The chromium_experimental_primitive_id is an experimental extension that allows the use of the primitive_id builtin in fragment shaders in WGSL.
This extension is experimental and the syntax is being discussed. No official WebGPU specification has been written yet.
The usage is restricted to fragment shaders.
| Platform | Type | Notes |
|---|---|---|
| SPIR-V | 32-bit integer | Requires geometry shaders, mesh shaders, or raytracing. Available as the PrimitiveID builtin |
| HLSL | u32 | Requires D3D10. Available as the SV_PrimitiveID semantic |
| GLSL | i32 | Requires GLSL 1.50 and later, ESSL 3.2. (ESSL 3.1 with GL_EXT_geometry_shader). Available as the gl_primitiveID builtin |
| Metal | u32 | Requires Metal 2.2 on MacOS or Metal 2.3 on iOS. Available as [[primitive_id]] |
Due to the limited availability, this will need a enable statement to be used. For this experimental extension it would be enable chromium_experimental_primitive_id.
All of the topologies in GPUPrimitiveTopology are supported. (Generally, adjacency topologies would not be supported but WebGPU does not have any adjacency topologies).
| Topology | Primitive |
|---|---|
| point-list | Each vertex is a primitive |
| line-list | Each vertex pair is a primitive |
| line-strip | Each adjacent vertex pair is a primitive |
| triangle-list | Each vertex triplet is a primitive |
| triangle-strip | Each group of 3 adjacent vertices is a primitive |
This extension adds a new builtin_value_name entry for primitive_id.
An entry is added to the Built-in input and output values table:
Name: primitive_id
Stage: fragment
Direction: input
Type: u32
Extension: chromium_experimental_primitive_id
The index of the first primitive is zero, incrementing by one for each subsequent primitive.
primitive_id resets to zero between each instance drawn.
If the primitive id overflows (exceeds 2^32 – 1), it wraps to 0. (This is from HLSL, is it true for other backends?)
There is no support for automatically generating a primitive id for adjacent primitives.
The primitive_id value is uniform across the primitive.
Primitive restart has no effect on the value of variables decorated with primitive_id.
@vertex fn vs_main(@builtin(primitive_id) my_id: u32) -> @builtin(position) vec4f { return vec4f(f32(my_id)); }