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Add and name Features for all Compat Mode limitations. #5018

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Add and name Features for all Compat Mode limitations. #5018

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40 changes: 39 additions & 1 deletion proposals/compatibility-mode.md
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Expand Up @@ -60,6 +60,8 @@ if desc.dimension is "3d":
set textureBindingViewDimension to "3d"
```

Feature: `flexible-texture-views`.

**Justification**: OpenGL ES 3.1 does not support texture views.

### 2. Color blending state may not differ between color attachments in a `GPUFragmentState`.
Expand All @@ -68,33 +70,47 @@ Each `GPUColorTargetState` in a `GPUFragmentState` must have the same `blend.alp

**Justification**: OpenGL ES 3.1 does not support indexed draw buffer state.

Feature: `per-color-target-fragment-state`. Supported via `GL_EXT_draw_buffers_indexed`, `GL_OES_draw_buffers_indexed`, or OpenGL ES 3.2.

### 3. Disallow `CommandEncoder.copyTextureToBuffer()` and `CommandEncoder.copyTextureToTexture()` for compressed texture formats
`CommandEncoder.copyTextureToBuffer()` and `CommandEncoder.copyTextureToTexture()` of a compressed texture are disallowed, and will result in a validation error.

**Justification**: Compressed texture formats are non-renderable in OpenGL ES, and `glReadPixels()` requires a framebuffer-complete FBO, preventing `copyTextureToBuffer()`. Additionally, because ES 3.1 does not support `glCopyImageSubData()`, texture-to-texture copies must be worked around with `glBlitFramebuffer()`. Since compressed textures are not renderable, they cannot use the `glBlitFramebuffer()` workaround, preventing implementation of `copyTextureToTexture()`.

Feature: `copy-compressed-texture-to-buffer`.

Feature: `copy-compressed-texture-to-texture`. Supported via `GL_EXT_copy_image` or OpenGL ES 3.2.

### 4. Disallow `GPUTextureViewDimension` `"CubeArray"` via validation

**Justification**: OpenGL ES does not support Cube Array textures.
**Justification**: OpenGL ES 3.1 does not support Cube Array textures.

Feature: `cube-map-array`. Supported via `GL_EXT_texture_cube_map_array` or OpenGL ES 3.2.

### 5. Views of the same texture used in a single draw may not differ in aspect or mip levels.

A draw call may not bind two views of the same texture differing in `aspect`, `baseMipLevel`, or `mipLevelCount`. Only a single aspect and mip level range per texture is supported. This is enforced via validation at draw time.

**Justification**: OpenGL ES does not support texture views, but one set of these parameters per texture is supported via glTexParameteri(). In particular, one depth/stencil aspect may be specified via `GL_DEPTH_STENCIL_TEXTURE_MODE`, and one mip level subset via the `GL_TEXTURE_BASE_LEVEL` and `GL_TEXTURE_MAX_LEVEL` parameters.

Feature: `flexible-texture-views`.

### 6. Array texture views used in bind groups must consist of the entire array. That is, `baseArrayLayer` must be zero, and `arrayLayerCount` must be equal to the size of the texture array.

A bind group may not reference a subset of array layers. Only views of the entire array are supported for sampling or storage bindings. This is enforced via validation at bind group creation time.

**Justification**: OpenGL ES does not support texture views.

Feature: `flexible-texture-views`.

### 7. Disallow `sample_mask` and `sample_index` builtins in WGSL.

Use of the `sample_mask` or `sample_index` builtins would cause a validation error at shader module creation time.

**Justification**: OpenGL ES 3.1 does not support `gl_SampleMask`, `gl_SampleMaskIn`, or `gl_SampleID`.

Feature: `sample-variables`. Supported via `GL_OES_sample_variables` or OpenGL ES 3.2.

### 8. Disallow two-component (RG) texture formats in storage texture bindings.

The `rg32uint`, `rg32sint`, and `rg32float` texture formats no longer support the `"write-only" or "read-only" STORAGE_BINDING` capability by default.
