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SP #005: Write-Only Textures
=================
Add Write-Only texture types to Slang's core module.
Status
------
Status: Design Review.
Implementation: N/A
Author: Yong He
Reviewer:
Background
----------
Slang inherits HLSL's RWTexture types to represent UAV/storage texture resources, this works well for HLSL, GLSL, CUDA and SPIRV targets.
However Metal has the notion of write only textures, and WebGPU has limited support of read-write textures. In WebGPU, a read-write texture can only have
uncompressed single-channel 32bit texel format, which means a `RWTexture2D` cannot be used to write to a `rgba8unorm` texture.
To provide better mapping to write-only textures on Metal and WebGPU, we propose to add write-only textures to Slang to allow writing portable code
without relying on backend workarounds.
Proposed Approach
-----------------
Slang's core module already defines all texture types as a single generic `_Texture<T, ..., access, ...>` type, where `access` is a value parameter
representing the allowed access of the texture. The valid values of access are:
```
kCoreModule_ResourceAccessReadOnly
kCoreModule_ResourceAccessReadWrite
kCoreModule_ResourceAccessRasterizerOrdered
kCoreModule_ResourceAccessFeedback
```
We propose to add another case:
```
kCoreModule_ResourceAccessWriteOnly
```
to represent write-only textures.
Also add the typealiases prefixed with "W" for all write only textures:
```
WTexture1D, WTexture2D, ...
```
These types will be reported in the reflection API with `access=SLANG_RESOURCE_ACCESS_WRITE`.
Write-only textures support `GetDimension` and `Store(coord, value)` methods. `Load` or `subscript` is not defined for write-only texture types,
so the user cannot write code that reads from a write-only texture.
Write only textures are supported on all targets. For traditional HLSL, GLSL, SPIRV and CUDA targets, they are translated
exactly the same as `RW` textures. For Metal, they map to `access::write`, and for WGSL, they map to `texture_storage_X<format, write>`.
|
