This contains the Metal HAL changes required to support external
textures. When creating a bind group we create resource bindings for
each of the 3 textures and parameters buffer that the external texture
has been lowered to. When creating the pipeline layout we fill the
`BindTarget` accordingly, so that the Naga MSL backend can bind each
of the global variables to which the the external texture has been
lowered to each of these resources.
We must also ensure the size of the buffer bound to the parameters
global matches the size of the MSL type, else metal validation
complains. We do this by adding a padding field to the rust-side
ExternalTextureParams struct, the size of which is used as the size of
the buffer to allocate.
Lastly we enable `Features::EXTERNAL_TEXTURE` on the Metal backend.
This adds MSL backend support for `ImageClass::External`. (ie WGSL's
`external_texture` texture type). This is implemented very similarily
to the HLSL implementation in #7826.
Each external texture global variable is lowered to 3 `texture2d`s and
a buffer of type NagaExternalTextureParams. As usual in Naga's MSL
backend, these are passed as arguments to the entry point. The
bindings for each of these arguments are provided via the usual
binding map, using a new `BindExternalTextureTarget` variant of
`BindTarget`.
Unlike HLSL, MSL allows textures to be used as fields in structs. We
therefore immediately wrap these variables in a
`NagaExternalTextureWrapper` struct. This wrapper can then
conveniently be passed to either user-defined functions or builtin
implementations that accept an external texture.
The WGSL builtins `textureDimensions()`, `textureLoad()`, and
`textureSampleBaseClampToEdge()` are implemented using wrapper
functions using the regular `write_wrapped_functions()` machinery.
The HLSL external texture implementation didn't have to do anything in
particular to handle both coordinate types, as int2 automatically gets
promoted to uint2. But in MSL this is not the case, and it is
therefore important to test that we correctly handle both coordinate
types.
textureSampleBaseClampToEdge() is valid when called on a
`texture_external` texture, but all other sample operations are
invalid. Previously validation was succeeding for these operations on
an external texture in cases where the same operation on a
`texture_2d<f32>` would be allowed. We must therefore explicitly check
for ImageClass::External and disallow invalid sample operations.
It makes sense for a function to be `FnMut + Send`, or `Fn + Send + Sync`,
but not `Fn + Send` because that is overly restrictive for the caller and
the callee.
P010 is a 4:2:0 chroma subsampled planar format, similar to NV12. Each
component uses 16 bits of storage, of which only the high 10 bits are
used. On DX12 this maps to DXGI_FORMAT_P010, and on Vulkan this maps to
G10X6_B10X6R10X6_2PLANE_420_UNORM_3PACK16.
The existing "nv12" gpu test module has been renamed to
"planar_texture", and a new test P010_TEXTURE_CREATION_SAMPLING has
been added similar to the existing NV12_TEXTURE_CREATION_SAMPLING. The
remaining tests in this module have been converted to validation tests,
and now test both NV12 and P010 formats.
