Description

There is a regression or change in the Build Lighting process when using Lightmass in Unreal Engine 5.6:

  • When building lighting with Lightmass calling the CalculatePointShadowing function, the result does not match with the UE5.5.
  • When Embree is disabled in DefaultLightmass.ini, the U5.6 results from logging the shadow sample visibility values match those obtained with UE5.5.
Steps to Reproduce
  • In UE 5.5, add this line below CalculatePointShadowing is called in TextureMapping.cpp:

UE_LOG(LogLightmass, Log, TEXT("Shadow Sample %f"), ShadowSample.Visibility);

  • Build UE5 Editor and UnrealLightmass
  • Create a game project
  • Add a DefaultLightmass.ini file in the Config folder and add the lines :
    [DevOptions.StaticShadows]
    ; Using area shadows by default instead of filtering in texture space
    bUseZeroAreaLightmapSpaceFilteredLights=True
  • Open the project and in the project setting check allow static lighting
  • Add a Stationary Spotlight actor and check "Cast Shadows", "Cast Static Shadows", "Cast Dynamic Shadows" and "Use Area Shadows for Stationary Light precomputed shadowmaps"
  • Add two plane actors. Place one plane above the other plane so that it creates a shadow in the second plane
  • Build Lightmass data with Build Lighting Only
  • Open swarm agent and check the log. (Increase the MaximumJobApplicationLogLines to 10K to contain the entire log)
  • Observe the value of ShadowSample.Visibility
  • Perform the same steps for UE 5.6 and observe the value of ShadowSample.Visibility. The UE5.6 and UE5.5 results don't match.
  • Disabling Embree by adding the following lines to DefaultLightmass.ini will make the result match between UE5.5 and 5.6:
    [DevOptions.StaticLighting]
    bUseEmbree=false

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Unresolved
ComponentUE - Graphics Features
Affects Versions5.6
CreatedOct 14, 2025
UpdatedOct 14, 2025
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