cesium-native/doc/implicit-tiling.md

In 3DTILES_implicit_tiling, the root availability subtree describes:

1. Which tiles exist at levels 0-n.
2. Which tiles have content at levels 0-n.
3. Which tiles at level n+1 have further subtrees.

cesium-native assumes that any tile holding a further subtree exists, but it doesn't assume that it has content. To determine if a tile at level n+1 has content, that tile's subtree must first be loaded.

This "gate" is implemented in Tileset::addTileToLoadQueue. A tile without a loaded subtree will not actually be added to the load queue, and will remain in the Unloaded state. However, it will - by other mechanisms - be added to the subtree load queue.

Thus, a tile which is the root of a subtree will wait in the Unloaded state until the subtree data is fully loaded, and only then will will the content be loaded, if it exists. If the availability data indicates the tile does not have content, it will be immediately transitioned into the ContentLoaded state without any network requests.

This is really important, because it ensures:

1. We don't call createImplicitChildrenIfNeeded before we know whether the implicit children exist. And that we do call it after we know whether or not the implicit children exist. This method is only called once on the transition from ContentLoaded to Done, but the mechanism above ensures we never arrive at that state too early.
2. We don't erroneously upsample tiles that seemingly have no children, but that will once we load availability.

Application to quantized-mesh and layers

In quantized-mesh, we already know that a tile has content available, but then we learn about availability in deeper levels from the content of that tile. We can't prevent content load until we have availability data, because then we'd never have content or availability data. However, getting both back at the same time is nearly as good. We just need to make sure we load the tile in any layers for which this tile is at an availability level, and add the availability data to the appropriate context, prior to allowing the tile to enter the ContentLoaded state. That way we will always have the necessary availability information prior to calling createImplicitChildrenIfNeeded. This currently happens in Tileset::requestTileContent.

Level skipping with implicit tiling and quantized-mesh

Because createImplicitChildrenIfNeeded is only called after a tile hits the ContentLoaded state, implicitly-tiled tilesets can't skip levels. They always have to load level zero tiles before they can load the level one tiles. This is in contrast to explicit tiling, which can skip levels and load more detailed tiles first.