If you've built a Web Driver scene containing complex geometry and lighting, you may notice performance hits. This is due to that fact that there are too many lighting calculations for the engine to make every render. The simplest way to remedy this is to make it so only certain lights affect certain objects in your scene. You can pull this off using light masks.
The light mask is a property of lights and geometry. Based on the relationship between a geometry's light mask and that of the light, the light may or may not affect the geometry.
The scene above contains three colored spotlights, a sphere, and a ground plane. The buttons above correspond to the first 6 bits of the light masks of each of the lights and of the sphere model. Press the buttons to toggle the binary values. You can read the base 10 value next to the binary representation. This would be the number you would pass to the setLightMask() method of WTLight, WTModel, or WTActor. You can play with the binary values to get the effect you want.
The simple way to think about how masks work is that if one or more pairs of 1's line up (reside at the same bit location) between the light's mask and the mask of a model, then the light will affect the model. If every bit location is such that either a pair of 0's line up or a 1 and a 0 line up, then the model will not be affected by that light.
Another way to think about it is to consider 0 as FALSE and 1 as TRUE. Between the light's mask and the mask of the model, each bit location is "anded." If any TRUE's are returned, then the light will affect the model.
WTLight::setLightMask, WTModel::setLightMask