UE4 lightmap index

UE4 lightmap index

Source Index (UV Channel 0): This is literally the source from which the lightmap will be created. Depending on how many UVs you have setup you can specify any one that is needed. Typically there is only one UV for your texture though. All that’s happening here is that when the lightmap is created this is the UV specific channel that will be used to create the lightmap UV.

Destination Lightmap Index (Typically channel 1, but can be specified): This is the UV destination you want to specify. the default is UV Channel 1 as most models only have a single UV for their texture (UV channel 0). If you have multiple UVs, you may not want the lightmap to be assigned to channel 1 and may want to make it channel 2 or more. SpeedTrees use multiple UVs, so having the lightmap be something different would make sense here.