General but important question about rendering/lightning building/Swarm.

Development Rendering
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Hello everyone! Sorry if i’m probably asking something obvious, but my researches with googling didn’t help me much. I’m trying to figure on the exact lightning building process details. I sincerely hoping to see some answers and discussion as i’m believing the topic could be potentially a great “pocket” piece of FAQ for anyone who might be looking in this direction for the answers as well.

I’ll put it straight with my question first: I’m having a test scene with no static or stationary lights (sky is also set to movable). Why the lightning building is taking a lot of time (approximately 3% per hour)? It’s the time while the Swarm is calculating some Process Mappings. What things are actually happening in the Swarm and the Lightmass processes, is there a documentation? By what criteria the Swarm kicks in (i’m compiling on a single machine)? Also, i’m getting the “LogGenericPlatformMisc:Warning: Failed to determine engine directory: Defaulting to …/…/…/Engine/” error on the v4.12, but the compiling seemingly going fine and showing no errors on. What are the possible flaws to the building process it might be giving?

A few tips to the others who might be having this kind of questions as me about lightning compiling issues, which i figured reading the other similar threads:

-Check the working processes when building. The Lightmass should be running and loading your CPU well. If you have a powerful CPU and believe there are unused hardware potential, you might want to set the Lightmass process’s priority to High (PC). Setting it to the Realtime is generally not the best idea though, and may actually slow your building as it may cause the system stutterings.

-Make sure the Lightmass is having the Network access, even though you making your build locally.

-In the Swarm settings (just double click it when the Swarm is running): check for the DeveloperSettings, set the priority to AboveNormal.

Well, this is all i can think on the topic so far. Please share your knowledge and thoughts, and if this discussion turns to be somewhat worthwhile to the community - i’ll be adding the vital parts to the sheet, hopefully making it a short community FAQ.

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I’m wondering myself what “Processing Mappings” means in Swarm. And why there ist a huge gap between Fetching Photons and beginning the processing in production quality (not present in preview quality).
And when there will be GPU processing Lightmass.

Thanks for the tipps with the higher priority.

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A bit of an UPDATE.

If the bForceNoPrecomputedLightning function is applied (World settings: “Force No Precomputed Lightning”), it’s returning the message: “…enabled, build lightning once to propagate the change (will remove existing precomputed lightning data).”. So obviously until this is done manually your level will hold some precomputed lightning data. Wondering if that junk cache files would migrate to the release compiled project if the option is not enabled (think of a mix of some dynamic lightning and some stationary in your level, and figure what possible junk cache the engine is adding to your final compilation and what is not a junk).

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Hi, interesting questions! I’m having some trouble with lightmass at the moment, in that I’m getting vastly varied build times. I have an answerhub question here: Difference in Lightmass build times/"lightmass stuck at %" - Rendering - Unreal Engine Forums

I am also wondering, like you, if there is some cache folder filled with junk somewhere. I’ve tried manually clearing the swarm cache, but it seems to have no effect. I would like to try the bForceNoPrecomputedLightning thing you mentioned to see if there’s a potential to remove junk that way, but at the same time if it’s building within a reasonable time on another similar machine, it shouldn’t really be a problem.