[Piglit] [PATCH] tex-miplevel-selection: Fix textureProj failures due to precision errors

[Ilia Mirkin]

On Wed, Mar 11, 2015 at 3:56 AM, Iago Toral Quiroga <itoral at igalia.com> wrote:
> The textureProj tests multiply expected texture coordinates by the projector
> in advance so that when the driver does the division we obtain the same
> coordinates. However, the division can lead to small rounding errors that
> can affect the selected layer and fail the tests. This is currently happening
> on Intel hardware for all projector tests involving 3D textures.
>
> When we test a 3D texture for texture level 0 we have 32 layers, which
> means that each layer takes 1/32 = 0.03125 space in the [0, 1] texture
> coordinate space. The test uses 0.5 for the Z coordinate, which is exactly
> the boundary between layers 15 and 16 (16 * 0.03125 = 0.5). Because we
> first multiply 0.5 by the projector in CPU and then we divide the coordinate
> by the driver in the GPU, the result may be subject to rounding/precision
> errors and if the result of this operation is even slighly smaller than 0.5
> the hardware will select layer 15 instead of layer 16, leading to the
> test failures we currently see, at least on Intel hardware, for all piglit
> tests that involve textureProj with 3D textures.
>
> The patch prevents this rounding from affecting the result of the test by
> using 0.51 as the target texture coordinates instead of 0.5. Because 0.51
> is 0.01 into layer 16, we are giving a small room for rounding/precision
> errors that won't lead the hardware to select a different layer.
>
> This fixes all projector tests on Intel hardware:
> bin/tex-miplevel-selection *ProjGradARB 3D -fbo -auto
> bin/tex-miplevel-selection *ProjLod 3D -fbo -auto
> bin/tex-miplevel-selection textureProj 3D -fbo -auto
> bin/tex-miplevel-selection textureProjGrad 3D -fbo -auto
> bin/tex-miplevel-selection textureProjGradOffset 3D -fbo -auto
> bin/tex-miplevel-selection textureProjOffset 3D -fbo -auto
> bin/tex-miplevel-selection "textureProj(bias)" 3D -fbo -auto
> bin/tex-miplevel-selection "textureProjOffset(bias)" 3D -fbo -auto

Neat! This also fixes the (apparently lower-precision) "p" modifier on
the sample instruction on a3xx -- previously we just assumed it
couldn't be used with 3D textures, and worked around it by doing the
projection by hand (in 32-bit fp land). I guess sam.p does it in fp16
land?

>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81405
> ---
>  tests/texturing/tex-miplevel-selection.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/tests/texturing/tex-miplevel-selection.c b/tests/texturing/tex-miplevel-selection.c
> index 54592f9..959bab2 100644
> --- a/tests/texturing/tex-miplevel-selection.c
> +++ b/tests/texturing/tex-miplevel-selection.c
> @@ -1252,10 +1252,10 @@ draw_quad(int x, int y, int w, int h, int expected_level, int fetch_level,
>                 SET_VEC(c3, s0, t1, TEST_LAYER, z);
>                 break;
>         case TEX_3D:
> -               SET_VEC(c0, s0*p, t0*p, 0.5*p, p);
> -               SET_VEC(c1, s1*p, t0*p, 0.5*p, p);
> -               SET_VEC(c2, s1*p, t1*p, 0.5*p, p);
> -               SET_VEC(c3, s0*p, t1*p, 0.5*p, p);
> +               SET_VEC(c0, s0*p, t0*p, 0.51*p, p);
> +               SET_VEC(c1, s1*p, t0*p, 0.51*p, p);
> +               SET_VEC(c2, s1*p, t1*p, 0.51*p, p);
> +               SET_VEC(c3, s0*p, t1*p, 0.51*p, p);
>                 break;
>         case TEX_1D_ARRAY:
>                 SET_VEC(c0, s0, TEST_LAYER, 0, 1);
> --
> 1.9.1
>
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