Rendering a CG Scene - An Introduction to Computer Graphics for Artists - page 225

Although all lights will cast shadows whenever the path of travel for its photons
is blocked, sometimes the shadows are so faint that they are not discernible. It is due
to this observation, and the desire to optimize rendering wherever possible, that cg
lights may have their shadows turned off. Lights that do not cast shadows but do
project light, can be very useful at providing fi ll light, or refl ected light. What this
does is to illuminate surfaces that would otherwise be indistinguishable due to
heavy shadow. Because they are already in shadow, there is no need to cast more;
confl icting shadows and the fi ll light can approximate the result of true bounced
light. This is a lighting optimization.
CG lights differ from real world lights in many ways, but the only differences of
any import to an artist are related to optimization. The number of photons actually
bouncing around in any given environment is so large that it would be physically
impossible for any computer to accurately calculate every interaction between every
photon and every scene element. Because of this, light calculations are optimized in
such a way that a scene may be credibly rendered with a much smaller number of
photons, fewer interactions, and various cutoff points beyond which further calcula-
tions will not be made. To light a scene properly, you must adjust your lights to
compensate for the differences between real and optimized CG lights.

12.5.5.1 Ambient Light

An ambient light provides even light to everything in your scene, regardless of loca-
tion. This light can be thought of as representing the shadow side of a scene. Whatever
you set the ambient value to, nothing in the scene will be darker. This should not be
set to black, because nothing is truly black, nor should it be set to a completely neutral
color, because colors in shadow tend to be more saturated than their neighbors in the
light. If the ambient light is a neutral gray, the scene will look ashen.

12.5.5.2 Point Light

A point light projects rays in every direction from a single user-specifi ed coordinate.

12.5.5.3 Directional Light

Directional lights cast parallel rays. The direction of the rays can be adjusted by the user.

12.5.5.4 Spot Light

A spot light has an origin and an aimpoint. In addition, the aimpoint is surrounded
by a circle of light defi ned by a user-specifi ed radius. The penumbra, the edge
between light and shadow, is also user-specifi ed. Although all CG lights can have
shadow casting turned off, spot lights are often used specifi cally to cast shadows.