Available Lights in CG Systems

Most current computer graphics systems use either scan-line rendering or ray tracing. This means that they treat the diffuse interreflections component of light as a constant and call it ambient light. Programs that render using the radiosity method compute this term and therefore produce much more realistic images. Aside from ambient light, most systems provide at least the following types of lights with the associated parameters (These are from Pixar Renderman):

• "ambientlight", intensity (float), color[R,G,B]
• "distant_light" intensity, light color, from, to
There is no distance dependence, e.g. the sun, but light goes in the direction from -> to. The default from -> to direction is 0 0 0 --> 0 0 1 (if default is default intensity and color is 1, then it is a white light straight along + Z axis)
• "pointlight", intensity, color, from. The intensity is a function of 1/(d*d), where d is the distance from the pointlight, and it shines equally in all directions, e.g., a light bulb.
• "spotlight", intensity, lightcolor, from, to, coneangle, conedeltaangle, beamdistribution

A spotlight is more complex than the other light sources. The figure below shows the different parameters of the spotlight:

 Similar to a pointlight, a spotlight has a position and a direction. The intensity falls off with angle such that the intensity = 1 in the direction of the spotlight (from--> to) and intensity = 0 at coneangle radians away. The fall off function F(ca) = F1 * F2, where F1= (L*A)^beamdistribution. A = from->to direction and L = light vector from surface point to spot light. So the larger the beamdistribution the faster the faster fall off. F2 = 1 from A to coneangle - conedeltaangle radians and has a smooth fall-off to 0 at coneangle radians. This is shown in the plots.

Here is a set of four images that illustrate these different types of light sources:

 Ambient light onlyIntensity = 0.5 Distant light onlyIntensity = 1.0 Point light onlyIntensity = 30 and 6 units away Spot light onlyIntensity = 30 and 6 units away

CG systems frequently have other types of lights, e.g. area lights that are not point light sources. Renderman does this by means of shaders (functions that are written in the Renderman shading language).

Lights in 3D Studio Max

Last changed November 02, 1998, G. Scott Owen, owen@siggraph.org