Geometric Area Lights in Katana

Geometric Area Lights in Katana

About Geometric Area Lights

One of the most valuable features of RenderMan for Katana is the ability to create and manage Geometric Area Lights, introduced in PRMan 18. RfK's handling of geometric area lights has evolved and will continue to evolve as we see continued changes in both PRMan and Katana. RfK for 1.5 and 1.6 and PRMan 18 relied on the RMSGeoAreaLight shader for both analytical geo area lights and arbitrary emissive geometry. With the advent of PRMan 19 and Katana 2.0 RfK is extending its management of geo area lights to take advantage of new features available.

Note: For RIS style shading the only lights supported are geo area lights. RIS has no support for pseudo area lights.

The sections below provide a brief overview of how to take advantage of these capabilities.

"PRMan Light" vs. "Light"

RfK offers a PRMan-specific light package for creating "PRMan Lights" in GafferThree. These lights are identical to standard lights but have an added set of light-specific attributes from PrmanObjectSettings. This will allow certain attributes to be controlled per-light without needing to add the extra PrmanObjectSettings node. Currently a PRMan Light offers control per-light of the following settings:

  • Visibility (camera, transmission and indirect are all 'Off' for lights)
  • Shading Rate (general default is 1, light default is 10.0)
  • Sides (general default is 2, light default is 1)

Creating Geometric Area Lights

Analytic Geometry Lights

The standard analytical geometric area light is created by simply adding a light to the gaffer with a geo area light shader. There are currently two built-in geo area light shaders:

  • PxrAreaLight (RIS)
  • RMSGeoAreaLight (Reyes)

With both of these shaders there are several options for the shape of the light ("rect", "disk", "sphere", etc) with the default being "rect".

Mesh Lights

Handling of geo area lights was significantly rewritten for RfK 2.0 and as a result the creation of geometric area lights using arbitrary geometry (i.e. "mesh lights") differs between the RfK 1.5/1.6 plugin and RfK 2.0:

Shaders for Geometric Area Lights

There is a certain amount of special handling that has to happen in the plugin in order to properly render and edit geometric area lights. To that end any shader which is to be used for geo area lights must be able to inform the plugin that it is to be treated as a geo area light. Currently this is accomplished via the "shaderType" tag in the args file for the shader. If the shader is tagged as being of type "light" the plugin will check for an extra detail hint on the type tag. Please refer to RfK Shader Configuration for details on configuring args files for geo area lights.

Light Sampling

With Reyes rendering Geometric Area Lights samples are managed by the renderer through the directlightingsamples option. This insures proper importance sampling and frees the user from needing to manage light samples on a per-light basis. The same capabilities are provided in RIS mode through the sample balancing in the integrator, typically with the numLightSamples option. RIS Light Sampling provides more detail about how light sampling is managed in RIS.