GEOMETRY_TYPE_TRIANGLE vs GEOMETRY_TYPE_USER BVH build times

[mrhe]

Hi,

I'm running collision tests between two triangular meshes in embree4. The rtcCollide function only accepts GEOMETRY_TYPE_USER as input. I got it to work and successfully find intersections between my meshes but noticed significant increase in build times for the BVH.

When creating a BVH for raytracing with the same geometry, I can use GEOMETRY_TYPE_TRIANGLE and the BVH is built almost instantaneously.

Here are the results of the rtcCommitScene for a 25k tris mesh

GEOMETRY_TYPE_USER : 14.3882 ms

GEOMETRY_TYPE_TRIANGLE : 0.0342 ms

The relevant code below. Do you have any ideas why this is the case?

Collision:

void createBaseMesh(float vertices[], int verticesLength, int faces[], int facesLength)
{
  RTCDevice device = rtcNewDevice(NULL);
  RTCScene scene = rtcNewScene(device);
  rtcSetSceneBuildQuality(scene, RTC_BUILD_QUALITY_LOW);
  rtcSetSceneFlags(scene, RTC_SCENE_FLAG_DYNAMIC);

  meshes.clear();
  std::unique_ptr<Mesh> baseMesh(new Mesh());
  baseMesh->x_.resize(verticesLength / 3);
  baseMesh->tris_.resize(facesLength / 3);

  int j = 0;
  for (int i = 0; i < verticesLength; i += 3) {
    baseMesh->x_[j].x = vertices[i];
    baseMesh->x_[j].y = vertices[i + 1];
    baseMesh->x_[j].z = vertices[i + 2];
    j++;
  }

  j = 0;
  for (int i = 0; i < facesLength; i += 3) {
    baseMesh->tris_[j].v0 = faces[i];
    baseMesh->tris_[j].v1 = faces[i + 1];
    baseMesh->tris_[j].v2 = faces[i + 2];
    j++;
  }

  RTCGeometry geom = rtcNewGeometry(device, RTC_GEOMETRY_TYPE_USER);
  unsigned int geomID = rtcAttachGeometry(scene, geom);
  rtcSetGeometryUserPrimitiveCount(geom, baseMesh->tris_.size());
  rtcSetGeometryUserData(geom, (void*)(size_t)geomID);
  rtcSetGeometryBoundsFunction(geom, triangle_bounds_func, nullptr);
  rtcSetGeometryIntersectFunction(geom, NULL);
  rtcSetGeometryOccludedFunction(geom, NULL);
  rtcCommitGeometry(geom);
  rtcReleaseGeometry(geom);
  meshes.push_back(std::move(baseMesh));
  rtcCommitScene(scene);
  rtcCollide(scene, scene, CollideFunc, &sim_collisions);
  rtcReleaseScene(scene);
  rtcReleaseDevice(device);
}

Raytracing:

void RaytracerTest(float vertices[], int verticesLength, int faces[], int facesLength)
{
  RTCDevice device = rtcNewDevice(NULL);
  RTCScene scene = rtcNewScene(device);
  RTCGeometry geom = rtcNewGeometry(device, RTC_GEOMETRY_TYPE_TRIANGLE);

  float* vb = (float*)rtcSetNewGeometryBuffer(geom,
    RTC_BUFFER_TYPE_VERTEX, 0, RTC_FORMAT_FLOAT3, verticesLength * sizeof(float), 3);
  memcpy(vb, vertices, verticesLength * sizeof(float));

  unsigned* ib = (unsigned*)rtcSetNewGeometryBuffer(geom,
    RTC_BUFFER_TYPE_INDEX, 0, RTC_FORMAT_UINT3, facesLength * sizeof(unsigned), 1);  
  memcpy(ib, faces, facesLength * sizeof(unsigned));
  rtcCommitGeometry(geom);
  rtcAttachGeometry(scene, geom);
  rtcReleaseGeometry(geom);
  rtcCommitScene(scene);
  rtcReleaseScene(scene);
  rtcReleaseDevice(device);
}