7.4 File format
ASAM OpenCRG does not define its own 3D model format. However, the geometry specification defines coordinate systems, units, node hierarchies, and more, which can be applied across different 3D model formats.
ASAM OpenCRG supports the 3D model file formats glTF, FBX, and USD with the following considerations:
- glTF
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3D models of ASAM OpenCRG in glTF format shall comply with the official glTF specification. The glTF specification has two delivery options: glTF + bin + textures, and the container format glb. Both delivery options are supported in ASAM OpenCRG.
The glTF coordinate system is right-handed, with +Y as the up axis, +Z as forward, and -X as right. The front of a glTF asset faces +Z.
This setup differs from how coordinate systems are defined in ASAM OpenCRG, see Section 7.2.2, "Coordinate systems". While modeling, the ASAM OpenCRG coordinate definitions shall be used. During export to and import from glTF, a coordinate transformation shall be applied to conform with the glTF specification. Common modeling tools like Blender handle this conversion automatically.
- FBX
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Although FBX lacks a formal specification, 3D models of ASAM OpenCRG in FBX format shall be compatible with the official AUTODESK FBX SDK.
- USD
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3D models of ASAM OpenCRG in USD format shall comply with the official USD specification.
The USD coordinate specification allows either +Y or +Z as the up axis, defined in the upAxis metadata. To adhere to the ASAM OpenCRG geometry specification, +Z shall be used as the up axis.