README.adoc

Composite

Warning

Composite was deprecated in 3D Tiles 1.1. See cmpt migration guide.

Overview

The Composite tile format enables concatenating tiles of different formats into one tile.

3D Tiles and the Composite tile allow flexibility for streaming heterogeneous datasets. For example, buildings and trees could be stored either in two separate Batched 3D Model and Instanced 3D Model tiles or, using a Composite tile, the tiles can be combined.

Supporting heterogeneous datasets with both inter-tile (separate tiles of different formats that are in the same tileset) and intra-tile (different tile formats that are in the same Composite tile) options allows conversion tools to make trade-offs between number of requests, optimal type-specific subdivision, and how visible/hidden layers are streamed.

A Composite tile is a binary blob in little endian.

Layout

Composite layout (dashes indicate optional fields):

Figure 1. Data layout of a Composite tile

Padding

A tile’s byteLength shall be aligned to an 8-byte boundary. All tiles contained in a composite tile shall also be aligned to an 8-byte boundary.

Header

The 16-byte header section contains the following fields:

Table 1. Header fields for Composite tiles

Field name	Data type	Description
magic	4-byte ANSI string	"cmpt". This can be used to identify the content as a Composite tile.
version	uint32	The version of the Composite format. It is currently 1.
byteLength	uint32	The length of the entire Composite tile, including this header and each inner tile, in bytes.
tilesLength	uint32	The number of tiles in the Composite.

Inner tiles

Inner tile fields are stored tightly packed immediately following the header section. The following information describes general characteristics of all tile formats that a Composite tile reader might exploit to find the boundaries of the inner tiles:

• Each tile starts with a 4-byte ANSI string, magic, that can be used to determine the tile format for further parsing. See tile format specifications for a list of possible formats. Composite tiles can contain Composite tiles.

• Each tile’s header contains a uint32 byteLength, which defines the length of the inner tile, including its header, in bytes. This can be used to traverse the inner tiles.

• For any tile format’s version 1, the first 12 bytes of all tiles is the following fields:

Table 2. Common header fields for tile formats

Field name	Data type	Description
magic	4-byte ANSI string	Indicates the tile format
version	uint32	1
byteLength	uint32	Length, in bytes, of the entire tile.

Refer to the specification for each tile format for more details.

File extension and media type

Composite tiles use the .cmpt extension and application/octet-stream media type.

An explicit file extension is optional. Valid implementations may ignore it and identify a content’s format by the magic field in its header.

Implementation examples

This section is informative

• Python packcmpt tool in gltf2glb toolset contains code for combining one or more Batched 3D Model or Instanced 3D Model tiles into a single Composite tile file.

• Code for reading the header can be found in Composite3DTileContent.js in the CesiumJS implementation of 3D Tiles.