On Thursday, September 8, 2016 10:33:06 PM PDT Francisco Jerez wrote: [snip] > Heh, right, my concern was that this smells strongly like a test relying > on not terribly well-defined behavior... AFAICT the problem addressed > here is ultimately caused by the discontinuity that the COLORBURN > blending equation has at the point Cd = 1, Cs = 0, and the test authors > had the awesome idea [not necessarily being sarcastic here ;)] of > testing the blending function at precisely that point, even though the > function is guaranteed to be numerically unstable and vary wildly given > the slightest rounding error. > > Does the extension impose any requirements on the precision of the > division by alpha operation done on pre-multiplied color components? > The test case may be valid assuming that IEEE precision rules apply, but > AFAIK GLSL has considerably looser requirements on the division > operation, and the KHR_blend_equation_advanced lowering code is > implemented in terms of GLSL division so the result could potentially be > farther off than 1 - epsilon (though AFAICT this change would be correct > assuming the result of GLSL division is guaranteed to be within ~1.5 ULP > of the exact value, which I don't think is the case).
FWIW, I finally filed a spec bug about this: https://cvs.khronos.org/bugzilla/show_bug.cgi?id=16042 We'll see what people think.
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