Thanks Roland and everyone else for the input. Based on the feedback I'll include the fix in thew next round of deqp fixes.
Iago On Thu, 2015-01-15 at 15:32 +0100, Roland Scheidegger wrote: > Am 15.01.2015 um 10:05 schrieb Iago Toral: > > Hi, > > > > We have 16 deqp tests that fail, at least on i965, because of > > insufficient precision of the mod GLSL function. > > > > Mesa lowers mod(x,y) to y * fract(x,y) so there can be some precision > > lost due to fract operation. Since the result is multiplied by y the > > total precision lost usually grows together with the value of y. > Did you mean fract(x/y) here? > > > > > Below are some examples to give an idea of the magnitude of this error. > > The values on the right represent the precision error for each case: > > > > mod(-1.951171875, 1.9980468750) => 0.0000000447 > > mod(121.57, 13.29) => 0.0000023842 > > mod(3769.12, 321.99) => 0.0000762939 > > mod(3769.12, 1321.99) => 0.0001220703 > > mod(-987654.125, 123456.984375) => 0.0160663128 > > mod( 987654.125, 123456.984375) => 0.0312500000 > > > > As you see, for large enough values, the precision error becomes > > significant. > > > > This can be fixed by lowering mod(x,y) to x - y * floor(x/y) instead, > > which is the suggested implementation in the GLSL docs. I have a local > > patch in my tree that does this and it does indeed fix the problem. the > > down side is that this implementation adds and extra ADD instruction to > > the generated code (besides replacing fract with floor, which I guess > > have similar cost). > > > > Since this is a case where there is some trade-off to the fix, I wonder > > if we are interested in doing this or not. Is the precision fix worth > > the additional ADD? > > > > Well I can tell you that llvmpipe implements frc(x) as x - floor(x), so > this change looks good to me :-). > On a more serious note though, it looks to me like the cost of this > expression would be mostly dominated by the division, hence some add > more shouldn't be that bad. And if the test is legit, I don't think > there's much choice (unless you could make this optional for some old > glsl versions if they didn't require that much precision but even then > it's probably not worth bothering imho). > > Roland > > _______________________________________________ mesa-dev mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/mesa-dev