Expand All @@ -103,12 +119,16 @@ Calls to `createTexture()` or `createBindGroupLayout()` with this combination ca

**Justification**: GLSL ES 3.1 (section 4.4.7, "Format Layout Qualifiers") does not permit any two-component (RG) texture formats in a format layout qualifier.

Feature: `rg32-storage-texture`.

### 9. Depth bias clamp must be zero.

During `createRenderPipeline()` and `createRenderPipelineAsync()`, `GPUDepthStencilState.depthBiasClamp` must be zero, or a validation error occurs.

**Justification**: GLSL ES 3.1 does not support `glPolygonOffsetClamp()`.

Feature: `depth-bias-clamp`. Supported via `GL_EXT_polygon_offset_clamp`.

### 10. Lower limits.

The differences in limits between compatibility mode and standard WebGPU
Expand Down Expand Up @@ -152,18 +172,26 @@ If either are used in the code passed to `createShaderModule` a validation error

**Justification**: OpenGL ES 3.1 does not support `linear` interpolation nor `sample` sampling.

Feature: `interpolation-type-linear`. Supported via `GL_NV_shader_noperspective_interpolation` and desktop GLSL.

Feature: `interpolation-mode-sample`. supported via `GL_OES_shader_multisample_interpolation` or OpenGL ES 3.2.

### 12. Disallow copying multisample textures.

`copyTextureToTexture` will generate a validation error if the sampleCount of the textures is not 1.

**Justification**: OpenGL ES 3.1 does not support copying of multisample textures.

Feature: `copy-multisampled-texture-to-texture`. Supported in OpenGL ES 3.2.

### 13. Disallow texture format reinterpretation

When calling `createTexture`, the `viewFormats`, if specified, must be the same format as the texture.

**Justification**: OpenGL ES 3.1 does not support texture format reinterpretation.

Feature: `texture-format-reinterpretation`.

### 14. Require `depthOrArrayLayers` to be compatible with `textureBindingViewDimension` in `createTexture`.

When creating a texture you can pass in a `textureBindingViewDimension`.
Expand All @@ -175,10 +203,14 @@ When creating a texture you can pass in a `textureBindingViewDimension`.
**Justification**: OpenGL ES 3.1 cannot create 2d textures with more than 1 layer nor can it
create cube maps that are not exactly 6 layers.

Feature: `flexible-texture-views`.

## 15. Disallow bgra8unorm-srgb textures

**Justification**: OpenGL ES 3.1 does not support bgra8unorm-srgb textures.

Feature: `bgra8unorm-srgb`.

## 16. Disallow `textureLoad` with `texture_depth?` textures

If a `texture_depth`, `texture_depth_2d_array`, or `texture_depth_cube` are used in a `textureLoad` call
Expand All @@ -188,6 +220,8 @@ in code passed to `createShaderModule` a validation error is generated.

Note: this does not affect textures made with depth formats bound to `texture_2d<f32>`.

Feature: `depth-texture-load`.

## 17. Disallow `@interpolation(flat)` and `@interpolation(flat, first)`

If code is passed to `createShaderModule` that uses `@interpolation(flat)` or `@interpolation(flat, first)`
Expand All @@ -197,6 +231,8 @@ generate a validation error.
other APIs only support the first vertex so only `@interpolation(flat, either)` is supported in
compatibility mode.

Feature: `provoking-vertex-first`.

## 18. Introduce new `maxStorageBuffersInVertexStage` and `maxStorageTexturesInVertexStage` limits.

If the number of shader variables of type `storage_buffer` in a vertex shader exceeds the `maxStorageBuffersInVertexStage` limit, a validation error will occur at pipeline creation time.
Expand Down Expand Up @@ -228,6 +264,8 @@ sampler in a shader will generate a validation error at pipeline creation time.

**Justification**: OpenGL ES 3.1 says such usage has undefined behavior.

Feature: `depth-texture-sample`.

## 21. Limit the number of texture+sampler combinations in a stage.

If the number of texture+sampler combinations used a in single stage in a pipeline exceeds
Expand Down