Geographic Polygon Collision Clipping works in 5.1 but not in 5.4


we use Cesium in Unreal 5.1 (Cesium 2.4.0) and want to upgrade to 5.4 (Cesium 2.5.0) . We found out, that the clipping with cartographic polygons does not remove the collision anymore.

We need the collision removed because we are placing an airport with it’s own collision at the clipped position. Outside the polygon the aircraft should still be able to collide with the landscape.

Is this a bug and will this be fixed?

The behavior in Unreal 5.1

The behavior in Unreal 5.4

We found out, that we get a properly clipped collision when the polygon is above a certain size. That means a polygon under a certain size threshold that clipped perfectly in 5.1 does not clip in 5.4 anymore.

The clipping is a Material trick that sets the Opacity Mask to 0 for the area that should be removed. So it doesn’t remove geometry in any Unreal Engine version.

However, as long as ExcludeSelectedTiles is checked on the CesiumPolygonRasterOverlay, tiles that are completely clipped (no part of them is still visible) will not be rendered at all, which effectively means their collision won’t exist in the level, either.

So I’d suggest first checking that the state of that flag is the same in both UE 5.1 and 5.4.

Also, anything that can affect the selected Level of Detail of the terrain surface can also affect this, because tiles at different levels of detail are different sizes. So if you’re using a slightly different tileset, or if the Maximum Screen Space Error property is set differently, or if the view is a little different, then that could do it.

If that still doesn’t explain it, it might be worth trying to use Cesium for Unreal v2.5.0 in UE 5.1 to see if that makes UE 5.1 break, too. v2.5.0 for UE 5.1 is not available from the Unreal Marketplace due to Epic’s rule that you can only publish updates for the three most recent Engine versions, but you can install it by following the instructions here:

Hey everyone!

Did you find any solution for your problem?

We ran into the same. Cutting Tilesets in 5.1 worked properly, but as soon as upgrade to 5.2 was done, the collision won’t be disabled at these cutted areas.

Hi @Riwa_Unreal, welcome to the community!

I would first refer to @Kevin_Ring’s previous post. We actually never implemented support for collision clipping in Unreal.

You may have experienced no collisions because of the Exclude Selected Tiles property on the CesiumPolygonRasterOverlay. Tiles that are completely clipped by the polygon (i.e., no part of them is visible) will not be rendered at all, so their meshes won’t exist in the level. I would double-check that the state of that flag is the same in both UE 5.1 and 5.4.

As Kevin also suggested, you can try to compile later versions of the plugin for UE 5.1, to test if something did truly break.

Let us know if you have any follow-up questions or concerns!

Thank you for the welcoming Janine and the fast reply!

I will try downgrade the project again and following these instructions.
That’s funny and good to know. To walk trough external imported IFC-Buildings we used these polygons to cut small pieces out of the tilesets. Worked great till the engine update.

At red position there is a big house which haven’t blocked the customer from walking inside^^

We now have tested all matching Cesium versions (2.4.0, 2.4.1, 2.5.0, 2.6.0) for UE5.1 and 5.2 and came to a conclusion. This might be help for the cesium team:

Cartographic Polygons in Cesium 2.4.0 definitively cuts geometry (or at least the collision), even if it is not intended as you said:) Testet both, in UE5.1 and 5.2

C.P. in Cesium 2.4.1 and all later versions does not cut. Also it does not set the Opacity mask 100% correctly. If the user walks nearby the cutted parts, these partly appear visible with collision. See on screenshots with Ces4.2.1 in UE5.1
The behavior is unrelated to the imported IFC-Building.

For further ongoing, we want to ask if you plan to include such feature as “clipping geometry” in any future CesiumForUnreal version?
Since our project is growing and this feature is a must-have for our customers it would be great to know.