Manual LODs and matching UVs
polycounter lvl 6
I can't believe I'm still struggling with this after all these years 
I'm working on a large prop and preparing to create the UVs, bake the base texture maps, and create the LODs. What I can't seem to figure out is the process of doing it. I'm using Houdini after switching from 3ds Max. The base model is pretty much done, and I intend to use UVLayout to make the UVs, and Substance Designer to bake and create the textures. So far so good.
But at this point I'm also trying to figure out how to set up the LODs. The thing is, automated LODs in UE4 work fine for at distance, but I still need one or two manual LODs for close up. The base model has quite a lot of tris and verts, as it's a Hero prop, so I intend to have at least one, maybe two, LODs that serves as the main in-game version. So I created LOD1 manually, which was a cinch with Houdini's workflow.
But that's just the mesh itself. Anything I do to simplify the base LOD will either remove or ruin the UVs anyway. Making the UVs is one thing, but how can I make sure that the UVs of the manual LODs match the UVs of the base LOD? Just unwrapping each LOD the same way (same seams), and then flatten, move, and align the different islands to the UVs of the base LOD? Is that practical?
How do other artists create manual LODs and unwrap them?
Replies
I did pure LOD'ing for a year, and never had much of a problem apart from realising the edge I had just removed was a border in the UV's (resulting in stretching).
@Violet makes good points
I've built / supervised the building of thousands of AAA assets with multiple (up to 7) manual LODs and am confident in saying that the quality and efficiency of your LODs is determined almost entirely by the UV layout.
I generally give this advice to new starters etc..
Build a low LOD, UV it and follow that layout for the high one. ( You can shove the bits that aren't in the low off to one side)
This makes manual Lodding easy because you can simply tear geometry out without breaking UVs and also means autolodders can do a better job as they're not hamstrung by disparate UV layouts.
It also means your low LODs look good so you can bring them in earlier and make the game go fasterer
You will get shit and/or inefficient low LODs if you layout UVs based on the high LOD without planning for the structure of the low LOD.
Everything works better if you have the low lod in place before you start unwrapping.
Most of the problems you are trying to solve will simply disappear using this approach.
The main reason for using the low lod is to define the major seams - which you have identified.
Our priorities may differ a little -
My experience is that a player spends more of their time looking at medium and low LODs than they do high LODs - as such I tend to prioritise hiding lod transitions and maintaining quality in the middle of the lod chain over perfect, undistorted UVs on the higher LODs.
In terms of how I usually do it - I use the planar map gizmo on selected faces a lot and accept some distortion on bevels - using a constant sized mapping gizmo means texel density remains consistent so its easy to lay mapped sections over top of each other.
Any faces in the higher lod that are perpendicular or at steep angles to the planar gizmo get torn off and shoved into gaps where they fit - Ill try to overlap and reuse uv space on those as much as possible. Anything that significantly affects silhouette will be defined in the lower LOD so should already have some space allotted.
Mesh normals can be projected in the same way to minimise lighting artefacts during LOD transitions.
I also work on the assumption that texel density will be constant across all similar assets so I'm not trying to fill the page, I'm making things the right size and taking a guess at how much extra space I'll need for the extra bits of high LOD when I decide how big my map needs to be.