Alembic / OBJ Export Convention

Different DCCs and renderers expect very different things from a mesh or particle file: some want positions in meters, others in centimeters; some read velocity from the schema-native .velocities slot, others want it under a specific name, as a vector, or even as a color set.

The Convention dropdown is shared by the Export: Mesh node (Alembic, OBJ) and the Export: Particles node (Alembic). It bundles all of those choices together, so that picking a target package gets you a file you can import without unit fixups or attribute renaming.

Picking a preset sets:

Preset	Positions[1]	Velocity binding[2]	Velocity units[2]	Notes[2]
Blender (m)	m	Native Schema	m/s
Cinema 4D (cm)	cm	Native Schema	cm/s
Houdini (m)	m	Arbitrary Data	m/s	Matches Houdini’s v point/vertex attribute convention.
Maya/Redshift (cm)	cm	Arbitrary Data	cm/s	Maya pipes the color set into velocityPP, so Arnold, Redshift and V-Ray pick it up for motion blur with no further setup.[3]
Unity (m)	m	Native Schema	m/s
Unreal (cm)	cm	Native Schema	cm/s
Custom	any	any	any	Exposes the underlying knobs so you can target a setup the presets don’t cover, for instance centimeter positions paired with cm/ms velocity.

[1]

Only for Alembic and OBJ.

[2] (1,2,3)

Only for Alembic, when exporting velocities is enabled.

[3]

Maya’s documentation on Motion Vector Source

Custom mode reveals the following parameters:

• Positions scale: Multiplier applied to every exported position. 1.0 writes meters, 100.0 writes centimeters.

• Export velocity: (Alembic only) Same toggle as on the format itself. When checked, also reveals:

  • Binding: Where in the Alembic file the velocity is written.

    • Native Schema: The schema-native .velocities slot of OPolyMesh (mesh) or OPoints (particles). This is what most DCCs read automatically.

    • Arbitrary Data: A user-named attribute under .arbGeomParams. Used when the consumer reads from a specific name, like Houdini’s v or Maya’s velocity color set.

      • Data type: (mesh only) Vector writes an OV3fGeomParam with interpretation=vector; Color writes an OC3fGeomParam with interpretation=rgb, which is what Maya needs to pipe the values into velocityPP for motion blur. Particles are always written as Vector.

      • Attribute name: Name of the attribute. Common values are v for Houdini and velocity for Maya.

  • Velocity multiplier: Multiplier applied on top of the simulation’s m/s output. 1.0 writes m/s, 100.0 writes cm/s, 0.1 writes cm/ms. This is independent from Positions scale so engines that mix units can be hit exactly.

This mode is intended for power users that want to export to DCCs not included in the presets list, or that require a different convention for other reasons (eg. they use non-default units). Please refer to your DCC’s documentation to find out which combination should work for it.

A final note: Autodesk FBX, Mesh Flipbook and Vertex Animated Texture are not affected by Convention. FBX declares its own units in the file header (system_unit, always m) and consumers convert on import; Mesh Flipbook and VAT have their own dedicated units pipelines.