Oh, I see, thanks Matt. Ok, so I can see the extra tile you’re talking about. I spent a lot of time digging into it, and wrote some bounding volume debug visualizations (I might try to get this into the product proper). My conclusion is that it’s working as expected, even though that may be surprising. Take a look at this screenshot from your level, with the added bounding volume visualization:
The large tile is the unexpected one that appears to be well outside the the CesiumBoxExcluder. The excluder itself is centered near the red/green/blue coordinate axes on the left side of the image, and is too small to see. So why are we rendering this tile that appears to be nowhere near the excluder? Shouldn’t it be excluded? Unfortunately, no, it should not. Let me explain.
The red wireframe box in the screenshot is the oriented bounding box that is actually expressed in the tileset.json for this particular tile. It’s a bit bigger than it needs to be, but it actually looks worse in this screenshot than it really is (a trick of perspective). The Google tileset has a reasonable - if not ideal - bounding volume for this tile.
The CesiumTileExcluder, however, doesn’t work with oriented bounding boxes. It uses a bounding box aligned with the Unity world axes. That aligned bounding box is shown with the green wireframe. It’s quite a bit bigger than the red one, but necessarily so. It’s the tightest-fitting axis-aligned bounding box that fully contains this tile’s oriented bounding box.
The CesiumBoxExcluder tests this axis-aligned box against the exclusion volume, and correctly determines (as we can see in the screenshot) that the excluder volume and the AABB overlap. So that is why this tile is not excluded. And, having not been excluded, it’s also rendered because you’ve turned off frustum culling.
You can actually improve the situation by turning on “Enforce Culled Screen Space Error”. With that option OFF, tiles outside the view frustum are not refined at all. So we tend to select very large, low detail tiles like the one in the screenshot. These large tiles have large bounding volumes (especially once they’re axis-aligned), so the CesiumTileExcluder tends to deem them not excluded.
With the option ON, however, these large tiles will be refined, so their smaller, higher-detail children will be selected instead. These child tiles have much smaller bounding volumes, and so they can be more readily excluded by the CesiumTileExcluder. The downside is that more tiles will need to be downloaded.