Ue4 create lightmap in third uv Channel, will it use it?

Development Rendering
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1

If I import an fbx model that contains two separate models with two unique uv texture channels but I use the import options “combine meshes” and “create Lightmaps”, I then get a single model with two texture uv channels (UV Channel0, UV channel 1) and a third channel (UV channel 2) created by ue4 for Lightmaps.

As we all know, the second uv channel (UV channel1) is reserved for LightMaps. However, since ue4 created the lightmap channel in the third uv channel slot, will ue4 be smart enough to use channel 3 instead of channel 2 for lightmaps?

I do understand that best practice is to combine meshes in my 3d software where the uvs are layout into UV Channel 0 and create a second channel for Lightmaps. However, On occasion, I want to edit objects without combining its parts as it makes it easier to adjust. Therefore, I export from maya together as an fbx and import into ue4 as a combined mesh. However, the uv channels do not get combines and you end up with multiple uv channels. When I tell ue4 to create the Lightmap uvs, it understandably creates a uv channel in the last position. This is where my question comes into play. If Ue4 creates the lightmap uv channel in a position that is NOT in the second uv channel, will it automatically set the lightmap channel to that new channel, or will it still default to looking at the second uv channel for Lightmaps?

2

any channel you tell ue4 to use as a lightmap will be the lightmap, this can be channel 0, can be channel 8. so no worries about that.

3

To answer my own question, it does appear that ue4 is smart enough to assign the correct lightmap channel to the newly created lightmap via the Destination Lightmap Index in that objects build settings. By default, this index is set to 1 for the second UV Channel. However, since One of my models was already using that slot, it automatically created the Lightmaps and set the destination index to 2.

It is the destination index that defines the lightmap channel address. Therefore, if you ever have a problem with your Lightmaps not showing up, check to make sure that your lightmap uv channel matches to the destination index. If not, switch the index.

4

Thanks, just noticed your answer after I submitted my own.
Just to elaborate on this answer, the way to tell UE4 which channel to use as the Lightmap uv channel is by setting the Destination Index located within that meshes build settings.