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# 1.7 Light Sources

A light source illuminates the objects in a scene. Light sources in mental ray are programmable and consist of a light source name, a named light shader function, and an optional origin and direction (exactly one of the two must be present). All light shaders also accept shader parameters that depend on the shader. All standard shaders require a light color parameter.

The lights available to a scene are defined outside materials and referenced by name inside materials. Only those lights which a material references will illuminate surfaces which use that material.

The shading function for light sources may be either a user written function linked at run time, or it may be one of the standard functions. There are standard shader libraries for Alias, Alias Maya, Autodesk 3D Studio MAX, Dassault CATIA, SOFTIMAGE, SolidWorks, and Wavefront compatibility, as well as generic shader libraries such as mental images' base, physics, and Lume shader libraries.

Some shaders accept a boolean parameter `shadow` that turns shadow casting on or off for that light source, and a floating point `factor` that is the shadow factor. The shadow factor controls penetration of opaque objects.

For example, the `mib_light_spot` shader in the base shader library implements a spot light. It has a `color` parameter that must be set to the color of the light, as three scalars for the red, green, and blue components. It also allows a boolean `shadow` parameter that enables shadows cast from this light if its value is `on`, and a `factor` parameter that controls shadow penetration, ranging from `0.0` for black shadows to `1.0` for invisible shadows. The shader also accepts a boolean `atten` parameter that turns distance attenuation on or off, and two scalar parameters `start` and `stop` that specify the range over which the attenuation falls off if `atten` is `on`. The spot light mode requires an origin, a direction, and a spread value in the light definition in the scene description file. The spread value in the light definition and the `cone` parameter of the shader together define the spot characteristics of the light. The `cone` parameter specifies the angle of the inner solid cone and the spread statement specifies the outer falloff cone. The spot casts a cone of light with a softened edge where the intensity falls off to zero between the cone and spread angles. This and most other shaders specify angles not in degrees but in the cosine of one-half of the opening angle to improve performance: 1.0 is an opening angle of 0 degrees (a laser beam too narrow to see), 0.707 specifies an opening angle of approximately 90 degrees (because cos 0.707) and 0.0 specifies an opening angle of 180 degrees.

Next: 1.8 Area Light Sources Up: 1. Functionality Previous: 1.6 Materials