glTF (Graphics Library Transmission Format or GL Transmission Format and formerly known as WebGL Transmissions Format or WebGL TF) is a standard file format for three-dimensional scenes and models. A glTF file uses one of two possible file extensions: .gltf (JSON/ASCII) or .glb (binary). Both .gltf and .glb files may reference external binary and texture resources. Alternatively, both formats may be self-contained by directly embedding binary data buffers (as base64-encoded strings in .gltf files or as raw byte arrays in .glb files).[3][4][5] An open standard developed and maintained by the Khronos Group, it supports 3D model geometry, appearance, scene graph hierarchy, and animation.[4] It is intended to be a streamlined, interoperable format for the delivery of 3D assets, while minimizing file size and runtime processing by apps.[4][6] As such, its creators have described it as the "JPEG of 3D."[4]
Overview
The glTF format stores data primarily in JSON. The JSON may also contain blobs of binary data known as buffers, and refer to external files, for storing mesh data, images, etc.[7] The binary .glb format also contains JSON text, but serialized with binary chunk headers to allow blobs to be directly appended to the file.
The fundamental building blocks of a glTF scene are nodes. Nodes are organized into a hierarchy, such that a node may have other nodes defined as children. Nodes may have transforms relative to their parent. Nodes may refer to resources, such as meshes, skins, and cameras. Meshes may refer to materials, which refer to textures, which refer to images. Scenes are defined using an array of root nodes.[8]
Most of the top-level glTF properties use a flat hierarchy for storage. Nodes are saved in an array called "nodes", and are referred to by index, including by other nodes. A glTF scene refers to its root nodes by index. Furthermore, nodes refer to meshes by index, which refer to materials by index, which refer to textures by index, which refer to images by index.[7]
All glTF data structures support being extended using an "extensions" JSON property. This allows adding arbitrary JSON data to any data structure, defined using an extension specification that is built upon the base glTF spec. See the Extensions section for more information.[9]
Releases
glTF 1.0
Members of the COLLADA working group conceived the file format in 2012.[10] At SIGGRAPH 2012, Khronos presented a demo of glTF, which was then called WebGL Transmissions Format (WebGL TF).[5] On October 19, 2015, Khronos released the glTF 1.0 specification.[5]
Adoption of glTF 1.0
At SIGGRAPH 2016, Oculus announced their adoption of glTF citing the similarities to their ovrscene format.[6][11] In October 2016, Microsoft joined the 3D Formats working group at Khronos to collaborate on glTF.[12]
glTF 2.0
The second version, glTF 2.0, was released in June 2017, and is a complete overhaul of the file format from version 1.0, with most tools adopting the 2.0 version.[4][5] Based on a proposal by Fraunhofer[13] originally presented at SIGGRAPH 2016, Physically based rendering (PBR) was added, replacing WebGLshaders used in glTF 1.0.[14] glTF 2.0 added the GLB binary format into the base specification. Other upgrades include sparse accessors and morph targets for techniques such as facial animation, and schema tweaks and breaking changes for corner cases or performance such as replacing top-level glTF object properties with arrays for faster index-based access.[15] There is ongoing work towards import and export in Unity[16] and an integrated multi-engine viewer / validator.[17]
In February 2020, the Smithsonian Institution launched their Open Access Initiative, releasing approximately 2.8 million 2D images and 3D models into the public domain, using glTF for the 3D models.[23]
In July 2022, glTF 2.0 was released as the ISO/IEC 12113:2022 International Standard.[24] Khronos stated they will make regular submissions to bring updates and new widely adopted glTF functionality into refreshed versions of ISO/IEC 12113 to ensure that there is no long-term divergence between the ISO/IEC and Khronos specifications.[25]
The open-source game engine Godot supports importing glTF 2.0 files since version 3.0 and export since version 4.0.[26]
Extensions
The glTF format can be extended with arbitrary JSON to add new data and functionality. Extensions can be placed on any part of a glTF, including nodes, animations, materials, textures, and on the entire document. Khronos keeps a non-comprehensive registry of glTF extensions on GitHub, including all official Khronos extensions and a few third-party extensions.[9]
PBR extensions model the physical appearance of real-world objects, allowing developers to create realistic 3D assets that have the correct appearance. As new PBR extensions are released, they continue to expand PBR capabilities within the glTF framework, allowing a wider range of scenes and objects to be realistically rendered as 3D assets.[27]
The KTX 2.0 extension for universal texture compression enables 3D models in the glTF format to be highly compressed and to use natively supported texture formats, reducing file size and boosting rendering speed.[28]
Draco is a glTF extension for mesh compression, to compress and decompress 3D meshes, to help reduce the size of 3D files. It compresses vertex attributes, normals, colors, and texture coordinates.[29]
Various glTF extensions for game engine interoperability have been developed by OMI group. This includes extensions for physics shapes, physics bodies, physics joints, audio playback, seats, spawn points, and more.[30][31]
The VRM consortium has developed glTF extensions for advanced humanoid 3D avatars including dynamic spring bones and toon materials.[32][33]
Derivative formats
On August 10, 2015, 3D Tiles, now a proposed OGC Community Standard, built on glTF to add a spatial data structure, metadata, and declarative styling for streaming massive heterogeneous 3D geospatial datasets.[34][35][36]
VRM, a model format for VR, is built on the .glb format.[37] It is a 3D humanoid avatar specification and file format.[38]
Software ecosystem
Khronos maintains the glTF Sample Viewer for viewing glTF assets. Khronos also maintains the glTF Validator for validating if 3D models conform to the glTF specification.[39][40]
3D Commerce, a working group of universal guidelines, standards and certifications for 3D content creation and distribution in e-commerce, incorporates glTF for transmission of photorealistic 3D assets.[47] In June 2021, the 3D Commerce viewer certification program was launched, enabling 3D viewers to demonstrate they can accurately and reliably display 3D products using the glTF file format.[48] 3D Commerce's asset guidelines, released in 2020, include best practices for use of the glTF file format in 3D Commerce.[49]
