Attachments only support sample counts 1 and 4

[phoekz]

Description
When starting a render pass which has multisampling enabled, I get this validation error:

[2021-08-19T16:28:25Z ERROR wgpu::backend::direct] wgpu error: Validation Error

    Caused by:
        In a RenderPass
          note: encoder = `Encoder`
        In a pass parameter
          note: command buffer = `Encoder`
        attachment's sample count 2 is invalid

Sample counts 1 and 4 work, while 2 and 8 do not, even though my GPU definitely supports those. I looked around and I suspect this condition is the problem:

wgpu/wgpu-core/src/command/render.rs

Lines 679 to 681 in 0b1657f

	if sample_count != 1 && sample_count != 4 {
	return Err(RenderPassErrorInner::InvalidSampleCount(sample_count));
	}

[kvark]

That's what WebGPU exposes today. We can think of a way to expose this in a non-portable native-only way.

[phoekz]

Ah my bad. I was actually trying to look up from the spec if this was intentional. Looks like I missed this under section 6.1.1:

[phoekz]

To avoid further confusion, it might be a good idea to throw a more specific error which states that only 1 and 4 are valid values.

Or you can go down Vulkan path of defining enums for sample counts, and only allowing for 1 and 4 for example.

[daxpedda]

Is this not fixed by #3140?

[cwfitzgerald]

It was!