With Opacity Micromap enabled, gl_PrimitiveID becomes invalid, how to get UV

[nitroglycerine]

Hello Forum,

I have a simple test program that renders one alpha-tested quad using vullkan RT, it should like this without applying OMM:

I have an any-hit shader that looks as follows.

#version 460
#extension GL_EXT_ray_tracing : require
#extension GL_GOOGLE_include_directive : enable

#include "raycommon.glsli"

layout(location=0) rayPayloadInEXT hitPayload prd;
hitAttributeEXT vec2 attribs;

layout(binding=3) uniform sampler2D texSampler1;  // alpha map

struct Vertex {
	vec2 pos;
	vec3 color;
	vec2 uv;
};
layout(std430, binding=4) readonly buffer VertexBuf {
	Vertex vertices[];
};

layout(std430, binding=5) readonly buffer IndexBuf {
    uint indices[];
};

void main() {
  // read UV
  uint idx0 = indices[gl_PrimitiveID * 3];
  uint idx1 = indices[gl_PrimitiveID * 3 + 1];
  uint idx2 = indices[gl_PrimitiveID * 3 + 2];
  vec2 uv0 = vertices[idx0].uv;
  vec2 uv1 = vertices[idx1].uv;
  vec2 uv2 = vertices[idx2].uv;
  
  const vec3 bary = vec3(1.0 - attribs.x - attribs.y, attribs.x, attribs.y);
  const vec2 uv = uv0 * bary.x + uv1 * bary.y + uv2 * bary.z;
  vec3 t = texture(texSampler1, uv).xyz;
  if (t.x < 0.5) {
    ignoreIntersectionEXT;
  } else {
  }
}

However, when I apply OMM to it, the render results become blank with all miss color. I took a closer look:

• I found the UV showing up in the any-hit shader are all (0, 0) instead of their normal values before applying the OMM. This causes the any-hit shader to think all rays fall on to the transparent part of the alpha map.
• The reason the UVs are all (0, 0) is the gl_PrimitiveId becomes a very large, invalid value, rather than 0 or 1 as it should before applying the OMM.
• If I remove ignoreIntersectionEXT in the any-hit shader and assign the payload with prd.hitValue = vec3(uv, 0);, I see an outline of the OMM:
  • 
• The OMM shows up in NSight Graphics correctly:
  • 
• The barycentrics (attribs) are correct.
• I checked the OMM integration guide, it suggests using attribs alone is enough for obtaining the UV: const float texCoords = GetTexCoords(barycentrics);. How is this achieved?
• I’m using the CPU OMM baker with a very low subdivision level of 2, the bake results match the appearance of the OMM in NSight.
  • 
• Making each BLAS containing only 1 triangle and using gl_InstanceID to index into the vertex array works, so it seems that only gl_PrimitiveID is broken:
  *
  
  

I’m using an RTX3060.

What could I have done wrong?
Thanks for your viewing this post and input!