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

[Iago Toral Quiroga]

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

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