The 3D Tiles category generalizes hierarchical geospatial data structures. It is being defined for the OGC 3D Tiles standard but may be generalized to handle the OGC i3s standard and the potree format as well.

Concepts

Tile Header Hierarchy - An initial, "minimal" set of data listing the hierarchy of available tiles, with minimal information to allow an application to determine which tiles need to be loaded based on a certain viewing position in 3d space.
Tile Header - A minimal header describing a tiles bounding volume and a screen space error tolerance (allowing the tile to be culled if it is distant), as well as the URL to load the tile's actual content from.
Tile Cache - Since the number of tiles in big tilesets often exceed what can be loaded into available memory, it is important to have a system that releases no-longer visible tiles from memory.
Tileset Traversal - Dynamically loading and rendering 3D tiles based on current viewing position, possibly triggering loads of new tiles and unloading of older, no-longer visible tiles.

Tileset Traversal Support

To start loading tiles once a top-level tileset file is loaded, the application can instantiate the Tileset3D class and start calling tileset3D.traverse(camera_parameters).

Since 3D tiled data sets tend to be very big, the key idea is to only load the tiles actually needed to show a view from the current camera position.

The Tileset3D allows callbacks (onTileLoad, onTileUnload) to be registered that notify the app when the set of tiles available for rendering has changed. This is important because tile loads complete asynchronously, after the tileset3D.traverse(...) call has returned.

Coordinate Systems

To help applications process the position data in the tiles, 3D Tiles category loaders are expected to provide matrices are provided to enable tiles to be used in both fixed frame or cartographic (long/lat-relative, east-north-up / ENU) coordinate systems:

cartesian WGS84 fixed frame coordinates
cartographic tile geometry positions to ENU meter offsets from cartographicOrigin.

Position units in both cases are in meters.

For cartographic coordinates, tiles come with a prechosen cartographic origin and precalculated model matrix. This cartographic origin is "arbitrary" (chosen based on the tiles bounding volume center). A different origin can be chosen and a transform can be calculated, e.g. using the math.gl Ellipsoid class.

Tileset Fields

Field	Type	Contents
`asset`	`Object` (Optional)
`root`	`Object`	The root tile header
`cartesianCenter`	`Number[3]`	Center of tileset in fixed frame coordinates
`cartographicCenter`	`Number[3]`	Center of tileset in cartographic coordinates `[long, lat, elevation]`
`webMercatorZoom`	`Number[3]`	A web mercator zoom level that displays the entire tile set bounding volume

TileHeader Fields

Field	Type	Contents
`boundingVolume`	`Object`

Tile Fields

Common Fields

Field	Type	Contents
`loaderData`	`Object` (Optional)	Format specific data
`version`	`Number`	See Header
`type`	`String`	See Mode
`cartesianOrigin`	`Number[3]`	"Center" of tile geometry in WGS84 fixed frame coordinates
`cartographicOrigin`	`Number[3]`	"Origin" in lng/lat (center of tile's bounding volume)
`cartesianModelMatrix`	`Number[16]`	Transforms tile geometry positions to fixed frame coordinates
`cartographicModelMatrix`	`Number[16]`	Transforms tile geometry positions to ENU meter offsets from `cartographicOrigin`.

PointCloudTile Fields

Field	Type	Contents
`attributes`	`Object`	Values are accessor objects.
`attributes.positions.value`	`Float32Array`
`attributes.normals.value`	`Float32Array`
`attributes.colors.value`	`Uint8Array`

TBA - batch ids?

Instanced3DModelTile Fields

Field	Type	Contents
`modelMatrix`

PointCloudTile Fields

Field	Type	Contents

CompositeTile Fields

Field	Type	Contents
`tiles`	`Object[]`	Array of parsed tile objects

Accessors

Following vis.gl conventions, attributes are represented by "glTF-style" accessor objects with the value field containing the binary data for that attribute stored in a typed array of the proper type.

Accessors Fields	glTF?	Type	Contents
`value`	No	`TypedArray`	Contains the typed array (corresponds to `bufferView`). The type of the array will match the GL constant in `componentType`.
`size`	No	`Number`	Number of components, `1`-`4`.
`byteOffset`	Yes	`Number`	Starting offset into the bufferView.
`count`	Yes	`Number`	The number of elements/vertices in the attribute data.