Units - Arnold User Guide

Scene Unit Scaling for Procedurals

If a procedural's units differ from the scene units, then the light intensity, volume density, and emission are scaled to match the scene units. ASS file procedurals are supported. USD, alembic, and other arbitrary procedural formats are not supported yet.

All plugins adjust the scale of the bounding box according to the scene units defined in their respective scenes. This means that the dimensions of the bounding box are automatically scaled based on the unit of measurement used in the scene, such as meters or centimeters. This ensures consistency and accuracy when transferring assets between different plugins.

Nits per Unit

The OpenPBR Surface parameter Emission Luminance (in nits) is internally scaled down by this value (defaulting to 1000) to bring it into Arnold units. This ensures that the default soft-maximum of Emission Luminance of 1000 nits corresponds to the former behavior of Standard Surface at Emission Weight 1.

Meters per Unit

Specifies the conversion factor from the scene units to meters. For instance, the default 1 meter means that 1 unit in Arnold equals 1 meter. This parameter affects all length unit values, such as vertex, camera and light positions, displacement height, and bump height. This is particularly useful for ensuring that lighting and other physical simulations behave correctly according to the intended scale of the scene.

1.0 (default): If your scene is already in meters, you would set meters_per_unit to 1.0.

0.01: If your scene is in centimeters, you would set meters_per_unit to 0.01 (since 1 centimeter = 0.01 meters).

0.001: If your scene is in millimeters, you would set meters_per_unit to 0.001 (since 1 millimeter = 0.001 meters).