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When do you need to lightmap meshes?

When exactly do meshes need to be uv unwrapped and lightmapped for Unreal Engine? Because every time I do unwrap a model and import it, the mesh then starts to have weird seems. I went to the lighting channels and unchecked all of the channels; now the objects look good. When are you suppose to actually make a lightmap?

Only / everytime you hit the “Build” button in the UE4 editor (and did not use the “auto lightmap function” of UE4 at import). Usually you need a UV map for each mesh (static, skeleton, …) so your texutres know where they should be placed. In this UVmap you could overlap like a boss. For example if you would create some bricks again and again UE4 is fine with it. However if you need some lightmap (if you hit that build button) it would suck if one block that is shadowed at some corner is even shadowed at some flat plane in the mid of your map just becaue the tiling is at the same place. That’s why we like a second UVmap for the lightmap. We could do every crazy stuff with our textures for color, metalic, AO, normals, … but the light gets its own place for every single face on our mesh. It could be painted bright or dark. It even does not matter that much if it’s scaled the same way like the color UVmap … it just have to take care that every edge and every corner got it’s own place for some bright light or dark shadow addition.

If your mesh only contains one UVset I believe UE4 will do an automatic second unwrapping for lightmaps. It can cause problems, so it’s best to do a lightmap UV channel manually. Do you need it? maybe not. And as far as I can tell it’s only used for static lighting anyway so if your game is fully dynamic lighting you could probably get away without it.

Whenever you have static/stationary lights and static Static Meshes.