Hey everyone!
I’ve previously brought this issue into the Speedtree forum, but they were sadly unable to identify it, which is why I’m trying my luck here.
I’ve currently been stuck with an issue regarding a couple of modified Speedtrees that were based on some models they sold at their store. Whenever I do a lighting build with basic skylight I’m getting some artifacts on the lightmaps, mainly the trunk. In more lighter areas they look like what appear to be visible UV seams, with a harsh transition from bright to dark areas. In more shadowy areas the artifacts are extremely present, resulting in thick black areas going through the entire trunk.
(Example scene)
(Lighting only)
The light map density of these was entirely green in both the Speedtree Modeller and UE4, with the light map UVs efficiently laid out. So far I’ve tried to specifically increase the trunk’s UV area, but even with a super high light map density and the light map resolution cranked to 1024 I’m still getting these issues. I’ve also tried increasing the lighting quality, as well as skylight bounces to some high numbers, but these artifacts remain.
These trees are modified versions of the Norway Spruce Species Pack from their store, and I first thought that this problem was related to the modifications I made. But to my surprise I had the same exact issue with the unmodified trees straight from the store.
So far I’ve tried checking Recompute Normals/Tangents, as well as cranking up the lightmass quality settings and increasing the skylight bounces, but without any luck. I had a suspicion that it might be related to the low poly nature of the assets, so I also tried increasing the segments on the trunk, but it didn’t make a difference. Something I’ve noticed though, is that these artifacts mostly occur in darker areas. In a bright, open area the trees are coming out just fine for me.
(Black areas on trunks of speed trees, notice how other tree assets look perfectly fine.)
(Artifacts only seem to appear in darker areas, trees in open areas look fine.)
All of these speed trees have a lightmap resolution of 512, I made sure to increase the UV coverage of the trunk until it was completely red to illustrate that it’s not a problem with the resolution itself. The Lightmass settings are turned up to 6 skylight bounces and an indirect lighting quality of 3.0, which is certainly higher than Unreal’s default settings. In addition, here’s a look at the tree itself.
Any help would be greatly appreciated, been stuck with this problem for about 2 weeks now. 
