Issues with textureGather on texture_2d<u32>

[JMS55]

Edit: Can't reproduce on naga master. However, now a different validation error is produced in wgpu:

Caused by:
    In Device::create_compute_pipeline
      note: label = `ambient_occlusion_denoise_pipeline`
    error matching shader requirements against the pipeline
    unable to filter the texture (ResourceBinding { group: 0, binding: 1 }) by the sampler (ResourceBinding { group: 1, binding: 0 })
    integer textures can't be sampled

Caused by the following match ignoring the gather field in naga:

naga/src/valid/analyzer.rs

Lines 571 to 594 in df4e403

	E::ImageSample {
	image,
	sampler,
	gather: _,
	coordinate,
	array_index,
	offset: _,
	level,
	depth_ref,
	} => {
	let image_storage = GlobalOrArgument::from_expression(expression_arena, image)?;
	let sampler_storage = GlobalOrArgument::from_expression(expression_arena, sampler)?;
	match (image_storage, sampler_storage) {
	(GlobalOrArgument::Global(image), GlobalOrArgument::Global(sampler)) => {
	self.sampling_set.insert(SamplingKey { image, sampler });
	}
	_ => {
	self.sampling.insert(Sampling {
	image: image_storage,
	sampler: sampler_storage,
	});
	}
	}

---

According to the spec, this should be a valid operation. However, I get the following error in my project:

var depth_differences: texture_2d<u32>;

error: Entry point denoise at Compute is invalid
    ┌─ wgsl:106:18
    │
106 │     let edges0 = textureGather(0, depth_differences, point_clamp_sampler, uv);
    │                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ naga::Expression [17]
    │
    = Expression [17] is invalid
    = Unable to operate on image class Sampled { kind: Uint, multi: false }

Taking a look at naga's source code, it looks like this is an InvalidImageClass Err. There's only 3 places it's created, of which the following seems most likely:

naga/src/valid/expression.rs

Lines 382 to 389 in 2a11d83

	let image_depth = match class {
	crate::ImageClass::Sampled {
	kind: crate::ScalarKind::Float,
	multi: false,
	} => false,
	crate::ImageClass::Depth { multi: false } => true,
	_ => return Err(ExpressionError::InvalidImageClass(class)),
	};

It looks like originally this match statement assumes a float texture only, which is a valid assumption when this was originally used for only regular samples, but when gathers were added, this assumption was broken.

[JMS55]

@teoxoy this commit 9ddf38e turned out to be necessary for my shader https://github.com/JMS55/bevy/blob/gtao/crates/bevy_pbr/src/ssao/denoise_ssao.wgsl#L25. The tests I wrote just didn't catch it :/.

[teoxoy]

What is the error exactly?

[JMS55]

Ah sorry, the error is:

Caused by:
    In Device::create_compute_pipeline
      note: label = `ssao_denoise_pipeline`
    error matching shader requirements against the pipeline
    unable to filter the texture (ResourceBinding { group: 0, binding: 1 }) by the sampler (ResourceBinding { group: 1, binding: 0 })
    integer textures can't be sampled

[teoxoy]

This looks like it's caused by this validation error over in wgpu: https://github.com/gfx-rs/wgpu/blob/f14bee6740b8755afa362c3be6ef4b225209871a/wgpu-core/src/validation.rs#L1071

I think we should allow those through if the SamplerBindingType is NonFiltering.

Please open an issue/PR on the wgpu repo.