2.1 KiB
RasterOverlayTileProvider
Loads or creates a RasterOverlayTile to cover a given geometry tile.
The rest of the 3D Tiles engine calls getTile, giving it the exact rectangle that the returned tile must cover. The tile provider has a Projection, and the provided rectangle is expressed in that projection.
getTile returns a RasterOverlayTile, but does not immediately start loading it. The 3D Tiles engine calls RasterOverlayTileProvider::loadTile to kick off the loading process. getTile may return nullptr if the provider has no data within the given rectangle.
While the returned RasterOverlayTile is loading, the 3D Tiles engine will use the geometry from the parent geometry tile. The RasterOverlayTileProvider doesn't need to concern itself with this at all. It is handled by RasterMappedTo3DTile.
RasterOverlayTileProvider also, in general, does not need to do any caching. The 3D Tiles engine will only call getTile once per geometry tile.
getTile internally calls the polymorphic loadTileImage. Derived RasterOverlayTileProvider classes implement this method to kick off a request, if necessary, then decode the result and provide the decoded pixels as a LoadedRasterOverlayImage. All the lifecycle management is handled automatically by RasterOverlayTileProvider, so that derived classes only need to implement this one async method.
QuadtreeRasterOverlayTileProvider
Derives from RasterOveralyTileProvider and provides an implementation for RasterOverlayTileProvider::loadTileImage. This implementation looks a bit like the old mapRasterTilesToGeometryTile. It figures out which quadtree tiles fall inside the rectangle passed to loadTileImage. Then, it starts an async load of each of these tiles and waits for them all to either finish loading or fail to load. For the ones that fail to load, we try a parent tile, which may require more waiting. Eventually we have tiles that cover the entire rectangle, and we can blit them to the output texture. loadTileImage is an async method start to finish, so this is straightforward. It doesn't require any tricky lifetime management.