Multi-source terrain provider feature

[juunie-roh]

Feature

Mixed terrain example with worldTerrain and Ellipsoid.

This issue was originated from Cesium Community.

What is it for:

Enables terrain to be loaded from multiple sources according to configured areas(with specific zoom level and tile coordinates) as the image shown above. It also enables setting fallback terrain, an error boundary for terrain provider.

Concept

I have implemented the feature on @juun-roh/cesium-utils, with name HybridTerrainProvider. It works as below:

1. The HybridTerrainProvider extends existing TerrainProvider class.
2. Configure areas to override terrain source with zoom levels, tile coordinates, and TerrainProvider having different terrain sources.
3. Override requestTileGeometry method with configured area inclusion checks.
4. Delegate the method to configured terrain provider's requestTileGeometry, with configured areas.
5. If returns false, proceed with default/fallback provider's.

The overall concept of this feature is to redirect the requestTileGeometry with different providers.

Implementation

You can see my full implementation here.

Briefly, how I redirected the method is by this:

requestTileGeometry(
    x: number,
    y: number,
    level: number,
    request?: Request,
  ): Promise<Awaited<TerrainData>> | undefined {
    if (!this._ready) return undefined;

    // Check regions for a match
    for (const region of this._regions) {
      if (this._regionContains(region, x, y, level)) {
        return region.provider.requestTileGeometry(x, y, level, request);
      }
    }

    // Fall back to default provider
    if (this._defaultProvider.getTileDataAvailable(x, y, level)) {
      return this._defaultProvider.requestTileGeometry(x, y, level, request);
    }

    // Final fallback
    return this._fallbackProvider.requestTileGeometry(x, y, level, request);
  }

Request for Guidance

My implementation does not follow Cesium's API design, since it has a separate development environment, so I'm expecting it to take some time to open a direct pull request about this feature.

Meanwhile, I'd appreciate feedback on this feature:

• API design preferences
• Any architectural considerations I've missed

[lukemckinstry]

Thanks @juunie-roh for opening this issue to guide work on a PR to implement your proposed HybridTerrainProvider feature as discussed on the Cesium forums. This sounds like a useful feature that will help users trying to load terrain from multiple sources for different areas into a single scene. I think the API as you designed it in your external library will translate well to the Cesium library. I also believe your approach for the requestTileGeometry method makes sense. Once you get a prototype working we can assess performance and compatibility with other features, but it bodes well that you created a working demo with your external library implementation.

[juunie-roh]

I've opened a draft PR on this feature! #12822
It's just a translation of a TypeScript implementation; Excluding the experimental Rectangle approach.