Iago Toral <ito...@igalia.com> writes:
> On Thu, 2016-03-03 at 01:30 -0800, Kenneth Graunke wrote:
>> From: Francisco Jerez <curroje...@riseup.net>
>>
>> The adjusted polynomial coefficients come from the numerical
>> minimization of the L2 norm of the relative error. The old
>> coefficients would give a maximum relative error of about 15000 ULP in
>> the neighborhood around acos(x) = 0, the new ones give a relative
>> error bounded by less than 2000 ULP in the same neighborhood.
>>
>> Fixes four dEQP subtests:
>> dEQP-GLES31.functional.shaders.builtin_functions.precision.acos.
>> highp_compute.{scalar,vec2,vec3,vec4}
>
> Cool! I thought we would have to use higher order polynomials. Series
> is:
>
> Reviewed-by: Iago Toral Quiroga <ito...@igalia.com>
>
> What about asin? It looks like the new coefficients should be useful
> there as well.
>
That's likely to be more harmful than helpful, the new polynomial
coefficients are optimized to minimize the relative error of acos
*only*, which has a different magnitude profile from asin -- The
relative error of acos(x) used to explode around x=1, while the relative
error of asin(x) is likely to be highest around x=0, because that's
where its magnitude is lowest. If you switch asin over to the acos
coefficients its overall relative error will likely increase, because it
will improve its accuracy around x=1 where it barely matters and worsen
it around x=0 where it's the most sensitive. Feel free to put my R-b on
the series:Reviewed-by: Francisco Jerez <curroje...@riseup.net> >> --- >> src/compiler/glsl/builtin_functions.cpp | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> Curro originally wrote this code for the SPIR-V front-end when working >> on Vulkan conformance. I copy and pasted it over to GLSL, but all of >> the actual work is Curro's. >> >> diff --git a/src/compiler/glsl/builtin_functions.cpp >> b/src/compiler/glsl/builtin_functions.cpp >> index 0d289c1..10fb21b 100644 >> --- a/src/compiler/glsl/builtin_functions.cpp >> +++ b/src/compiler/glsl/builtin_functions.cpp >> @@ -3262,7 +3262,7 @@ builtin_builder::_acos(const glsl_type *type) >> ir_variable *x = in_var(type, "x"); >> MAKE_SIG(type, always_available, 1, x); >> >> - body.emit(ret(sub(imm(M_PI_2f), asin_expr(x, 0.086566724f, >> -0.03102955f)))); >> + body.emit(ret(sub(imm(M_PI_2f), asin_expr(x, 0.08132463f, >> -0.02363318f)))); >> >> return sig; >> } > > > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
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