Cannot use builtin ObjectToWorldKHR variable

[jcmnn]

Expected Behaviour

ObjectToWorldKHR should be able to be used, but throws an error.

Example & Steps To Reproduce

#[spirv(closest_hit)]
pub fn chit(
    #[spirv(object_to_world)] object_to_world: [Vec3; 4],
) {
    // ...
}

object_to_world fails to compile with the error:

[VUID-ObjectToWorldKHR-ObjectToWorldKHR-04307] According to the Vulkan spec BuiltIn ObjectToWorldKHR variable needs to be a matrix with 4 columns of 3-component vectors of 32-bit floats. ID <5> (OpVariable) has columns 0 and rows 0 not equal to expected 4x3.

Are matrix types implemented? I know Vector is implemented for glam types allowing them to be assigned to builtin variables, but there doesn't appear to be an equivalent matrix trait.

It does not work for any combination: [f32; 12], &mut [Vec3; 4], &mut [f32; 12]

System Info

• Rust: rustc 1.84.0-nightly (b19329a37 2024-11-21)
• OS: Windows 11
• GPU: RTX 3060
• SPIR-V: SPIRV-Tools v2024.4 v2024.4.rc2-0-g4d2f0b40

[LegNeato]

The glam Mat types are supported, though I have never used this API before so I am unsure if they work for it? Try and let me know!

[jcmnn]

glam only provides square matrix types, so it will not work for this - ObjectToWorldKHR is mat4x3.

Looking at the generated SPIR-V, even with a glam Mat4 type, the code uses a struct of vectors:

OpDecorate %5 BuiltIn ObjectToWorldKHR

%13 = OpTypeFloat 32
%28 = OpTypeVector %13 4
%11 = OpTypeStruct %28 %28 %28 %28
%29 = OpTypePointer Input %11
%5 = OpVariable %29 Input

whereas it should be OpTypeMatrix similar to this function:

rust-gpu/crates/spirv-std/src/ray_tracing.rs

Lines 941 to 949 in 30024f8

	pub unsafe fn get_candidate_intersection_object_to_world<V: Vector<f32, 3>>(&self) -> [V; 4] {
	let mut result = Default::default();
	asm! {
	"%u32 = OpTypeInt 32 0",
	"%f32 = OpTypeFloat 32",
	"%f32x3 = OpTypeVector %f32 3",
	"%f32x3x4 = OpTypeMatrix %f32x3 4",
	"%intersection = OpConstant %u32 0",

Generated SPIR-V from a GLSL file using gl_ObjectToWorldEXT looks like:

OpDecorate %32 BuiltIn ObjectToWorldKHR

OpName %32 "gl_ObjectToWorldEXT"
%6 = OpTypeFloat 32
%30 = OpTypeMatrix %7 4
%31 = OpTypePointer Input %30
%32 = OpVariable %31 Input

I'm assuming OpTypeMatrix does not get autogenerated? If not, is it possible to use builtin variables using the asm! macro?

[jcmnn]

It works with the #[spirv(matrix)] attribute as shown here: https://github.com/Rust-GPU/rust-gpu/blob/main/tests/compiletests/ui/spirv-attr/matrix-type.rs#L7