Unreal Engine 5 - Clipping Raster Polygon and GI Problems

Hi everyone,

I’m encountering an issue with the Cesium Georeference plugin in Unreal Engine 5. I’ve set up a clipping raster polygon that clips Google Maps data, but I’m running into problems with Global Illumination (GI). When the Google tiles data is visible, the GI on the surface of my model appears black. However, when I hide the Google tiles data, the GI works normally, and the model is properly lit.

Has anyone experienced a similar issue or have any solutions for this problem? Any advice or suggestions would be greatly appreciated!

Thanks in advance.

I’m not very familiar with Unreal’s GI support. Can you walk us through how to set it up in the Cesium for Unreal Samples project so that we can see the problem ourselves?


I’m not an expert on Unreal’s GI implementation either. However, since I’m a 3D enthusiast and actively learning UE5 myself, I’d like to share some thoughts on this issue from my perspective.

Global Illumination (GI) is controlled by quality settings.

As you can see, at medium settings, GI, powered by Lumen Scene as I understand it, is significantly simplified compared to High and Cinematic settings.
For some reason, the clipping zone defined as a polygon (Cesium Cartographic Polygon → CesiumPolygonRasterOverlay1) seems to hinder GI from receiving lighting data, as if a transparent polygon is still blocking light.
Here is - LightingOnlyPass.

The geometry of 3dTiles is still here and blocking the light.

So. If the Google 3D Tiles landscape is placed above or obstructs the imported geometry, Lumen Scene GI is not calculated correctly. Since the clipping zone in the form of a polygon is a rasterized transparency window on the scanned landscape, it still works as geometry for GI for Google 3D Tiles in the sense of casting shadow.

This is just my subjective take. I might be completely wrong.

If you have any ideas on how to fix this, please give advice)

I hope this helps! Let me know if you have any other questions

Your theory makes sense to me. You’re right that the clipping polygons are essentially a material trick, where the opacity mask value is set to 0.0 inside the polygon. The geometry is still, technically, present. I would hope Unreal wouldn’t allow completely transparent geometry to block light, because that would cause artifacts in other cases, such as opacity-masked leaves on a tree, for example. But maybe it’s challenging to do otherwise. If your theory is correct, though, it’s something that would have to be fixed in Unreal Engine, right?

We do plan to eventually make clipping polygons actually cut the geometry, but don’t currently have a timeline for that. It’s inherently a much more costly operation.