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Modeling Global Illumination with Radiosity 51
each pixel based on the model information and a
specific viewpoint (camera).
The color of any specific point on a surface in
amodelisafunctionofthephysicalmaterial
propert ies of that surface and the light that
illuminates it. Two gene ral shading algorithms:
local illumination and global illumination are used
to describe how surfaces reflect and t ransm it light.
Local Illumination
Local illuminat ion algorithms describe only
how individual surfaces reflect or transmit light.
Givenadescriptionoflightarrivingatasurface,
these mathematical algorithms, called
shaders
in3dsMax,predicttheintensity,color,and
distribution of the light leaving that surface. In
conjunction w ith a material description, different
shaders w ill determine, for example, if a surface
will appear like plastic or metal or if it will appear
smooth or rough. 3ds Max prov ides a robust
interface for defining a wide array of different
surface materials.
After defining how an indiv idual sur face interacts
with light at the local level, the next ta sk is to
determine where the light arriving at the surface
originates. With the standard
scanline renderi ng
system (page 3–1100)
of 3ds Max, only the light
coming directly from the light sources themselves
is considered in the shading.
Formoreaccurateimages,however,itisimportant
to take into account not only the light sources,
but also how a l l the surfaces and objects in the
environment interact with the light. For example,
some sur faces block light, casting shadows on
other surfaces; some surfaces are shiny, in which
case we see in them the reflections of other
surfaces; some sur faces are transparent, in which
case we see other surfaces t hrough them; and some
surfaces reflect light onto other surfaces.
Global Ill umination
Renderingalgorithmsthattakeintoaccount
thewaysinwhichlightistransferredbetween
surfaces in the model are called global illumination
algorithms. 3ds Max offers two global illumination
algorithms as an integral part of its production
rendering system:
ray-tracing and radiosity .
Before an explanation of how ray-tracing and
radiosity work, it’s useful to understand how light
is distributed in the ph ysical world. Consider, for
example, the room shown in the illustration b elow.
Kitchen lit by two lights
This kitchen above has two light sources. One
theory of light considers the light in terms of
discrete particles called photons, that travel from
the light s ource until they encounter some surface
in the kitchen. Depending on the surface material,
someofthesephotonsareabsorbedandothers
are scattered back out into the environment.
Thefactthatphotonstravelingataparticular
wavelength are absorbed while others are not is
what determines the color of the surface.
Surfaces that are very smooth reflect the photons
in one direction, at an angle equal to the angle
at which they arrive at the surface, the angle of
incidence.Thesesurfacesareknownasspecular
surfaces, and this type of reflection is known as
specular reflection. A mirror is an example of
a perfectly specular surface. Of course, many