Light Sources
Every object in a scene is potentially a source of light. Light may be either be emitted or reflected
from objects. Generally, in computer graphics we make a distinction between light emitters and light reflectors.
The emitters are called light sources, and the reflectors are usually the objects being rendered.
Light sources are characterized by their intensities while reflectors are characterized by their material properties.
Most computer graphic rendering systems only attempt to model the direct illumination from the emitters to the
reflectors of the scene. On the other hand most systems ignore the geometry of light emitters, and consider only
the geometry of reflectors.
The rationalization behind these simplifications is that most of the light from a scene results from a single bounce
of a emitted ray off of a reflective surface. This is, however, a very questionable assumption. In most computer
generated pictures you will not see light directly emitted from the light source, nor the indirect illumination from a
light reflecting off on surface and illuminating another.
First we will consider some very simple lighting models.
Ambient Light
Eventhough an object in a scene is not directly lit it will still be visible.
This is because light is reflected from nearby objects. A simple hack that
is commonly used to model this indirect illumination is ambient light.
Ambient light has no spatial or directional characteristics. The amount of ambient
light incident on each object is a constant for all surfaces in the scene.
The amount of ambient light that is reflected by an object is independent
of the objects position or orientation. Properties often determine how much
of this ambient light is reflected.
Directional Light Sources
All of the illumination examples that I have shown to this point have used
directional light sources. All of the rays from a directional light source
have the same direction, and no point of origin. It is as if the light source
was infinitely far away from the surface that it is illuminating. Sunlight is
an example of an infinate light source. We consider the direction of the light
source when computing both the diffuse and specular components of illumination.
With a diectional light source this direction is a constant.
Point Light Sources
The rays emmitted from a point light radially diverge from the source. A point
light source is a fair approximation to a local light source such as a light
bulb. The direction of the light to each point on a surface changes when
a point light source is used. Thus a normalized vector to the light
emmiter must be computed for each point that is illuminated.
This page last modified on Tuesday, November 12, 1996