Graphics Reference
In-Depth Information
we discuss some application areas, how conventional graphics systems work, and
how each of these disciplines influences work in computer graphics.
A narrow definition of computer graphics would state that it refers to taking a
model of the objects in a scene (a geometric description of the things in the scene
and a description of how they reflect light) and a model of the light emitted into the
scene (a mathematical description of the sources of light energy, the directions of
radiation, the distribution of light wavelengths, etc.), and then producing a repre-
sentation of a particular
view
of the scene (the light arriving at some imaginary eye
or camera in the scene). In this view, one might say that graphics is just glorified
multiplication: One multiplies the incoming light by the reflectivities of objects in
the scene to compute the light leaving those objects' surfaces and repeats the pro-
cess (treating the surfaces as new light sources and recursively invoking the light-
transport operation), determining all light that eventually reaches the camera. (In
practice, this approach is unworkable, but the idea remains.) In contrast, computer
vision amounts to
factoring
—given a view of a scene, the computer vision system
is charged with determining the illumination and/or the scene's contents (which
a graphics system could then “multiply” together to reproduce the same image).
In truth, of course, the vision system cannot solve the problem as stated and typ-
ically works with assumptions about the scene, or the lighting, or both, and may
also have multiple views of the scene from different cameras, or multiple views
from a single camera but at different times.
In the field of computer graphics, the word “model” can refer to a geometric
model or a mathematical model. A
geometric model
is a model of something
we plan to have appear in a picture: We make a model of a car, or a house, or
an armadillo. The geometric model is enhanced with various other attributes
that describe the color or texture or reflectance of the materials involved in the
model. Starting from nothing and creating such a model is called
modeling,
and the geometric-plus-other-information description that is the result is called
a
model.
A
mathematical model
is a model of a physical or computational process.
For instance, in Chapter 27 we describe various models of how light reflects
from glossy surfaces. We also have models of how objects move and models of
things like the image-acquisition process that happens in a digital camera. Such
models may be faithful (i.e., may provide a predictive and correct mathemat-
ical model of the phenomenon) or not; they may be physically based, derived
from first principles, or perhaps empirical or phenomenological, derived from
observations or even intuition.
In actual fact, graphics is far richer than the generalized multiplication pro-
cess of rendering a view, just as vision is richer than factorization. Much of the
current research in graphics is in methods for creating geometric models, methods
for representing surface reflectance (and subsurface reflectance, and reflectances
of participating media such as fog and smoke, etc.), the animation of scenes by
physical laws and by approximations of those laws, the control of animation,
interaction with virtual objects, the invention of nonphotorealistic representa-
tions, and, in recent years, an increasing integration of techniques from computer
vision. As a result, the fields of computer graphics and computer vision are grow-
ing increasingly closer to each other. For example, consider Raskar's work on a
