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[spv-in] Sampling results for depth textures need to be changed to scalar floats #4551

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expenses opened this issue May 25, 2023 · 3 comments · May be fixed by #6384
Open

[spv-in] Sampling results for depth textures need to be changed to scalar floats #4551

expenses opened this issue May 25, 2023 · 3 comments · May be fixed by #6384
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area: naga front-end lang: SPIR-V Vulkan's Shading Language naga Shader Translator type: enhancement New feature or request

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@expenses
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This is a followup to gfx-rs/naga#2341. Consider the following SPIR-V shader snippet:

<snip>
%type_2d_image = OpTypeImage %float 2D 1 0 0 1 Unknown
<snip>
         %68 = OpSampledImage %type_sampled_image %66 %67
         %69 = OpImageSampleExplicitLod %v4float %68 %65 Lod %float_0
         %70 = OpCompositeExtract %float %69 0
<snip>

OpTypeImage has a 1 after 2D to indicate that it is a depth texture. When it is sampled the sample typed is a 4-component vector as required by the spec.

This shader doesn't pass naga validation though because the wgsl spec requires that depth textures sample to scalar floats:

[2023-05-25T23:00:43Z ERROR naga::proc::typifier] Access index type Scalar { kind: Float, width: 4 }
Function [1] 'dof_downsample_with_coc' is invalid: 
        Expression [45] is invalid
        Type resolution failed
        Invalid access into expression [44], indexed: true
Generating wgsl output requires validation to succeed, and it failed in a previous step

If I manually mess with the the SPIR-V assembly to change the result type of the sample to a scalar float then naga processes it fine (note that this fails SPIR-V validation):

<snip>
%type_2d_image = OpTypeImage %float 2D 1 0 0 1 Unknown
<snip>
         %68 = OpSampledImage %type_sampled_image %66 %67
         %70 = OpImageSampleExplicitLod %float %68 %65 Lod %float_0
<snip>

What we need to do in the above case is, upon sampling a depth texture, we need to change the sample type to be a scalar float then insert a composite instruction that creates a 4-component vector with each field set to the scalar sample result (to not break later instructions).
@teoxoy teoxoy added lang: SPIR-V Vulkan's Shading Language area: naga front-end type: enhancement New feature or request labels May 31, 2023
@cwfitzgerald cwfitzgerald transferred this issue from gfx-rs/naga Oct 25, 2023
@cwfitzgerald cwfitzgerald added the naga Shader Translator label Oct 25, 2023
@schell
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schell commented Oct 7, 2024 •
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I just got bit by this one while attempting to use OpImageFetch to read a depth texture. @expenses is this still on your radar? I might try to tackle it as it's preventing me from finishing occlusion culling. I imagine @LPGhatguy would be interested in getting this one done, as well.

schell added a commit to schell/wgpu that referenced this issue Oct 8, 2024
@schell
spv-in sampling from depth textures returns a scalar, and is splatted…
fa26449 
… to a vector for SPIR-V compatibility, fixes gfx-rs#4551
@schell schell linked a pull request Oct 8, 2024 that will close this issue
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@schell
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schell commented Oct 8, 2024

In the case of OpImageSampleDref[Explicit | Implicit]Lod the result type is a scalar, we have to account for that.

@schell
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schell commented Oct 8, 2024

Actually it's any of the OpImage variants that use depth references.

schell added a commit to schell/wgpu that referenced this issue Oct 8, 2024
@schell
spv-in sampling from depth textures returns a scalar, and is splatted…
dfc3783 
… to a vector for SPIR-V compatibility, fixes gfx-rs#4551
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area: naga front-end lang: SPIR-V Vulkan's Shading Language naga Shader Translator type: enhancement New feature or request
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[spv-in] sampling from depth textures returns a scalar and is splatted schell/wgpu
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@schell @cwfitzgerald @expenses @teoxoy