How to rebuild UnrealLightmass.cpp

I get an error message saying “Light build fail. Swarm failed to kick off. Rebuild UnrealLightmass.”

I try to rebuild by opening the visual studio file, but there is no UnrealLightmass. I remember i have seen it before, but this time it is totally missing.

Where is UnrealLightmass.cpp?

There should be a compilation target for it.

It seems you are missing some files.


Why am i missing all those files? Is it because i am running on the VR template? Did i just discover a bug??

**The “Programs” section in my Visual Studio file is TOTALLY missing. This is a screenshot of my visual studio file. **


I am surprised nobody knows the solution to this issue/bug.

Can Epic tech support assist, please? Otherwise i am foreever stuck here unable to build lighting. I believe there are many others out there experiencing the same bug.

Just to be sure, your engine is the source version cloned from Github?

You need to get the Unreal Engine Source from Github if you want to recompile modules.

That is what i suspected. The one i have now is downloaded from Epic Games Launcher.

So only the Github version can compile Lightmass! Jeeeezz, i wasted so many days trying to find a solution. Wasted hours of my precious life. Why this big hole for people to fall into?

Only the Github version should -need- to compile lightmass. Something with your launcher build is bugged, and yes, it -is- only you, I’ve never seen this before. Try just deleting the engine, manually deleting any files left over in the engine folder, and redownloading it, should fix it.

You mean a normal Epic Launcher downloaded 4.16 can compile lightmass? My one just happened to be bugged, but otherwise non-bugged ones is able to compile lightmass?

Or is it only and only the github version can compile lightmass? If this is true, then i shall stay far far away from Epic Games Launcher in the near and far future.

The launcher version has it compiled already. You do not need to compile anything. You don’t even need Visual Studio installed to use Unreal from the launcher (as long as you stick to blueprint).

Your launcher download sounds corrupted, that’s all. Kill it, redownload it.

So in the future whenever “Light build failed. Swarm failed to start. Rebuild Unreal Lightmass” happens again(it has already happened many times), then deleting Epic Launcher and reinstalling it will solve the problem correct? Don’t need to get the github to compile lightmass?

Unless you’re using the Git version of Unreal, which you DO need to compile lightmass, it shouldn’t be happening in the first place. Something’s messed up with your downloads, maybe a firewall or something. Hard to say from here without actually looking at it.

Without compiling lightmass, you can’t build ANY lighting in your project. Which is exactly the same problem I’m having now. I migrated my level into a new C++ project just so I can be able to rebuild unreallightmass.cpp in VS but the stupid "programs folder’ is also missing. Its been a year since the last post and this freaking problem still exists.

I’m having this issue and I’m getting it routinely, so is a team member. We aren’t using a custom launcher, just the normal one. It’s coming up randomly in different projects.

It’s so frustrating. When I google it people tell me to recompile this stuff but I can’t as i’m not using a custom engine. We’ve redownloaded the engine, this problem happened in 4.19, 4.20, 4.21, with new projects…

It’s seriously starting to drive us crazy.

Anyone have any ideas?

I’m having this problem now as well, I have none of these folders/files other people are talking about, it’s still an issue in 2019

Thanks a-million! This is what got me sorted out. I believe I had done a full compile of the UE4 solution, but maybe it didn’t include the projects folder. The first time I have ever tried to build lighting with a source version of the editor. This is a reliable failure of lightmass with UESource. I had the same issue back when I had 4.22S, but didn’t dig deep into the fix until now.

For those having other issues with lightmass, there is good documentation here. It even goes over cleaning and re-validating cache, as well and having to compile the UnrealLightmass solution from within Visual Studio, for UES.