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Stencil reads are S,X,X,X. (but expect S,S,S,S or S,0,0,1) #1978

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Merged
kdashg merged 1 commit into from
Aug 4, 2021
Merged

Stencil reads are S,X,X,X. (but expect S,S,S,S or S,0,0,1) #1978

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21 changes: 18 additions & 3 deletions spec/index.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -2810,7 +2810,7 @@ or a 32-bit IEEE 754 floating point value ("depth32float").

Issue: add something on GPUAdapter(?) that gives an estimate of the bytes per texel of "stencil8"

The {{GPUTextureFormat/stencil8}}) format may be implemented as
The {{GPUTextureFormat/stencil8}} format may be implemented as
either a real "stencil8", or "depth24stencil8", where the depth aspect is
hidden and inaccessible.

Expand Down Expand Up @@ -4019,7 +4019,7 @@ any specific point in the code at all.
: <dfn>message</dfn>
::
A human-readable string containing the message generated during the shader compilation.

: <dfn>type</dfn>
::
The severity level of the message.
Expand All @@ -4029,7 +4029,7 @@ any specific point in the code at all.
The line number in the shader {{GPUShaderModuleDescriptor/code}} the
{{GPUCompilationMessage/message}} corresponds to. Value is one-based, such that a lineNum of
`1` indicates the first line of the shader {{GPUShaderModuleDescriptor/code}}.

If the {{GPUCompilationMessage/message}} corresponds to a substring this points to
the line on which the substring begins. Must be `0` if the {{GPUCompilationMessage/message}}
does not correspond to any specific point in the shader {{GPUShaderModuleDescriptor/code}}.
Expand Down Expand Up @@ -9524,6 +9524,7 @@ None of the depth formats can be filtered.
<th>Bytes per texel
<th>Aspect
<th>{{GPUTextureSampleType}}
<th>Returned in shaders as...
<th>Copy aspect from Buffer
<th>Copy aspect into Buffer
</thead>
Expand All @@ -9532,38 +9533,52 @@ None of the depth formats can be filtered.
<td>1 &minus; 4
<td>stencil
<td>{{GPUTextureSampleType/"uint"}}
<td>`vec4<u32>(S, X, X, X)`
<td colspan=2>&checkmark;
<tr>
<td>{{GPUTextureFormat/depth16unorm}}
<td>2
<td>depth
<td>{{GPUTextureSampleType/"depth"}}
<td>`f32(D)`
<td colspan=2>&checkmark;
<tr>
<td>{{GPUTextureFormat/depth24plus}}
<td>4
<td>depth
<td>{{GPUTextureSampleType/"depth"}}
<td>`f32(D)`
<td colspan=2>&cross;
<tr>
<td rowspan=2 style='white-space:nowrap'>{{GPUTextureFormat/depth24plus-stencil8}}
<td rowspan=2>4 &minus; 8
<td>depth
<td>{{GPUTextureSampleType/"depth"}}
<td>`f32(D)`
<td colspan=2>&cross;
<tr>
<td>stencil
<td>{{GPUTextureSampleType/"uint"}}
<td>`vec4<u32>(S, X, X, X)`
<td colspan=2>&checkmark;
<tr>
<td>{{GPUTextureFormat/depth32float}}
<td>4
<td>depth
<td>{{GPUTextureSampleType/"depth"}}
<td>`f32(D)`
<td colspan=1>&cross;
<td colspan=1>&checkmark;
</table>

Stencil formats must sample as `vec4<u32>(S, X, X, X)`, where S is the stencil value and each X is an implementation-defined unspecified value.
Authors must not rely on these `.y`, `.z`, and `.w` components, as their behavior is non-portable.

Note:
Short of adding a new more constrained stencil sampler type (like depth), it's infeasible for implementations to efficiently paper over the driver differences for stencil reads.
As this was not a portability pain point for WebGL, it's not expected to be problematic in WebGPU.
In practice, expect either `vec4<u32>(S, S, S, S)` or `vec4<u32>(S, 0, 0, 1)`, depending on hardware.

<dfn dfn>Copies of depth and stencil textures</dfn> can only happen within the following sets of formats:
- {{GPUTextureFormat/stencil8}}, {{GPUTextureFormat/depth24plus-stencil8}} (stencil component), {{GPUTextureFormat/r8uint}}
- {{GPUTextureFormat/depth16unorm}}, {{GPUTextureFormat/r16uint}}
Expand Down