Good day, Pendali
Let me clear up how auto lightmap UVs work in UE4.
It takes existing UV islands from UV channel you specify, and packs them according to the settings. In the static mesh viewer you have the following things to tweak:
-
Min Lightmap Resolution is the value for intended minimum lightmap
resolution, that will be used with
this mesh. It determines amount of
padding between UV islands. -
Source Lightmap Index is the index of UV channel, that will
provide existing UV islands for auto
pack. -
Destination Lightmap Index is the index of UV channel, where generated
lightmap will be placed.
So your workflow should not be much different. Export your model with two sets of UVs. UV channel 0 should contain whatever mapping you are using for your textures. UV channel 1 should contain UV islands, to be used for lightmap. In general case, angle-based unwrap(I haven’t worked with 3D max, but I assume that is what you are referring to as flatten UVs) will suffice for UV channel 1.
You can check results of automatic unwrap in static mesh viewer.
Lastly, be sure to increase lightmap resolution for the mesh to a sufficient level.
Hope that helps.