Autodesk LIGHTSCAPE, Manual

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Computer Graphics Rendering
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case we see other surfaces through them; and some
surfaces reflect light onto others.
Global illumination
algorithms are rendering algorithms that take into
account the ways in which light is transferred
between the surfaces in the model.
Lightscape uses two global illumination algorithms:
ray tracing and radiosity . Before explaining how
these techniques work, it is useful to have a basic
understanding of how, in the physical world, light is
distributed in an environment. Consider, for
example, the simple room illustrated as follows.
This room has one light source. One theory of light
considers light in terms of discrete particles called
photons , which travel out from the light source until
they encounter some surface in the room.
Depending on the material of the surface, some of
these photons, traveling with particular wave-
lengths, are absorbed, while others are scattered
back out into the environment. The fact that photons
traveling at a particular wavelength are absorbed
while others are not is what determines the color
(also referred to as the
spectral reflectance ) of the
surface.
The way a surface reflects photons depends prima-
rily on its smoothness. Surfaces that are rough tend
to reflect photons in all directions. These are known
as
diffuse surfaces, and this type of reflection is
known as diffuse reflection . A wall painted with flat
paint is a good example of a diffuse surface.
Very smooth surfaces reflect the photons in one
direction, at an angle equal to the angle at which they
arrive at the surface
(angle of incidence) . These
surfaces are known as specular surfaces, and this
type of reflection is known as specular reflection . A
mirror is an example of a perfectly specular surface.
Of course, many materials display some degree of
both specular and diffuse reflection.
The final illumination of the room is determined by
the interaction between the surfaces and the billions
of photons that are emitted from the light source. At
any given point on a surface, it is possible that
photons have arrived directly from the light source
(direct illumination) or else indirectly through one or
more bounces off some other surfaces (indirect
illumination) .
If you were standing in the room, a very small
number of the total photons in the room would enter
your eye and stimulate the rods and cones of your
retina. This stimulation would, in effect, form an
image that is perceived by your brain. Computers
replace the rods and cones of a retina with the pixels
of the computer screen. One goal of a global illumi-
nation algorithm is to recreate, as accurately as
possible, what you would see if you were standing in
a real environment. A second goal is to accomplish
this task as quickly as possible, ideally in
real time
(30 images per second). There is currently no single
global illumination algorithm that can accomplish
both of these goals.
Global illumination in a room
Diffuse reflection Specular reflection