3.10.2 Lighting
Yorick's pseudocolor model cannot describe a shaded color object. (You
need to be able to specify arbitrary colors for that.) Therefore, if
you want shading to suggest a 3D shape, you must settle for black and
white (or any single color -- the `heat.gp' palette works about as
well as `gray.gp' for shading).
You also need to worry about more variables -- namely the position and
relative brightness of any lights illuminating your object. You call
the light3 function to set up light sources.
The 3D surfaces comprising your object (plotted by either plf or plfp
for the `plwf.i' or `slice3.i' interfaces) consist of a number
of polygonal facets. Yorick assigns a 3D normal vector to each facet
(for non-planar polygons, the direction is somewhat arbitrary). The
lighting model, which you set up by light3 calls, maps normal directions
to surface brightness. If the facet is oriented so that it reflects one
of your light sources directly toward your camera, that facet will
appear very bright.
The functional form of this specular reflection model is
(1+cos(alpha))^n, where alpha is the angle between your camera and
the light source, as seen from the center of the facet. You can adjust
the power n. Large n gives a polished metallic look; small n gives a
matte look. You can specify as many light sources as you want.
However, the light sources are all infinitely far away. Also, although
you can specify a different n for each light source, you cannot get
dfferent values of n for different surfaces or parts of surfaces.
Instead of or in addition to these specular light sources, you can
also get diffuse lighting. In this model, the brightness of a surface
is simply proportional to the cosine of its angle relative to your
viewing direction. Thus, a surface you view face on is brightest,
while one viewed edge on is dimmest (the effect is called limb
darkening).
All of these effects are controlled by keywords to the light3
function: specular (brightness of specular light sources), sdir
(directions to specular light sources), spower (the powers n for the
specular light sources), diffuse (brightness of diffuse light source),
and ambient (an overall additive brightness). The light3 function
also returns a value, which you can pass back to it in a later call
in order to restore a previous lighting specification.
These lighting models -- which are actually the standard models you
find in most 3D graphics packages -- bear little relationship to the
appearance of real world scenes. There are no shadows, and surfaces
have no intrinsic texture. Your goal is excitement, not realism.
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