Framebuffer Fetch (experimental)

This extension enables support for the chromium_experimental_framebuffer_fetch WGSL extension.

The extension is experimental and might change for example to gain new validation rules (with extension struct) in the future.

It is available on tiler Metal GPUs and Vulkan with the InputAttachment capability.

Validation

In Device::CreateRenderPipeline or Device::CreateRenderPipelineAsync:

  • For each @color(N) in : T fragment in:
    • color target N must exist
    • color target N's format must match T in both component count and base type

WGSL

Validation

  • Requires enable chromium_experimental_framebuffer_fetch
  • Requires fragment shader
  • Requires T to be i32, u32, f32, f16 or vec{2,3,4}{i,u,f,h}.
  • Requires N to be unique per color
  • The N value must be in the range of [0..7]

Metal

  • @color(N) entries translate directly to [[color(N)]] annotations on the input data.

Vulkan

  • Requires a group, binding value provided for each color N value from Host

  • Add the OpCapability InputAttachment

  • For each color(N) entry

    • Create an OpTypeImage %type SubpassData 0 0 0 2 Unknown where %type is f32, i32 or u32. For f16, the %type will be f32 and the OpImageRead will be decorated with RelaxedPrecision.

    • Create a OpTypePointer UniformConstant %image

    • Create a OpVariable %ptr_uniform_image UniformConstant

    • Decorate the var with OpDecorate %inputVar InputAttachmentIndex N

    • We will then replace the value of the input variable with the value read from the attachment

      • OpImageRead %v4float %img %zero where %zero is OpConstantComposite %v2int %int_0 %int_0
      • For f16, the result of the RelaxedPrecision-decorated OpImageRead is converted to f16.

  • For Dawn, this would probably map to creating the image in ShaderIO and then calling the image_read intrinsic for SPIR-V