Instanced 3D Model and Composite Tile Formats about 3d-tiles

[pjcozzi]

We'll also want to be able to support LOD where instanced models are rendered when zoomed in close, and billboards are rendered when zoomed out. I think the right tile.json layout (e.g., with i3dm tiles at the leafs and vector tiles with billboards in non-leafs) will work.

[lilleyse]

Updated batchTableLength to be the length in bytes rather than the numbers of elements.

[lilleyse]

Some questions:

• glTF can be embedded binary or a url. Should Batched3DModel also allow both for consistency? Should Instanced3DModel only allow a url?
• Naming conventions: usually we use length to specify byte length but instancesLength refers to the number of instances. Should we use byteLength instead?

[pjcozzi]

+1 on byteLength. We should also rename it for b3dm.

[mramato]

Would we be able to specify any kind of scaling factor for each instance as part of the tile, or would that have to be done client side after-the fact? For example, I may have 100k trees that all use the same model but each tree's height and radius might be different based on their size and age. It would be convenient to bake that information into the tile rather than have to store the meta-data and do extra work to scale them every time the tile is loaded.

[pjcozzi]

Yes, scale will be a semantic in the Batch Table just like, for example, b3dm has per-building heights.

[pjcozzi]

• Initial Cesium implementation for i3dm tile: CesiumGS/cesium#3059
• Initial Cesium implementation for composite: CesiumGS/cesium#3158

[pjcozzi]

• Composite spec: #31

[pjcozzi]

glTF can be embedded binary or a url. Should Batched3DModel also allow both for consistency? Should Instanced3DModel only allow a url?

Discussed offline with @lilleyse. No changes here. b3dm embeds binary glTF since it is unlikely that tiles would reuse the same model; i3dm has the uri option to share models across tiles and the embed option for a single tile use case.