I am in the process of creating some models to be put into udk. Its working towards my dissertation experiment and I am very short on time and it seems I have alot to learn :'). I have already created some and the question just popped into my head while I am creating a stair-well. I wish I had asked earlier because it might have saved me on some of the other stuff I have created xD. So..

UDK's lightmass does not like over-lapping UVs. Are those just the lightmap uvs or can I for example at the moment I am creating some stairs can I just overlap the "texture" uv map to obviously save space as you would in a typical model.

So i.e for the stairs I am doing. can i just overlap the "texture/diffuse" uvs (create one stair texture) but for the actual lightmap uvs which I have apperently forgot to do for ALL of my models so far xD unpack and re-pack them all individually. So every stair would have its own lightmap uv space.

Thank you for reading and if you reply thank you in advance

Also hope everyone had an awesome christmas :P

Yes you are right.

Lightmass does not like overlapping on the UV channel that will be used for the lightmap. For example, if you have 3 UV sets, the one that you have assigned in UDK to be used by lightmass, must be the one without overlapping, the others can have whatever they like.

So generally, people create two UV sets, one for their diffuse/spec/etc which can contain overlapping UV's, and one for the lightmap (no overlapping).

You could in some situations, use your diffuse UVs for lightmass if you did not overlap anything in them, and just have the one UV set.

Yeah, overlapping UVs is fine for your texture UVs. It's only the lightmap UVs that need to be unique with no overlapping. Don't forget to put a lot of padding around the lightmap UV shells. I typically put 4 pixels between shells.

yeah so UV channel 1 in max etc should be your texture UVs where you can have overlaying islands. but your UV channel 2 for your lightmap UV's shouldn't be overlapping. remember in UDK channel 1 in max is translated to 0, and 2 in max is to channel 1.

edit: shit moar replies as i was typing so what i said is probally said

I forgot to mention that you should be really careful about mirroring UVs in UDK. If you split a surface down the middle and mirror it's UVs then it's very likely you'll get a visual specular seam after baking lightmass, especially if you mirrored the UVs in a up/down direction and your material is very shiny. It's better to mirror UVs horizontally <-|->.

Ah thank you very much, I am supprised I got so many replies so quickly haha. I am just annoyed I didnt think of it sooner :P.

Yeah I have seen that you need to "pad" them out a bit else I could get bleed through or whatever the term was and I also saw about the max and udk 0 and 1 channel mix up as well :P but thank you anyway. I would have been back here asking why it wasn't working :P.

@Ben Apuna: ah cheers, I have never heard of that sort of thing before. -_- hopefully I dont run into any of those problems although I have never done a "bake" so god knows how many problems I will run into xD.

Here's a thread about mirroring from a while back. http://www.polycount.com/forum/showthread.php?t=75993 Maybe someone has a better solution nowadays.

EDIT:

And some more posts about mirroring in UDK on this page.

n.b.: according to the udn docs, padding towards the outer edges of 1/0 space isn't an issue, since the engine does its own padding here. take that as you will