writing to storage buffers/textures in fragment shaders with multisample targets is not portable. #4696
Description
IIUC the spec says or implies if you're using a non-multisampled target you get 1 invocation of the fragment shader for each pixel being rendered
If rendering to a multisampled target then then you still only get 1 invocation per pixel, unless you're using sample_index or @interpolate(..., sample) in which case you'll get 1 invocation per sample
But, testing, that's not the case. We can test how many times a fragment shader is called with something like
@group(0) @binding(0) var<storage, read_write> count: atomic<u32>;
@fragment fn fs() -> @location(0) vec4f {
atomicAdd(&count, 1);
...
}
So, testing that
M1 Mac
PASS: sampleCount: 1, @interpolate(perspective) , , count = 1, should be 1
PASS: sampleCount: 1, @interpolate(perspective) , sample_index, count = 1, should be 1
PASS: sampleCount: 1, @interpolate(perspective, sample), , count = 1, should be 1
PASS: sampleCount: 1, @interpolate(perspective, sample), sample_index, count = 1, should be 1
FAIL: sampleCount: 4, @interpolate(perspective) , , count = 4, should be 1 <---
PASS: sampleCount: 4, @interpolate(perspective) , sample_index, count = 4, should be 4
PASS: sampleCount: 4, @interpolate(perspective, sample), , count = 4, should be 4
PASS: sampleCount: 4, @interpolate(perspective, sample), sample_index, count = 4, should be 4
AMD/Intel Mac
PASS: sampleCount: 1, @interpolate(perspective) , , count = 1, should be 1
PASS: sampleCount: 1, @interpolate(perspective) , sample_index, count = 1, should be 1
PASS: sampleCount: 1, @interpolate(perspective, sample), , count = 1, should be 1
PASS: sampleCount: 1, @interpolate(perspective, sample), sample_index, count = 1, should be 1
PASS: sampleCount: 4, @interpolate(perspective) , , count = 1, should be 1
FAIL: sampleCount: 4, @interpolate(perspective) , sample_index, count = 1, should be 4 <--
FAIL: sampleCount: 4, @interpolate(perspective, sample), , count = 1, should be 4 <--
FAIL: sampleCount: 4, @interpolate(perspective, sample), sample_index, count = 1, should be 4 <--
Windows NVidia 2070 Super
PASS: sampleCount: 1, @interpolate(perspective) , , count = 1, should be 1
PASS: sampleCount: 1, @interpolate(perspective) , sample_index, count = 1, should be 1
PASS: sampleCount: 1, @interpolate(perspective, sample), , count = 1, should be 1
PASS: sampleCount: 1, @interpolate(perspective, sample), sample_index, count = 1, should be 1
PASS: sampleCount: 4, @interpolate(perspective) , , count = 1, should be 1
FAIL: sampleCount: 4, @interpolate(perspective) , sample_index, count = 1, should be 4 <--
FAIL: sampleCount: 4, @interpolate(perspective, sample), , count = 1, should be 4 <--
FAIL: sampleCount: 4, @interpolate(perspective, sample), sample_index, count = 1, should be 4 <--
Does this need to be called out in the spec in some way? Like one idea is to mention it's not portable to use storage buffers/texture in a fragment shader because number of invocations in undefined. Another idea would be to generate a validation error if storage buffers/textures are written to a fragment shader with a mulitsample target.
It's also possible these are bugs in my code or in dawn or my understanding 😅