[Bug] Lightmass Build Fails



Install Path:



Win7, GTX580, 16GB, UAC: on


I was building a basic map with lightmass, and suddenly I get this error telling me that it’s failed to build lighting.

Here are the logs.

  • [Log file.][1]
  • [Lightmass log.][2]

Repro Steps:

I was building and baking the lights to a slightly modified ExampleMap when this happened. Slightly modified means that I only alt dragged a couple of the floor’s blocks to create some simple extra level geometry. From there, go to Build > Build Light > All Levels and bake as you normally would.

Update: This appears to happen on any map, even a blank slate examplemap file.

I don’t know what it is, but bugs tend to have a knack for finding their way to me. I can’t use lightmass at all until I find the underlying cause of the problem. This has not been a problem with past builds; it appears to be exclusive to this one.

Although I have to say, if this was an issue specific to this version, it would have been reported by someone already. So it begs the question: What’s different here that would cause Lightmass to go haywire specifically on my machine like this?


I get an error prompt, followed by a map check dialog basically telling me that lighting still needs to be built on the map. Then I get this really terrible lighting setup with harsh shadows.


Perfect bake with no hassles.


After a couple hours of some digging, it turns out that Avast antivirus was intercepting lightmass, treating it as a suspicious program and then attempting to run it in a sandbox. AV normally regards Lightmass as an innocuous program… I wonder what changed with Lightmass in this version to make it go off the wall like that?

I’m glad you figured this out. I’ll let the team know that this happened so we can work on the issue.