Sustainability of Digital Formats: Planning for Library of Congress Collections

3D Tiles, standardized in the Open Geospatial Consortium (OGC) document 18-053r2 is "designed for streaming and rendering massive 3D geospatial content such as Photogrammetry, 3D Buildings, BIM/CAD [Building Information Modeling/Computer-Aided Design], Instanced Features, and Point Clouds. It defines a hierarchical data structure and a set of tile formats which deliver renderable content." Cesium GS, Inc. first introduced 3D Tiles in 2015 and shepherded its acceptance as an OGC Community Standard in 2019 with version 1.0. The specification and supporting information is also held at the CesiumGS 3D Tiles GitHub public repository which is updated more frequently than the OGC site. Note that a new draft version of the specification is under development in 2022. 3D Tiles, version 1.1, with the OGC document number 22-025r2, is a "Work Item Draft." In addition, development is also underway for 3D Tiles Next. See History for more information.

Benefits of 3D Tiles

Some of the benefits of 3D Tiles, according to Cesium, include:

• interactive selection and styling which allows for individual model interaction such as highlighting on mouseover, or removing a 3D building;
• adaptive spatial subdivision which can subdivide a dataset, for example, based on the cost of rendering each model and the distribution of models, resulting in a balanced data structure;
• flexible refinement options allowing for both replacement refinement (in which visible map tiles are replaced with new higher-resolution map tiles as a viewer zooms in) and additive refinement (in which only new buildings are streamed when a viewer zooms in);
• full-precision geometry to avoid jittering artifacts;
• time-dynamic visualizations, such as of satellites and unmanned aerial vehicles (UAV or drone).

Structure of 3D Tiles

The 3D Tiles specification defines a hierarchical data structure and a set of tile formats which deliver renderable content. In this context, a tileset is a set of tiles organized in a spatial data structure, the tree. A tile is defined as "a subset of a tileset containing a reference to renderable content and the metadata, such as the content’s bounding volume, which is used by a client to determine if the content is rendered." Each tileset is described by at least one tileset JSON file containing tileset metadata and a tree of tile objects. Tile objects can be one of four types of renderable data content as described in the OGC specification:

• Batched 3D Model: Heterogeneous 3D models. E.g. textured terrain and surfaces, 3D building exteriors and interiors, massive models.
• Instanced 3D Model: 3D model instances... [According to section 10.2 of 18-053r2, instanced models or features "refer to a large number of models, called instances, with slight variations. In the simplest case, the same tree model, for example, may be located—or instanced—in several places. Each instance references the same model and has per-instance properties, such as position. Using the core 3D Tiles spec language, each instance is a feature. In addition to trees, Instanced 3D Model is useful for exterior features such as fire hydrants, sewer caps, lamps, and traffic lights, and for interior CAD features such as bolts, valves, and electrical outlets."]
• Point Cloud: Massive number of points [in which each point is defined by a position and by optional properties used to define its appearance, such as color and normal, as well as optional properties that define application-specific metadata. See section 10.3 of 18-053r2 for more details.]
• Composite: Concatenate tiles of different formats into one tile.

Both the Batched 3D Model and Instanced 3D Model formats are built on glTF (GL Transmission Format) 2.0 in which the tile's model geometry and texture information is embedded as a glTF asset. Point Cloud does not embed glTF.

Tiles are organized in a tree structure, following the concept of Hierarchical Level of Detail (HLOD) which means support for "decreasing the complexity of a 3D representation according to metrics such as object importance or distance from the tile to the observation point or camera position" for optimal rendering of spatial data. Enabling HLOD means that "only visible tiles are streamed - and only those tiles which are most important for a given 3D view." Each tile has a bounding volume, an object defining a spatial extent completely enclosing its content. The tree has spatial coherence; the content for child tiles are completely inside the parent’s bounding volume.

More granularly, each tile has a 28-byte header which contains the 4-byte ANSI string magic number of the tile object data type (b3dm, i3dm, pnts and cmpt respectively), version number of the tile data format, length of the tile and more. This header is immediately followed by the body section, composed of a Feature Table and Batch Table. [A Feature Table describes position and appearance properties required to render each feature in a tile whereas the Batch Table contains additional, application-specific properties for each feature which are not necessarily used for rendering.] For Batched 3D Model and Instanced 3D Model data, the embedded glTF asset comes next in the structure.

3D Tiles is widely adopted in 3D geospatial content applications including leading software, service, and data providers across industry and government, and by developers building end-user applications. Cesium, a major open 3D platform provider and sponsor of the 3D Tiles specification work, is built on 3D Tiles integration. Their products support the Australian New South Wales Spatial Digital Twin project which incorporates 3D Tiles into 'web-mapping and visualizing data in 3D + time (the ability to look forward and back in time) to build a real-world ‘digital twin’ prototype" for use during emergencies such as brush fires. This facilitates mapping the locations of telecommunication towers and assets which can enable emergency service organizations to better understand and protect these vital locations before and during an emergency event. Other adopters include Reveal Technology's Farsight software which, according to a August 1, 2022 press release uses 3D Tiles to enable Farsight users "to explore areas of interest in greater detail and complexity without slowing down operations. 3D Tiles support rapid map creation and interoperability with various viewers, enhancing situational awareness and collaboration." Adoption is across multiple domains including technology companies such as Nearmap, offering high-resolution aerial imager; Safe Software, creators of FME, a data integration platform; and MapTiler, a mapping platform for developers; gaming industries such as Epic Gaming, a leading interactive entertainment company and O3DE, a cross-platform 3D engine.

Cesium introduced the 3D Tiles initiative in 2015 at SIGGRAPH, a special interest group of the Association for Computing Machinery (ACM). 3D Tiles gained traction and earned several adoptions, leading to Cesium submitting 3D tiles for an OGC vote to start the Community Standard in 2016. The OGC document 18-053r2 Version 1.0 3D Tiles Specification 1.0 is the current standard, which was submitted in 2018 and published in 2019.

While 3D Tiles, version 1.1, with the OGC document number 22-025r2, is a "Work Item Draft." and still under development as of this writing in late 2022, there are some potentially impactful changes in the draft. Most notable is the sunsetting of the four tiles types from version 1.0 (Batched 3D Model [b3dm]; Instanced 3D Model [i3dm]; Point Cloud [pnts]; Composite [cmpt]) - all of which are superseded by the glTF Tile Format (see also glTF (GL Transmission Format) 2.0). Other significant changes to version 1.1 include updating of the Annexes with Annex A describing the Objects and Properties required to implement 3D Tiles, Annex B defining the 3D Tiles JSON Schema and more. A list of changes is documented in the 3D Tiles v1.1 Community Standard Justification document (PDF download). See also Public comment requested: proposed v1.1 update to 3D Tiles Community Standard from OGC.

In addition, 3D Tiles Next is under development in the Cesium 3D Tiles Next GitHub repository, which states that these features may be incorporated into 3D Tiles 2.0 in the future. The relationship between 3D Tiles Next and 3D Tiles version 1.1 is unclear. Comments welcome.