Vector Tile: Polygons

[pjcozzi]

See the originial notes for Vector Data.

Considering only polygons and no styling:

Header
------
magic            // uchar[4], "vctr"
version          // uint32,   1
byteLength       // uint32,   length, in bytes, of the entire tile.  See #23
batchTableLength // uint32,   length, in bytes, of the batch table.  Can be zero.

Batch Table
-----------
// Same as Batched 3D Model.  Optional.

Polygons
--------
polygonsLength    // uint32, number of polygons in the tile

There are polygonsLength of these (each is variable length):
* batchId         // uint32, index into batchTable, zero when Header.batchTableLength === 0
* minimumHeight   // float,  for shadow volume extrusion optimization
* maximumHeight   // float,  for shadow volume extrusion optimization
* positionsLength // uint32, number of vertex positions in this polygon
* x, y, z ...     // doubles in WGS84 coordinates
* indicesLength   // uint32, number of triangle indices
* i0, i1, i2, ... // uint16, every three indices form a triangle

This design assumes server-side triangulation and client-side subdivision and shadow volume extrusion. It includes metadata to optimize shadow volume rendering (which makes a huge difference).

Polygons with holes can be represented as multiple polygons (with or without the same batchId) or as one polygon with disconnected triangles.

The geometry is well behaved: there are no degenerate triangles nor are the duplicate vertex positions.

Cesium should render these tiles with RTC instead of GPU RTE.

Easy Questions

• batchId is the first field here, but the last field in i3dm (Instanced 3D Model and Composite Tile Formats #23). Which do we prefer?
• Should indices be uint16 or uint32? When using uint16, a big polygon could be represented as several polygons with the same batchId.
• minimumHeight and maximumHeight are for rendering optimization. Most likely, they should be a semantic in the Batch Table instead.
• minimumHeight and maximumHeight just need to be conservative. Is uint16 more appropriate?

Less Easy Questions

• Tight representation for vertex positions, e.g., RTC, EXT_quantized_attributes, delta encoding, Open3DGC SOA vs. AOS, cartographic, etc.
• As we add more geometries, including geometries with optional fields, will a hybrid JSON/binary approach, like Binary glTF, be better?
• As we add more geometries, do we need to introduce multiple Batch Tables - one per geometry type. Or do we break this into many tile formats and use a Composite (Instanced 3D Model and Composite Tile Formats #23) tile?
• How bad are cracks and what are we going to do about them?
• Can additive refinement just add new vertices to polygons? Perhaps this is a completely different progressive tile format.

[pjcozzi]

Initial Cesium implementation: CesiumGS/cesium#4186

[pjcozzi]

Largely addressed in #124.

For easier tracking, moved the rest of the issues to #25.