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Automatic Lightmap UVs aren't snapped to texture edges?

So Yes, ‘another lightmap thread’ it is…

I’m wondering why the automatic lightmap build tool doesn’t snap the UV edges to the texel edges? Or is it not doing so because my default UV co-ordinates aren’t snapped?

See this:

There’s clearly some lightmap bleeding going on on one of the edges there. When viewing the lightmap density, you can see that the auto-generated UVs aren’t snapped to the pixel grid:

Any idea what’s going on here? Is this a missing feature, intentional, or have I done something wrong with my original UVs to mess it up this way?

It’s not done automatically by the tool.

The Unreal generation of light maps adds some margin between areas to avoid bleed. I think you can even configure how much in the mesh inspector tool!
However, if you get extreme MIP map reduction, you may start seeing bleed.

Snapping to the pixel grid isn’t super important, it only works on rectangular geometry and most geometry isn’t like that.