That’s a crazy amount of UV islands for such a simple mesh… You should probably merge a lot of those if you care at all about efficiency… Every UV seam doubles the vertices along the seam.
Yes, your new issue is just an artifact of how lightmass works. Basically it has to process each mesh on a different thread that doesn’t have information about what the other threads are doing, so you end up with discontinuities in the lighting. You can read more about it here but unfortunately all the images are missing as they didn’t survive the forum migration.
There are 4 main ways to deal with this:
- Use higher quality lightmass settings (will have a huge impact on your light build times, especially in complex scenes).
- Design your mesh pieces in a way that the seams won’t be noticeable
- Merge the meshes where the seams are occuring into a single mesh. This will ensure Lightmass treats them as a single mesh. Lightmass won’t be able to distribute its work as efficiently if you do this, but it’s still most likely going to be way faster than using higher quality settings.
- Use GPU Lightmass instead of CPU lightmass. It doesn’t suffer from this issue. But it is still in development and is missing a lot of features, especially if you are in UE4. It also requires a raytracing capable GPU.
Some examples of higher quality settings and their results (including the light build time on my 3900x):
Most of the time my advice would be to either design around it (usually impossible for floors) or merge your meshes. But it’s your call.