Overlapping UVs

I’m trying to make tree but when I build lighting I get the UV overlapping error. I’ve generated unique UVs on Channel1 within the editor, but I still get the error and the lighting still appears strange

Is there some setting I need to set to avoid this error for two-sided materials? The Material Properties page mentions “Two Sided does not work correctly with static lighting, since the mesh still uses only a single UV set for lightmapping. As a result, two sided materials with static lighting will be shaded the same on both sides.”


Are you sure that you have a 2nd UV setup correctly? All the materials properties is saying is that you will have the same shadow map on the front that you have on the back. You could try re-packing the atlas using the UV generation tool inside of the editor to see if that help. If that does not work I would suggest that you do this inside of 3Ds Max, Maya, or Blender. You will get much better results. The in-engine tool is more for doing things quickly and is not for generating super awesome results. I use it more when I am testing something out and I want to see what baked shadows will look like.

It says it’s showing UV channel 1. So I know that’s your second uv channel. Look in the details panel to the right and make sure that your lightmap coordinates is set up to be channel 1. The default will sometimes be channel 0 which is your texture UV.

I would recommend you to create it in your 3d program :slight_smile: :

if figured out, if you import your Mesh without a second uv-channel in the beginning, the lightmap index in UE4 stays on 0, if you reimport a new one with Lightmap (2. uv channel). You always need to put the channel from 0 to 1 manually in mesh editor.

Thanks guys. I upgraded to 4.2 and completely re-imported after unwrapping in 3DsMax instead. No overlap errors.

This issue is probably the most frustrating for me right now.

I use Blender to do my meshes, but I have no idea how to prevent UV wrapping errors. All my meshes have two UV maps, one for textures and a second one for the light map I’m assuming. I painstakingly make sure that the 2nd channel does not have have overlapping charts, but building the map with light still says I do.

The thing that throws me off is a mesh that never gave me problems, all of a sudden just starts showing up as overlapped UVs.

What am I doing wrong?

If you build lighting and you get those error messages, check what your mesh looks like, it may look just fine. In those cases I just ignore the error.

After import, you may need to open the mesh up in the mesh viewer, and search for the “light map uv coordinate” property (it’s in an advanced roll-out) and flip it to your other set.

Hi Gibdion,

There are two UVs you’ll need to have for any static meshes (non-movable). The first is used for your textures. The second channel is for your Lightmap and needs to be made (preferably) in your 3D software for the best results.

World of Level Design covers this topic quite extensively to give you a good indication of what is needed!.

Once you’ve created the second UV with no overlapping faces, you’ll need to go into the mesh viewer > in the details panel type in “lightmap” > Change the coordinate index from 0 to 1.

This will tell it to look at the second UV as your lightmap. You can also adjust the resolution here for a default if you’re using this mesh more than once in your game. That way you don’t have to set it for each instance in your scene.

If you want to view your lightmap in the viewer you can click on the UV tab in the toolbar. Then in the dropdown that says UV Channel X change it to 0 for your textures and 1 for the lightmap. This will overlap the layout on your screen so you can see that it’s there.

Thank you!


I had the same problem because I didnt want to create lightmaps for my million trees.

After about a week of fluffing around i found that if i made the tree a blueprint and then added the blueprint tree to the scene, the light worked and no errors were thrown.

Dont ask me why this works but we have been doing it for about a month with all our trees with no problems.