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

[Ilia Mirkin]

On Wed, Mar 11, 2015 at 9:59 AM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> 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?

Although the question is... is this loss of precision OK? Probably is
for lowp in gles, but what about highp or desktop gl?

Iago, what happens if you do the projection "by hand" in a lowering
pass, do those tests start passing then? Does the spec have anything
clever to say on the matter?

  -ilia