Graphics Reference
In-Depth Information
CHAPTER 9
Illumination and Shading
9.1
Introduction
Image synthesis
has been defined as the creation of images using an illumination
model for the propagation of light, with the goal being
photorealism
. Actually, pho-
tography operates under some constraints defined by the camera and film being used,
so that a much more general goal would be to produce a visual experience that
matches the one that one gets by viewing the real scene.
There are basically five steps to getting a realistic computer generated image of a
scene:
(1) Modeling the scene
(2) Projecting to the image plane and transforming to the viewport including
clipping
(3) Hidden surface and visibility computations
(4) Light intensity computations (shading) using illumination models
(5) Antialiasing
We have looked at all of these except (4). After the visibility of a surface has been
determined, one uses a shading model to establish the light intensity and color at its
points. These computations usually dominate the visibility computations unless the
number of objects is very large. Several shading models are used in computer graph-
ics. The two that we shall describe in great detail are the Phong model and ray tracing.
Good references for additional information about this subject are [Hall89], [FVFH90],
and [WatW92].
First of all, one distinguishes between an
illumination
or
reflectance model
and a
shading model
. The latter will be the more comprehensive term. A shading model
uses
illumination models. Two different shading models could use the same illumination
model. For example, one shading model could compute the light intensity at
every
point according to a fixed illumination model and another might only compute them
at
vertices
of a linear polyhedron (using that same illumination model) and then get
the rest of the values with interpolation.
Anyone in geometric modeling will sooner or later have to choose an illumination
model and one will have to decide between one of two approaches. On the one hand,
