Add size attributes to lights

[alanblevins]

Description of Issue

For a number of renderers, including Arnold, V-Ray, and also our internal proprietary renderer, these lights represent size as an attribute:

• DiskLight: radius
• RectLight: width, height
• SphereLight: radius

In UsdLux representation, the size of these lights is fixed at 1 (radius = 1, or width = height = 1), and it appears that the size of the light is controlled by scaling the transform on the light itself. This presents a conflict when representing a light for most renderers using UsdLux.

• If we apply the light size as part of the transform on the light itself, the light will be correctly represented in USD but loses the explicit differentiation between what is meant as the light size, and what is meant as the transform on the light.
• We could represent light size as its own custom transform attribute and add that transform to the transform stack on the light, but this feels error-prone. It would simply have to be known that the transform stack on the light needs to include this extra transform, and that the transform is only used for light size, and overrides should not remove that transform from the list. It becomes an internal standard by convention, with no means of enforcing the convention.
• If we apply the light size as a custom attribute, we no longer have a universal light representation, and USD importers and Hydra would need to be customized to support that attribute. To me, "U" is the most important letter in the USD acronym, so this solution is not appealing.

I propose updating the schemas for these lights to support radius (DiskLight and SphereLight), and width and height (RectLight) as first-class attributes. This change could be entirely backwards-compatible, if the default value for these new attributes is set at 1. I also feel like it would bring UsdLux in line with UsdGeom - for example, Cylinder defines height and radius, Sphere defines radius, etc.

Proposed improvement

Add these attributes to these light schemas:

Schema	New Attributes
SphereLight	radius (float)
DiskLight	radius (float)
RectLight	width (float) height (float)

System Information (OS, Hardware)

all systems, all hardware

Package Versions

v0.8.0+

[jtran56]

Filed as internal issue #151437.