dFdy(coords.xy)) == half2(0, 1)) ? colorGreen : colorRed;
}