About

GLB (GL Transmission Format Binary) stores 3D scenes in a single binary blob, embedding JSON metadata, geometries, textures, and animations. When debugging assets or integrating with text-based pipelines, you need the human-readable GLTF variant. This converter parses the GLB container structure - validating the 0x46546C67 magic number, extracting the JSON chunk (type 0x4E4F534A) and binary chunk (type 0x004E4942) - then rebuilds the output as standard GLTF. Binary buffers can be embedded as Base64 data URIs or exported as separate .bin files. The parser enforces 4-byte chunk alignment per the Khronos specification, preventing corruption in WebGL runtimes that assume aligned memory access. Risk: Manual hex editing or improper chunk splitting breaks asset references. This tool validates chunk integrity and preserves buffer byte offsets.

Formulas

GLB File Structure:

Header = MagicNumber + Version + TotalLength

where MagicNumber = 0x46546C67 (4 bytes), Version = 2 (4 bytes), TotalLength = file size in bytes (4 bytes)

Chunk Structure:

Chunk = ChunkLength + ChunkType + ChunkData

ChunkLength (4 bytes, little-endian) = byte length of ChunkData

ChunkType (4 bytes) = 0x4E4F534A (JSON) or 0x004E4942 (BIN)

Base64 Encoding Overhead:

EncodedSize = OriginalSize × 43

If OriginalSize = 1 MB, then EncodedSize ≈ 1.33 MB

Chunk Alignment:

PaddedLength = ceil(ChunkLength4) × 4

JSON chunks padded with 0x20 (space), BIN chunks with 0x00

Reference Data

Property	GLB (Binary)	GLTF (JSON)	Notes
File Extension	.glb	.gltf	GLTF may have .bin sidecars
Magic Number	0x46546C67	N/A	ASCII "glTF"
Version	2	2.0	Both use GLTF 2.0 spec
JSON Chunk Type	0x4E4F534A	N/A	ASCII "JSON"
Binary Chunk Type	0x004E4942	N/A	ASCII "BIN\0"
Chunk Alignment	4 bytes	N/A	Padding with 0x20 or 0x00
Buffer Storage	Embedded	External or Data URI	GLTF allows both
Typical Size	Smaller	Larger (Base64 overhead)	~33% increase with Base64
Use Case	Web delivery	Debugging, editing	GLB for runtime, GLTF for dev
Texture Embedding	Yes	Optional	Data URIs or file references
Animation Data	Binary accessors	Binary or JSON	Keyframes in buffers
Node Hierarchy	JSON chunk	JSON root	Same structure
Material Definitions	JSON chunk	JSON root	PBR parameters
Khronos Validator	Supported	Supported	github.com/KhronosGroup/glTF-Validator
Three.js Loader	GLTFLoader	GLTFLoader	Same loader for both
Babylon.js Support	Yes	Yes	SceneLoader.ImportMesh
Blender Export	Native	Native	File → Export → glTF 2.0
Compression	Draco, KTX2	Draco, KTX2	Extensions for both
Morph Targets	Supported	Supported	Blend shapes in accessors
Skinning	Supported	Supported	Joint matrices in buffers
Lights	Extension	Extension	KHR_lights_punctual

Frequently Asked Questions

Textures stored in the GLB binary chunk are preserved. In embedded mode, they're converted to Base64 data URIs in the GLTF JSON (image.uri = "data:image/png;base64,..."). In external mode, they remain in the .bin file, and the GLTF references the buffer by index. The bufferView byte offsets are recalculated to maintain alignment.

This tool is unidirectional (GLB → GLTF). For reverse conversion, you need a GLTF-to-GLB packer that reconstructs the binary container, merges external .bin files, and writes the chunk headers. Tools like glTF-Pipeline or Blender's exporter handle bidirectional conversion.

Common causes: (1) Buffer URIs point to missing .bin files if you selected external mode but didn't download them. (2) Base64 data URIs are malformed due to incorrect padding. (3) The original GLB had custom extensions (KHR_materials_unlit, KHR_draco_mesh_compression) that aren't declared in the GLTF extensionsUsed array. Check the browser console for GLTFLoader errors.

The converter parses the GLB structure and extracts chunks, but it does not decode Draco geometry compression. The compressed buffers are passed through as-is. To use the output GLTF, your rendering engine must support the KHR_draco_mesh_compression extension (Three.js requires DRACOLoader).

Browser memory limits apply. Files under 100 MB process instantly. For 100-500 MB files, a Web Worker prevents UI freezing. Beyond 500 MB, you risk heap allocation errors. For massive models, use command-line tools like glTF-Transform (Node.js) that handle streaming buffers.

Use the Khronos glTF Validator (github.com/KhronosGroup/glTF-Validator). Upload your GLTF and .bin files. It checks for: invalid accessor bounds, missing required properties, incorrect buffer byte strides, and extension compatibility. The validator outputs a JSON report with errors/warnings.

Yes. The JSON is human-readable and can be modified in any text editor. However, if you change buffer byte offsets, accessors, or bufferViews, you must recalculate the indices manually. Tools like glTF-Transform or Blender's glTF editor provide safer APIs for programmatic editing without breaking references.