Scripting Guide - SAS

Chapter 13 Three-Dimensional Scenes 455
Lighting and Normals
Table 13.3 Light Parameters and Default Values
Parameter
Default
Value
Meaning
DIFFUSE (1, 1, 1, 1) diffuse RGBA intensity
SPECULAR (1, 1, 1, 1) specular RGBA intensity
POSITION (0, 0, 1, 0) (x, y, z, w) position
SPOT_DIRECTION (0, 0, -1) (x, y, z) direction of spotlight
SPOT_EXPONENT 0 spotlight exponent
SPOT_CUTOFF 180 spotlight cutoff angle
CONSTANT_ATTENUATION 1 constant attenuation factor
LINEAR_ATTENUATION 0 linear attenuation factor
QUADRATIC_ATTENUATION 0 quadratic attenuation factor
Note: The default values for DIFFUSE and SPECULAR in this table only apply to Light 0. For other lights,
the default value is (0, 0, 0, 1) for both parameters.
The first three parameters (AMBIENT, DIFFUSE, and SPECULAR) are used to color the light. DIFFUSE is the
parameter that is most closely associated with the physical color of the light. AMBIENT refers to the property
of the light when it functions as a background light. SPECULAR alters the way a light is reflected off a
surface.
Specify the position of the light using the POSITION parameter. Nonzero values of the fourth (w) coordinate
position the light in homogenous object coordinates.
Light in the real-world decreases in intensity as distance from the light increases. Since a directional light is
infinitely far away, it does not make sense to attenuate its intensity as a function of distance. However, JSL
attenuates a light source by multiplying the contribution of the source by an attenuation factor
1
attenuation factor = ---------------------------
c+ ld+
qd 2
where c = CONSTANT_ATTENUATION, l = LINEAR_ATTENUATION, and q = QUADRATIC_ATTENUATION.
To create a spotlight, limit the shape of the light to a cone. Use the SPOT_CUTOFF parameter to define the
side of the cone, as shown in the following illustration.