Lightmap UV
Second UV channel for baked lighting
Also known as: Lightmap UVs · UV channel 1 · Lightmap resolution · Overlapping UVs warning
Definition
A lightmap UV is a second UV channel used to store baked lighting on a mesh. Unlike texture UVs, it must not overlap: every face needs its own space inside the 0 to 1 area, and islands need padding between them. In Unreal Engine it is UV channel 1 by default. Overlapping or missing lightmap UVs cause black blotches, streaks and incorrect shadows in baked lighting.
In production
Unreal can generate lightmap UVs on import (Generate Lightmap UVs), which is fine for simple props, but hand-made ones give better control on hero assets. Keep the packing efficient and set the lightmap resolution on the mesh to match its importance on screen. Fully dynamic lighting with Lumen reduces the need for lightmaps, but projects with baked lighting, mobile and VR still depend on clean lightmap UVs.
