Square Light Artifacts


I’ve been working on this project for a while now in my spare time. It started in an earlier 4.xx version, but it’s in 4.24 now. I finally got around to rebuilding the lights, and I’ve noticed these strange square artifacts on the ceiling in this corner. It only happens in this one spot on that one mesh. I tried re-importing the mesh and it has the same problem. I have similar meshes throughout the project and the light is a blueprint that I have throughout the project as well.

It looks different than the usual splotch artifacts I’ve seen others post, so I’m assuming it’s something different.

Any help is greatly appreciated! Constantly rebuilding the lights and coming back hours later to find that whatever I tried had no effect is starting to wear on me :frowning:


Doesn’t exactly look like it, but what is the lightmap resolution set to on that ceiling?

This one is 1024, which is probably a higher than it should be. I believe the version I imported previously was set to 512 or 256 and it still had issues.

Well, I would knock that down gradually by dividing by 2 until it’s in the green.
however that shows that the artifacts are not caused by the lightmap.

Could it be backlighting? Is the mesh and material double sided?

The mesh and material are single sided. I tried using a double sided material at one point but that didn’t seem to make a difference.

Does the light position affect it? It coild be some issue with a light profile…
other than that really not sure.
maybe the light map UVs are bad.

I found something that works!

It could be that the lighting UV’s were bad. I admit that I’m still new to Unreal and am coming from a background of traditional rendering. The geometry comes from SketchUp and I’m using datasmith to import it, so it’s not the best content in the world.

I made two changes, so it could have been either one. I tried combining the geometry of the wall with the ceiling to make it one mesh; my theory is maybe there were some weird light bounces between the two since it’s at an acute angle. There was also a tiny hole where the wall met the floor near the window that I filled in…maybe a thing beam of light was coming in to cause the weirdness.

Either way, I’m happy it’s finally fixed and hopefully this post can help someone else down the line! Also, thank you for your help!


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I was thinking it appeared to correspond with the translucent glass surface’s position in terms of bounced / refracted / transmitted light. It could’ve picked up some of the lighter coloring in the translucent glass, scattered it on the ceiling some, and the calculations were off somehow so it rendered as an artefact.

One final insight on what the real cause has to be:

I just noticed after using Unreal’s mesh editing tools that the problem came back. I used Unreal’s mesh editing tools to change the material on the wall to tile originally. This is most likely the cause since the problem didn’t occur until after I did this the first time. Before I re-imported it combined with the ceiling, I fixed the mesh first. It was fine until I went to modify a couple faces on the new mesh and after another lighting rebuild, it had some new weird artifacts. Unreal’s mesh editing tool must have screwed up the lighting UV’s of the wall somehow causing weird light bounces on the ceiling.

I think I’ll be doing all of my mesh editing outside of Unreal from now on

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Hola!! es probable que algún plano esté flipeado en tu maya 3D, con flipear el plano por donde entre la luz se soluciona!!

I just did some testing, and it seems the biggest factor causing this pixelated lighting (square sparkle shapes in the shadowed areas) is from geo which is intersecting and especially having the full face bleeding out from the intersection point (likely the ceiling in OP’s example). This catches indirect/bounced lighting rays from the directional light, or other large light which touches the many surfaces.

The fix has been to delete stacked faces on geo. There should be no top face or thickness to that shower wall tile’s wall. The ceiling should also stick out as short as possible past the shower wall, if the ceiling and walls are not welded together. If there is a hamburger model, the interior buns and paddy would all have their interior faces deleted (the portions un-scene). Better yet, the mesh should be built air-tight with no interior faces, for optimization. It seems that lightmass, especially with directional lights, rewards artists for building their art very cleanly (i.e. welded together, no long triangle slivers, verts un-welded next to each other, submerged faces, etc.).

Shout-out to the Not Yet Lightmap Tools plugin for helping me troubleshoot this issue 50x faster.

I too have been getting those square artifacts in my datasmith imported Sketchup model though a lot worse than the image attached to this post… interesting… I did notice that making geometry double-sided help to reduce the issue a bit but not completely. I’m going to try that principle of “no internal faces” a go and see if that improves things. Thanks guys for investigating this and posting :pray:

If you’re getting that kind of artifact it is due to overlapping lightmap UV’s, not sure how Datasmith works with the lightmap UV’s, maybe it’s just a particular mesh that isn’t unwrapping well

I am having similar issue. I am guessing this is happening because Sketchup is not good with UV Mapping (I am not sure if sketchup texture mapping is even related to lightUV Mapping) and some meshes imported to Unreal get their LightMap generated by unreal and dont turn out good. Nevertheless was anyone here successful in finding a good fast solution to bypassing this? I have been struggling for 2 week. I have tried deleting the UV Map 1 and regenerating it, sometimes it works and sometimes it won’t, i have realised that the squares are like stamped into the lights in the level that cast onto the surface because i tried deleting the lights and adding new ones and that solved it sometimes but with generating a new lightuv map to the surface. There should be an easy way to bypass it i just cant figure out how yet.

You can always check what the lightmap UV’s look like by opening up the static mesh and enabling the UV view
If existing UV’s are bad, then the lightmap UV’s that Unreal generates will also be bad because it reuses existing UV’s to create them, it’s not a complete automatic solution due to that.
However, for you it kind of looks like your issue is more due to low lightmap resolution, the default is 64x64 which is pretty low.