On large-format ICVFX stages it is a standard that we support two frustums. This requires one node in the cluster to render three rasters simultaneously, with one GPU doing double duty rendering the two frustums. Because of this, we continue to rely on Lightmass baking to keep rasterization performance viable — we are pushing significantly more pixels than a single raster application such as a video game or smaller stage with only one frustum.
While newer standards such as ST 2110 and more powerful GPUs make it easier to move away from baked lighting, there is still a continuity issue: our legacy ICVFX environments need to match previous shoots. When re-opening these environments, we expect light bakes to remain consistent.
In Unreal Engine 5.6, all of our old baked environments are invalidated and require rebaking. However, when attempting to rebake with GPU Lightmass, the editor crashes consistently during the voxelization stage.
In our first crash log, the editor warned about Nanite meshes with unsupported translucent materials (e.g., glass). We removed those meshes, but the crash persists in the exact same stage, indicating that this is not a single-asset problem but a broader GPU Lightmass stability issue in 5.6. CPU Lightmass does not crash, but GPU Lightmass is unusable on these environments.
Based on the call stack (FVolumetricLightmapRenderer::VoxelizeScene, FCacheRayTracingSceneData::SetupFromSceneRenderState), we suspect the issue may be related to GPU Lightmass attempting to voxelize very large Nanite meshes, which are common in ICVFX environments (e.g. walls, floors, ceilings). CPU Lightmass does not exhibit this issue, but GPU Lightmass is currently unusable in these environments.
Workarounds / Potential Fixes:
- CPU Lightmass: Rebaking with CPU Lightmass completes without crashing, but is significantly slower and impractical for large ICVFX environments.
- Remove or split large meshes: Breaking down massive Nanite meshes (e.g., walls, ceilings) may reduce voxelization load.
- Adjust Lightmass settings: Increasing Volumetric Lightmap Detail Cell Size may reduce voxelization stress but lowers quality.
Luoshuang’s GPU Lightmass Plugin (community patch): The original author of GPU Lightmass maintains a patched plugin that improves stability and fixes several issues not addressed in Epic’s implementation, we tried this and it seemed to fix the issues but we worry about its long-term viability .
Pros:
Includes improvements and bug fixes from the original developer.
Reported to be more stable in some complex scenes.
May bypass voxelization crashes in 5.6.
Cons:
Not officially supported by Epic.
May require manual patching or source builds.
Compatibility with UE 5.6 uncertain; long-term maintenance not guaranteed.
At present, none of these options are a reliable production-ready substitute for official GPU Lightmass support in UE 5.6.