Re: [PATCH v2 0/4] MR602: vkd3d-shader/dxil: Inverse trigonometry

[Henri Verbeet (＠hverbeet)]

> +% Because sqrt isn't identical across platforms, there is some inaccuracy
> +% here even with an identical algorithm, and because it's so near zero,
> +% each ulp is really small. So, in order to pass there needs to be this
> +% enormous margin.
> +uniform 0 float4 0.0 0.0 0.0 0.0
> +todo draw quad
> +probe all rgba (0.0000675916672, 0.0, 0.0, 0.0) 131072

Which sqrt is that? Yeah, I know, the shader model 4 lowering for the asin() in the shader for this test involves a sqrt(). But that's a fairly obscure implementation detail, and doesn't seem obvious to someone reading the test.

The maximum difference here doesn't seem that excessive, but for what it's worth, an alternative would be to add a quantisation step to the shader; we use that approach in e.g. ddxddy.shader_test.