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Add the mip level and array layer consensus proposals. #4365

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Nov 10, 2023
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Add the mip level and array layer consensus proposals. #4365

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Add the mip level and array layer consensus proposals.
SenorBlanco Nov 3, 2023
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SenorBlanco Nov 7, 2023
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SenorBlanco Nov 8, 2023
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14 changes: 13 additions & 1 deletion proposals/compatibility-mode.md
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Expand Up @@ -71,12 +71,24 @@ Each `GPUColorTargetState` in a `GPUFragmentState` must have the same `blend.alp
### 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()`.
**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()`.

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

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

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

A draw call may not bind two views of the same texture differing in `baseMipLevel` or `mipLevelCount`. Only a single mip level range range per texture is supported. This is enforced via validation at encode time.
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How specific should we get in the proposal vs the actual spec?

This is valid

setBindGroup(0, subView1);  
setBindGroup(1, subView2);  // incompatible with subView1
draw()                      // pipeline that only uses subView1 (via group(0))

both setBindGroup and draw are "encoding". Not sure if that's being too nit-picky but the final spec will need to make that clear that it's only validated on draw and only cares about bindgroups actually used. (or even if it cares about all of them it still has to at draw cmd encoding, not setBindGroup cmd encoding)

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We should try to be fairly precise in proposals, so that translation to spec text doesn't require roundtrips to the group ideally. +1 that this should be checked on draw.

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Done.

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(I've changed it to "draw time". Now maybe I'm being overly picky, but to me draw time is when the draw command is executed, not when it is encoded. OTOH, no validation occurs on execution, so perhaps it's ok in this case. Futher wordsmithing welcome.)


**Justification**: OpenGL ES does not support texture views, but one mip level subset may be specified per texture using `glTexParameter*()` via the `GL_TEXTURE_BASE_LEVEL` and `GL_TEXTURE_MAX_LEVEL` parameters.

### 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.
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I don't know if this will come up or needs to be mentioned. I think it might have been brought up before. This is enforced at bind group creation and not create view time because it's still valid to make a sub range of 1 layer use as a render attachment


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

## Issues

Q: OpenGL ES does not have "coarse" and "fine" variants of the derivative instructions (`dFdx()`, `dFdy()`, `fwidth()`). Should WGSL's "fine" derivatives (`dpdxFine()`, `dpdyFine()`, and `fwidthFine()`) be required to deliver high precision results? See [Issue 4325](https://github.com/gpuweb/gpuweb/issues/4325).
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