Add an ellipsoid tileset loader for flat terrain generation

[jqntn]

Hi,

This is a proposal to implement an ellipsoid tileset loader. It leverages the custom loader API and allows the Cesium3DTileset to generate terrain by creating an ellipsoidal surface.

I believe this is equivalent to EllipsoidTerrainProvider in CesiumJS.

In my opinion, this can be very useful, especially on low-end devices, or if you simply don't have a tileset at your disposal.

Here is a Unity example using the raster overlay, which you can try on this branch using this version of the cesium-unity package:

CC @carlopiersanti

[azrogers]

Hi @jqntn - thanks for the pull request! Haven't dug through this yet, but I'd like to point you to PR #890, which is almost in, that adds custom ellipsoid support to cesium-native. For cesium-native, this means instead of passing the ellipsoid as a separate parameter, you can access it from TilesetOptions::ellipsoid. This will make sure it matches the ellipsoid that the georeference is using.

Alternatively, if you see a circumstance where you'd want to use a different ellipsoid for terrain generation than you use for georeferencing, I'll point you to CesiumGS/cesium-unity#465. With that PR, you can change "From WGS84 Ellipsoid" to "From Ellipsoid Asset" or something of the sort on Cesium3DTileset in Unity, and add a property to it that takes a CesiumEllipsoid asset (a ScriptableObject that contains the ellipsoid's radii).

Regardless, this looks good on a quick skim - please @ me when you're ready to have it reviewed!

[jqntn]

Hi @azrogers, thanks for pointing me to #890. I'm ready to make appropriate changes on my end as soon as your PR is in👍

[azrogers]

@jqntn Looks like #890 just made it in!

[kring]

Thanks @jqntn! I have a few suggestions for changes below. Let us know if anything is unclear, or if you want some help with any of it. We really appreciate your work on it so far!

[kring]

I'm moving this out of the July release for now, to give time to make a few more changes.

[jqntn]

Hi @kring! Any update on this?

[j9liu]

Hi @jqntn, sorry for the delay. Thank you for implementing @kring 's suggestions! We'll get a pair of eyes on the changes as soon as we can, thank you for your patience 🙏