3D model conversion to GLB/GLTF for Android AR

Converting 3D Models to GLB/GLTF Format for Android AR

GLB—binary glTF 2.0 container, Khronos Group standard. ARCore, Sceneform, model-viewer, WebXR—all use glTF/GLB. Unlike USDZ, GLB works cross-platform: Android AR, WebAR, Three.js, Babylon.js, Unity, Unreal.

But "open standard" doesn't mean "everything converts without issues."

GLB Structure and AR Requirements

glTF 2.0 describes scene via JSON: meshes, materials (PBR metallic-roughness workflow), animations, skins, cameras, light. GLB packages JSON + binary data (geometry, animations) + textures in single file.

Mandatory requirements for Android ARCore / Sceneform:

• Materials only via pbrMetallicRoughness—Blinn-Phong, Phong, Lambert unsupported
• Textures: PNG (with alpha) or JPEG (no alpha), max 2048×2048
• Geometry: triangles only (TRIANGLES primitive mode), not quads
• No unreferenced nodes and materials (validator complains)
• asset.generator—recommend setting (helps compatibility debugging)

Converting from Main Formats

FBX → GLB. Blender—best intermediate converter:

blender --background --python fbx_to_glb.py -- input.fbx output.glb

Python script for Blender:

import bpy
bpy.ops.import_scene.fbx(filepath=input_path)
bpy.ops.export_scene.gltf(
    filepath=output_path,
    export_format='GLB',
    export_materials='EXPORT',
    export_animations=True,
    export_apply=True  # applies modifiers
)

Main FBX → GLB problem via Blender: FBX materials often use Standard or Lambert shaders—Blender converts to Principled BSDF, but parameters (specular, shininess) map approximately. Results need visual verification.

OBJ → GLB. OBJ doesn't support animation, PBR materials (only MTL with Kd/Ks), skins. For static objects—convert via Blender or obj2gltf:

obj2gltf -i model.obj -o model.glb

MTL maps to glTF pbrMetallicRoughness approximately: Kd → baseColorFactor, map_Kd → baseColorTexture. Normal, roughness, metallic maps from MTL not picked up—add manually via glTF pipeline.

USDZ → GLB (iOS → Android). Apple Reality Converter can export to USD/OBJ, then via Blender to GLB. No direct USDZ→GLB converter—mandatory intermediate step via USD Python API or Blender.

Optimizing GLB for Mobile AR

Draco compression. Compresses geometry 60–90% without visible quality loss:

npx gltf-pipeline -i model.glb -o model_draco.glb --draco.compressionLevel 7

Important: ARCore supports Draco. Old Three.js without DracoLoader—no. Check compatibility with target renderer.

KTX2/Basis Universal textures. GPU-compressed textures, load faster, use less VRAM:

npx gltf-transform etc1s model.glb model_ktx2.glb
# or toktx for creating KTX2 files separately

Basis Universal decodes on GPU—no RAM unpacking. On Android AR, reduces memory 40–60% for textured models.

Mesh quantization. Pack float32 vertex coordinates to int16—precision loss < 0.01%, size 25–40% gain:

npx gltf-transform quantize model.glb model_quantized.glb

Validation

glTF Validator from Khronos—mandatory before deploy:

npx gltf-validator model.glb

Outputs errors and warnings with codes. Codes like ACCESSOR_ELEMENT_OUT_OF_MIN_BOUND—floating-point values outside allowed range, often from FBX conversion.

model-viewer (Google)—browser glTF viewer, shows model as in ARCore. Ideal for quick verification before device install.

Typical Errors

• Model without UV unwrap—in ARCore white or gray
• Double-sided materials not explicitly set (doubleSided: true in JSON)—inner faces invisible
• Coordinates in inches/centimeters instead of meters—object in AR microscopic or gigantic
• Animation with non-linear interpolation—glTF supports only LINEAR and STEP/CUBICSPLINE

Timeline

Cost calculated based on volume and source formats.