Sets lighting properties for 3D shading effects.
A surface will be shaded according to its to its orientation
relative to the viewing direction.
The ambient level A_LEVEL is a light level (arbitrary units)
that is added to every surface independent of its orientation.
The diffuse level D_LEVEL is a light level which is proportional
to cos(theta), where theta is the angle between the surface
normal and the viewing direction, so that surfaces directly
facing the viewer are bright, while surfaces viewed edge on are
unlit (and surfaces facing away, if drawn, are shaded as if they
faced the viewer).
The specular level S_LEVEL is a light level proportional to a high
power spower=N of 1+cos(alpha), where alpha is the angle between
the specular reflection angle and the viewing direction. The light
source for the calculation of alpha lies in the direction XYZ (a
3 element vector) in the viewer's coordinate system at infinite
distance. You can have ns light sources by making S_LEVEL, N, and
XYZ (or any combination) be vectors of length ns (3-by-ns in the
case of XYZ). (See source code for specular_hook function
definition if powers of 1+cos(alpha) aren't good enough for you.)
With no arguments, return to the default lighting.
light3, diffuse=.1, specular=1., sdir=[0,0,-1]
(dramatic "tail lighting" effect)
light3, diffuse=.5, specular=1., sdir=[1,.5,1]
(classic "over your right shoulder" lighting)
(two light sources combining previous effects)
interpreted function, defined at i/pl3d.i line 262