DirectionalLight bleeding through hollow BSPs


I created a simple scene, including a movable DirectionalLight and an hollow box BSP (with a wall thickness of 10). I don’t understand why is the light bleeding to the inside of the box. I took a screenshot, after switching the DirectionalLight color to red to highlight the problem:

It’s pretty much the same issue as in [this thread][2], except building the lighting did not help.
I’m surprised I couldn’t find a solution by myself given how simple the issue is to reproduce. Any hollow box with a wall thickness of 10 on a level with a DirectionalLight has the same problem. The only workaround I found is increasing the WallThickness to 40 at least, but I don’t want to have to use such thick walls everywhere.

Am I doing something wrong? Did I miss something? I’d appreciate your help a lot.

did you try and increase the lightmap resolution for the brush surfaces?
but remember the lower the number the better is the resolution for BSP geo

I did. It’s defaulted at 32, I tried 64, 128, 2, and as far as 1024. No change.

I’ve been looking for a solution today, reinstalled 4.5.1 and saw that the problem is not happening on that version (I’m using 4.6). Given there are other problems with BSPs on 4.6 (snap to ground doesn’t work anymore), I believe the lighting problem I have is caused by a bug in the current version of the engine.

When looking about the snapping I saw another thread with an answer from the staff saying the bug has been fixed in 4.7. I guess I’ll just wait and hope that the lighting one will be fixed too.

This might be obvious but, if you make a BSP object, each surface of the BSP object has a separate lightmap resolution setting - you have to change all the ones which might be causing the problem (I thought there was only one setting for each BSP object).