Large worlds are typically better using dynamic lighting. Trying to build lighting for very large worlds, even with multiple lightmass importance volumes, is a intense process. You will more than likely run into hardware limitations doing this where the computer will run out of memory. Using build farms will make this process easier to split the tasks.
Epics latest GDC Demo with Kite World, which is a large open world at 100 km squared, is using solely dynamic lighting. No pre-computed lighting at all. Likely you would run into texture streaming issues since all the lightmaps for the objects in the scene would need to be streamed in as well. This would probably cause more of a bottleneck or performance hit than using dynamic lighting.
This document goes over what a Lightmass Importance Volume is and why it’s useful along with all things Lightmass oriented. This was linked above.
https://docs.unrealengine/latest/INT/Engine/Rendering/LightingAndShadows/Lightmass/index.html#lightmassimportancevolume
Take the time to read through the documentation above and you should come away with a clearer understanding of how this works with UE4.
Some things to consider.
- Large worlds are a very specific thing and not specifically something that covers a lot of lightmass issues. Again, large worlds are typically using dynamic lighting (Skyrim, GTA, Fallout, Assassins Creed), as this is going to be a better solution than thousands of baked lightmap textures streaming.
- Moderately sized worlds can be OK to use lightmaps on and by using multiple Lightmass Importance Volumes you can focus the photons that are cast (explained in the documentation above) to a specific area rather than the entire level. Not everything needs to have the best looking shadows if the player is not able to access the area. This is the point of the lightmass importance volume.
- The trouble shooting guide that Eric and I have been working on is geared towards more common trouble topics for basic issues. This is not a guide that was asked for by Epic, but instead something that we put together to help the community since these questions come up often. While we plan to expand this with many more topics we do not plan to do anything beyond educating users in basic lighting issues and tips with clear examples.
Even though you added the edit I still wanted to address some of the concerns you had mentioned before hand.
Using some of the tips and troubleshooting topics that we’ve already addressed in the Wiki Lighting Troubleshooting and Tips guide should help with dynamically lit large worlds.
In time, I would like to do some simple troubleshooting with Distance Fields Shadows as well (some of this is already covered in the DFAO and Ray Traced Distance Field Soft Shadows documentation) along with more general lightmapping and UV things.
Currently the two that I’m more focused on getting examples together for is Lightmass/Swarm Agent Troubleshooting and Modular mesh seams/color variation.
My biggest suggestions out the gate would be:
- Use LODs for your meshes so that dynamic lighting doesn’t have to calculate complex meshes at distances. It’s a waste of resources.
- Don’t expect shadows to work for miles for every object. There is a hard limit for how far cascaded shadow maps can stretch. It’s better to use a combination of DFs and CSM in this respect. Keep CSM at a lower distance, 5000 to 10000 range then use DFs or Far CSM for objects that need them at far distances (Far CSM will be in 4.8).