glTF - gl Transfer Format
glTF is a standard file format for three-dimensional scenes and models, intended to be a streamlined, interoperable format for the delivery of 3D assets, while minimizing file size and runtime processing by apps. Sometimes described as the "JPEG of 3D."
An open standard developed and maintained by the Khronos Group, it supports 3D model geometry, appearance, scene graph hierarchy, and animation.
Variants
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).
Version History
glTF 2.0
-GLB was incorporated directly into glTF 2.0.
glTF 1.0
- GLB was introduced as an extension.
glTF Extensions
glTF extensions can be present in glTF files, and will be present in the parsed JSON. glTF extensions can be supported by applications by inspecting the extensions fields inside glTF objects, and it is up to each application to handle or ignore them.
loaders.gl aims to provide support for glTF extensions that can be handled completely or partially during loading, and article describes glTF extensions that are fully or partially processed by the @loaders.gl/gltf classes.
Note that many glTF extensions affect aspects that are firmly outside of the scope of loaders.gl (e.g. rendering), and no attempt is made to process those extensions in loaders.gl.
| Extension | Preprocessed | Description |
|---|---|---|
| KHR_draco_mesh_compression | Y | Decompresses draco-compressed geometries |
| EXT_meshopt_compression | Y | Decompresses meshopt-compressed geometries |
| KHR_texture_basisu | Y | Adds the ability to specify textures using KTX v2 |
| KHR_texture_transform | Y | Adds transformation properties (translation, rotation, scale) for TEXCOORD_ mesh attribute |
| KHR_texture_webp | Y | |
| EXT_mesh_features | Y | 3D tiles extension |
| EXT_structural_metadata | Y | 3D tiles extension |
| KHR_lights_punctual | Y* | Deprecated |
| KHR_materials_unlit | Y* | Deprecated |
| EXT_feature_metadata | Y* | Deprecated. 3D tiles extension |
Official Extensions
KHR_draco_mesh_compression
Supports compression of mesh attributes (geometry).
The GLTFLoader by default fully decompresses draco compressed geometries, removing the draco extension and the compressed data from the parsed glTF data structure.
Specification: KHR_draco_mesh_compression.
Parsing Support:
- By adding the
decompress: trueoptions to theGLTFParserany decompressed by theGLTFParser. - The expanded attributes are placed in the mesh object (effectively making it look as if it had never been compressed).
- The extension objects are removed from the glTF file.
Encoding Support:
- Meshes can be compressed as they are added to the
GLTFBuilder.
KHR_lights_punctual
Supports specification of point light sources and addition of such sources to the scenegraph node.
Specification: KHR_lights_punctual
Parsing Support:
- Any nodes with a
KHR_lights_punctualextension will get alightfield with value containing a light definition object with properties defining the light (this object will be resolved by index from the globalKHR_lights_punctualextension object'slightsarray) . - The
KHR_lights_punctualextensions will be removed from all nodes. - Finally, the global
KHR_lights_punctualextension (including its light list)) will be removed.
Encoding Support:
- N/A
KHR_materials_unlit
Specifies that a material should not be affected by light. Useful for pre-lit materials (e.g. photogrammetry).
KHR_texture_basisu
This extension adds the ability to specify textures using KTX v2 images with Basis Universal supercompression.
The GLTFLoader by default fully decompresses compressed textures, removing the basisu extension and the compressed data from the parsed glTF data structure.
KHR_texture_transform
Many techniques can be used to optimize resource usage for a 3d scene. Chief among them is the ability to minimize the number of textures the GPU must load. To achieve this, many engines encourage packing many objects' low-resolution textures into a single large texture atlas. The region of the resulting atlas that corresponds with each object is then defined by vertical and horizontal offsets, and the width and height of the region.
To support this use case, this extension adds offset, rotation, and scale properties to textureInfo structures.
Custom Extensions
EXT_meshopt_compression
This extension provides a support for the meshopt binary geometry data compression format that is tailored to the common types of data seen in glTF buffers.
The GLTFLoader by default fully decompresses meshopt compressed geometries, removing the meshopt extension and the compressed data from the parsed glTF data structure.
EXT_feature_metadata
3D tiles extension by Cesium. This extension allows batching features for efficient streaming to a client for rendering and interaction.
EXT_mesh_features
3D tiles extension by Cesium. This extension defines a means of assigning identifiers to geometry and subcomponents of geometry within a glTF 2.0 asset.
EXT_structural_metadata
3D tiles extension by Cesium. This extension defines a means of storing structured metadata within a glTF 2.0 asset.