Lightmap issue with modular pieces

Hi guys, having a small problem and not sure what’s causing it.

I’ve created a simple modular mesh with 2 UV channels, one obviously for lightmaps with each face it’s own shell, spaced out and snapped to grid (which was also set based on the 1 / Lightmass res)

I’ve attached 2 screen shots. The first is of the mesh tiled along a few times, and as you will see everything looks fine, no seams, etc.

The second is when I put one of the same pieces in, rotated 90 degrees to make a corner in the wall. Seams suddenly appear it between the pieces beside it.

First screen, everything is okay:

Second screen, seams due to rotation of one of the pieces.

As you can see, there’s a slight seam on the left hand side of the mesh that helps make the corner.

So, what causes this? I’ve upped the lightmass res (fixing UVs accordingly), also increased a bunch of the world settings (and, well…decreased, too). Nothing seems to fix this, and I can’t for the life of me figure out why.

Any thoughts/recommendations/help would be greatly appreciated.

Quick edit: Just thought of something that might help - when I rotate that one piece, it doesn’t even have to be touching/intersecting the other piece, and it will still cause a seam. E.g.

This is a known flaw with Lightmass. See:

You will just have to work around it. It doesn’t show up as strongly once you are using textured assets in a less sterile environment, but if you have a lot of uniform small pieces of static geometry with soft lighting, you may need to change the way you are assembling structures if it is a visible problem for you.

Thanks for this info, great help!