Arnold light_groups and shadow_groups


Light linking in Maya, include/exclude objects for lights in CINEMA 4D, light masks in Houdini, inclusive/exclusive lights in Softimage, they all are translated to light_groups and shadow_groups in Arnold.

Here’s a sphere on a plane, with two area lights (cyan and green).

default

If we add a light_group to the sphere polymesh:

 use_light_group on
 light_group "CyanAreaLight"

then we get this (see below). The sphere is lit only by the cyan area light: the light_group is the lights that affect the object.

light_group_on_cyan
Note that the sphere still casts shadows from the green light, even the sphere isn’t lit by the green light. That’s because there’s no shadow_group, so all lights cast shadows from the object.

If we add a shadow group, so that the light_group and shadow_group are both “CyanAreaLight”, then the sphere is lit by the cyan light only, and casts shadows from the cyan light only.

use_light_group on
 light_group "CyanAreaLight"
use_shadow_group on
 shadow_group "CyanAreaLight"

light_shadow_groups_on_cyan

And finally, here’s the sphere when light groups are on, but there’s no lights in the light group:

use_light_group on
use_shadow_group on
 shadow_group "CyanAreaLight"

light_group_empty

Rayswitching shadow rays


The ray_switch node lets you evaluate different shading trees for different ray types. In general, it’s used to provide a simpler shading tree for secondary rays, such as diffuse, glossy, or shadow rays. So when a camera ray hits the object, you can do one thing, but when a secondary ray hits the object, you can do another, simpler thing 🙂

For shadow rays, it’s not the color that matters, but whether the object blocks the shadow ray from a light. For example, if an object is opaque, then it doesn’t matter whether ray_switch returns red or blue or green or yellow for the shadow rays: the object is still opaque and blocking the light. So, you could use a ray_switch.shadow to plug in a simple standard shader that provides just the opacity mask.

ray_switch_shadow_opacity