What is a GLB file? (and how it differs from glTF)

What a GLB file contains

A GLB stores the same things as any glTF 2.0 asset. Not every file uses all of them:

• Meshes: vertex positions, normals, tangents, UV coordinates and optional vertex colours.
• Materials: glTF’s physically based metallic-roughness model, with slots for base colour, metallic-roughness, normal, occlusion and emissive textures. See PBR maps explained for what each one does.
• Textures: in the core spec the images are PNG or JPEG; other formats (such as KTX2) come through extensions.
• A scene: the node hierarchy, with each part’s position, rotation and scale, plus optional cameras.
• Animation: skins (skeletons), keyframe animations and morph targets.

The spec also fixes the space: distances are in metres, and +Y is up. That is why a GLB usually arrives the right size and the right way up, while formats without units often do not.

How a GLB file is built

GLB is a small container around glTF’s JSON and binary data. The spec defines it exactly, and it is little-endian:

Each chunk starts with its length and its type, and every chunk is padded to a 4-byte boundary. Because the binary data is stored as raw bytes, a GLB avoids the roughly 33% size overhead that comes from writing the same data into a .gltf as base64 text.

A quick way to check a file: open it in a hex viewer. A real GLB starts with the four characters glTF.

GLB vs glTF

GLB and glTF are not two different formats. They are two ways to package the same glTF 2.0 data, so a model looks the same in either.

Converting between them only repackages the data, so nothing about the model is lost. Use GLB when you send or publish a model, and glTF when you want to read or edit the JSON. Our GLB to glTF and glTF to GLB converters switch between the two.

Why some GLB files are much smaller

Two GLBs of the same model can differ a lot in size. The container is rarely the reason; compression is:

• Geometry compression: the KHR_draco_mesh_compression and EXT_meshopt_compression extensions shrink the mesh data.
• Texture compression: the KHR_texture_basisu extension stores textures as KTX2, which stays compressed on the GPU.
• Texture size: a 4096 px texture has four times the pixels of a 2048 px one, and textures are often most of a GLB’s weight.

A viewer has to support an extension to open a file that uses it. If a compressed GLB will not load somewhere, that is usually why.

How to open a GLB file

• In a browser: drop it into our free GLB viewer. The file is read on your device, not uploaded.
• Blender: File > Import > glTF 2.0 (.glb/.gltf). Blender’s exporter writes GLB the same way.
• Godot: imports glTF and GLB natively, and its docs recommend glTF 2.0.
• Unity: through a glTF importer package such as Unity glTFast.
• On the web: three.js, Babylon.js and the model-viewer web component load GLB directly.

iPhone AR Quick Look uses USDZ rather than GLB, so convert the model first with GLB to USDZ. For 3D printing, slicers want STL or 3MF: GLB to STL does that, but note STL drops all colour.

Why does my GLB look grey?

A GLB can carry textures, but it does not have to. A file is still a valid GLB if its material is one flat colour, if it uses vertex colours instead of an image, or if it has no material at all. Many CAD exports and many AI-generated meshes arrive this way.

To give a grey GLB a real texture, the model needs UVs and a base colour image. The AI 3D Texturing Studio does both in the browser: upload the GLB, describe the look, press Auto texture, and download a GLB with the texture packed inside. If your GLB already has good UVs, Keep my UVs paints straight into them and keeps the file’s own material.

Questions