Texture with multisample imposes too strict limits on `depth_or_array_layers`

[LaylBongers]

I am working on an OpenXR application that has to render to a multiview swapchain. I am attempting to support MSAA rendering, and have to create a framebuffer for doing this. To do this, I create a textures with depth_or_array_layers set to 2 (for multiview) and sample_count set to 4 (for multisample).

Currently, in WGPU, this will fail with a InvalidDimension, MultisampledDepthOrArrayLayer error.

wgpu/wgpu-core/src/device/resource.rs

Line 1434 in 0cb64c4

if desc.size.depth_or_array_layers != 1 {

However, the WebGPU standard does not impose this limitation. Instead the WebGPU standard specifies only "3D" (TextureDimension::D3) textures can't be multisampled:
https://www.w3.org/TR/webgpu/#gputexturedescriptor

"2d" Specifies a texture that has a width and height, and may have layers.
"3d" Specifies a texture that has a width, height, and depth. "3d" textures cannot be multisampled, and their format must support 3d textures (all plain color formats and some packed/compressed formats).

In fact, the limitations for 2D images says:
https://www.w3.org/TR/webgpu/#abstract-opdef-validating-gputexturedescriptor

descriptor.size.depthOrArrayLayers must be ≤ limits.maxTextureArrayLayers.

Thus, the limits should be extended to be restricted by Limits::max_texture_array_layers, instead of to a flat value of 1. Additionally, there should be a check that a multisampled texture cannot be 3D, which is not currently present.

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Edited for context:

I overlooked the part of the spec that does enforce this, here:
https://www.w3.org/TR/webgpu/#texture-creation

If descriptor.sampleCount > 1:
descriptor.size.depthOrArrayLayers must be 1.

Regardless, I do have a need for this feature, maybe as an extension similar to how multiview is supported?

[andyleiserson]

Additionally, there should be a check that a multisampled texture cannot be 3D, which is not currently present.

This is actually blocked by a different check, there is confusing redundancy. See #8568.

[LaylBongers]

It seems I've overlooked the part of the spec that does enforce this.
At
https://www.w3.org/TR/webgpu/#texture-creation

If descriptor.sampleCount > 1:
descriptor.size.depthOrArrayLayers must be 1.

I do have a need for this, and it's supported by the underlying Vulkan platform. Should this be an extension then, like multiview is?

[andyleiserson]

There is a TEXTURE_ADAPTER_SPECIFIC_FORMAT_FEATURES feature that enables things WebGPU doesn't support like multisample counts other than 4. It seems reasonable to use that to gate multisampled 2d arrays. (I'm not the expert on this stuff though.)