Questions about Light Map

ToRuKMactO

Hi guys, this is my first attempt to do light maps, I've read epic docs about it and made a couple researches and learn some rules and techniques to do such thing, but I still have some questions.

1 - My model has multiple UV sets for textures, can I also do multiple UV sets for light maps ??

2 - Can I have massive distortions on that UVs without any poor results ??

3 - I've read that low resolution light maps is more for modulate assets that will be repeatedly placed on level, so can I do light map resolution 512x512 or 1024 or even 2048 for assets that is going to be place just once ??

4 - Again about distortions, following the rules I should leave this part of my object all together, but doing so will result on a massive result, is that correct ?? If no, what would be a alternative for that exemple ??

An automatic light map generated by Unreal I get some issues, I think I can handle with the rest of object, but this very part I am not sure and have no idea what to do

musashidan

You would get much better results if you manually unwrapped the lightmap UVs. You can bump up the lightmap res but at a pretty exponential hit on performance.

Distortions don't really matter on lightmap UVs. Coverage, sufficient padding, and contiguous UV shells are more important. The more seams you have, the more padding you need, and consequently, the less pixel pixel coverage you have. Remember, lightmap UV res is significantly lower than texture res. In some case it could be as low as 8x8 pixels.
In your case I'm guessing there is a UV seam, where the lighting artifact occurs, with not enough padding to prevent the bleed.

ToRuKMactO

Ok, I did some researches and completely changed my layout, some stuffs I did understand some I didnt, but what actually cause those bleedings ?? Someone told me that UV Islands too small can bleed and like you said as well, not "enough" padding. Right now I need to optimize my UV sizes and positions, but I tested just my automatic layout and I do have bleeds, I will do this optimization but I am not quite sure if this will solve my problem since I don't know how and neither why these bleedings are occurring

ToRuKMactO

I still have some bleeding, I made a new layout with padding based in a grid for 32 resolution leaving 2 pixels horizontally and 1 pixel vertically, I re-scaled some small pieces but didn't work, I have less bleeding but I have a few really bad ones

musashidan

You still have a lot of seams. You should try to have as little seams as possible even if this causes distortions(but avoid overlaps)

Having a 1:1 layout isn't important and lightmap layout doesn't follow the usual rules for texture layout.
So you should stitch and relax as much as you can

Less seams= less padding= more coverage and higher resolution = not needing as much in your lightmap res

ToRuKMactO

Every Uv island that you see on the image is a continuous component, I can't have much less seams than this.
Could you tell me what exactly cause bleedings ?? what should be a good or enought padding ??

musashidan

Looking at your bus model you should theoretically be able to unwrap 90% of it into a single UV island. As it is you have about 100 islands. The more islands you have the more chance of light/shadow bleed between islands.

If you have a look here: UE4 Docs Lightmapping UVs
It gives a very good explanation of many different lightmap UV layout scenarios.