Lightmap Stuck on Simple Scene [Reproducible Case]

I’ve scene a few posts about the build lighting process getting stuck across multiple versions of UE4 and run into it fairly frequently myself in 4.9.1.

I have at least one easily reproducible case here:

1.) File → New Level → Empty Level.
2.) Add BSP box of 1000, 1000, 400 to scene and mark it hollow.
3.) Place a couple of point lights inside the hollow box and make both static.
4.) Convert BSP box to static mesh.
5.) Open static mesh and set it’s lightmap coordinate index to 1 and save it.

If I try to build lighting at this point it sticks at 66% and never finishes.

Hello tmek,

This is an issue that has been re occurring throughout 4.9.

After following your steps, I’m assuming the 1000,1000, 4000 is referring to the scale of the bsp, and building lights my lights built to 100% and finished.

Could you link me your DxDiag and I will see if this issue has been reported with your specs.

Thank you,


Could you confirm that your lightmass is building inside of swarm but your build sticks at 66%.

Thank you,

I replied just now but seems to have disappeared so apologies if this becomes a double post.

I’ve attached my dxdiag.txt

Also, for science, try this simpler scene I was able to make with just a static point light and basic cube shape.

File->New Level->Empty
Then copy paste the contents from CubeAndLightScene.txt

link text

link text

Yes, if you are replying to my simpler scene with the point light and cube, swarm starts and gets to 66.67% and just keeps running.

Attached is the log:

Note the last line says “Lighting 100.0%” but swarm status still says 66.67% and the blue bar keeps slowly progressing along seemingly forever.

link text

Hello Tmek,

I just tried this reproduction that you mentioned and I was able to build.

I believe this to be another issue we have been having with lighting builds. What will happen is the lightmass will run but the build will stick at a certain percentage. We are currently investigating this issue and your thread has been added to the list of users currently experiencing this issue. Hopefully with a large amount of issues from the community this will be resolved very soon.

The ticket number is UE-20782.

I will keep you up to date with this bugs progression.

Thank you for reporting this.