WebGPU/WGSL: Compiler Error, LightData struct member variable "type" is a reserved word.

[scotbrew]

When integrating MaterialX into USD WebGPU, we have come across a shader code issue in MaterialX.
In short, in the WebGPU/WGSL compiler, "type" is a reserved word, so cannot be used as a member variable name for the structure LightData GLSL code that WebGPU code is translated from.

As a test fix, "LightData.type" can be changed to "LightData.light_type" (or something similar). From sample shaders reviewed so far, this appears to be used internal to the shader source and is not set externally, so should minimize risk.

This is a blocker for USD WebGPU to use MaterialX materials as-is.

struct LightData
{
    int type;
    vec3 position;
    vec3 color;
    float intensity;
    float decay_rate;
    vec3 direction;
    float shadowOcclusion;
};

For the WGSL spec, see: https://www.w3.org/TR/WGSL/

Steps to Reproduce (standalone):

1. In Chrome browser, Launch https://jsfiddle.net/cx20/vmkaqw2b/
2. Switch bottom right shader code to "fragment"
3. Change the GLSL code to be:

#version 450
layout(location = 0) out vec4 outColor;

struct LightData
{
    int type;
    vec3 position;
    vec3 color;
    float intensity;
    float decay_rate;
    vec3 direction;
    float shadowOcclusion;
};

void main() {
    LightData s;
    s.type = 2;
    s.color = vec3(1.0, 1.0, 1.0);
    outColor = vec4(0.0, 0.0, 1.0, 1.0);
}

Resulting WGSL / WebGPU code:

struct LightData {
  type : i32,
  position : vec3<f32>,
  color : vec3<f32>,
  intensity : f32,
  decay_rate : f32,
  direction : vec3<f32>,
  shadowOcclusion : f32,
}

var<private> outColor : vec4<f32>;

fn main_1() {
  var s : LightData;
  s.type = 2i;
  s.color = vec3<f32>(1.0f, 1.0f, 1.0f);
  outColor = vec4<f32>(0.0f, 0.0f, 1.0f, 1.0f);
  return;
}

struct main_out {
  @location(0)
  outColor_1 : vec4<f32>,
}

@fragment
fn main() -> main_out {
  main_1();
  return main_out(outColor);
}

4. In a separate tab of the Chrome browser, open: https://google.github.io/tour-of-wgsl/. Copy the WGSL shader code to replace the @Fragment code on the right side. Change "fn main()" to "fn frag_main()".

Result:

error: 'type' is a reserved keyword
type : i32,
^^^^

Proposed Modification:

struct LightData
{
    int light_type;
    ...
};

and update other places that refer to it in the shader code.

[ashwinbhat]

@scotbrew Since type is a reserved keyword, I think your proposal to use light_type is a suitable replacement.
The changes should be fairly simple but I'm adding @niklasharrysson and @JGamache-autodesk for their thoughts if light_type would be an acceptable replacement.

[scotbrew]

As other applications that build upon MaterialX shader code (like USD) may use LightData.type, we probaby want to put the renaming change behind an #ifdef block. The identified changes in MaterialX look to be scoped to 4 one-liner changes in 4 files, though there may be an unidentified place where MaterialX is setting LightData.type.

[kwokcb]

I would suggest that logic be added to the derived shader generator functions that emit safe code instead of inserting any #ifdef code which would "pollute" other generater's code. e.g. override LightSamplerNodeGlsl::emitFunctionDefinition() .

[scotbrew]

@kwokcb , I will reach out to you over slack so we can iterate a bit on proposed solutions.

[scotbrew]

Issue being addressed by #2407 .