ray hits only occur on thread 0 when using OpenMP

crobar — Tue, 08 Jul 2025 23:24:58 GMT

I'm working with a code which parallelizes the detection of ray intersections with a geometry using an OpenMP parallel for. This is actually a code for simulating thermal images. The code is open source and available here:

https://github.com/crobarcro/ThRend

In this code there is a parallel for loop, i.e. with

#pragma omp parallel for

which contains the following (and other things):

RTCRay ray; ray.org_x = camOrig.x; ray.org_y = camOrig.y; ray.org_z = camOrig.z; ray.tfar = 1000.0f; ray.tnear = EPS; ray.time = 0; ray.id = 0; ray.flags = -1; ray.dir_x = dir.x; ray.dir_y = dir.y; ray.dir_z = dir.z; RTCRayHit query; RTCIntersectContext context; rtcInitIntersectContext(&context); query.ray = ray; query.hit.geomID = RTC_INVALID_GEOMETRY_ID; query.hit.primID = RTC_INVALID_GEOMETRY_ID; query.hit.instID[0] = RTC_INVALID_GEOMETRY_ID; rtcIntersect1(eScene, &context, &query); float t = 0; float aparentT = 0; if (query.hit.geomID != RTC_INVALID_GEOMETRY_ID) { <snip>

On Linux, when I use a single thread the full output image is created correctly. However, if I increase the threads to two, only half the image is created, the rest blank, and if I increase to 4, only 1/4 is created. Ther reason is that only thread 0 ever passes the if (query.hit.geomID != RTC_INVALID_GEOMETRY_ID) test.

Can anyone suggest what the problem might be? I'm certain it's not the geometry.

For further info, the scene is created in buildSceneEmbree in EmbreeUtils.h and the ray tracing is done in generateThermography in main.cpp

Re: ray hits only occur on thread 0 when using OpenMP

SvenW_Intel — Tue, 19 Aug 2025 07:08:42 GMT

The problem comes likely as you do not initialize the ray.mask to 0xFFFFFFFF. Please try again with that fix.

topic ray hits only occur on thread 0 when using OpenMP in Intel® Embree Ray Tracing Kernels

ray hits only occur on thread 0 when using OpenMP

Re: ray hits only occur on thread 0 when using OpenMP