Fundamentals of Computer Graphics, 3rd Edition

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William B. Thompson

Visual Perception

The ultimate purpose of computer graphics is to produce images for viewing by

people. Thus, the success of a computer graphics system depends on how well it

conveys relevant information to a human observer. The intrinsic complexity of the

physical world and the limitations of display devices make it impossible to present

a viewer with the identical patterns of light that would occur when looking at a

natural environment. When the goal of a computer graphics system is physical

realism, the best we can hope for is that the system be perceptually effective:

displayed images should “look” as intended. For applications such as technical

illustration, it is often desirable to visually highlight relevant information and

perceptual effectiveness becomes an explicit requirement.

Artists and illustrators have developed empirically a broad range of tools and

techniques for effectively conveying visual information. One approach to improv-

ing the perceptual effectiveness of computer graphics is to utilize these methods

in our automated systems. A second approach builds directly on knowledge of

the human vision system by using perceptual effectiveness as an optimization cri-

teria in the design of computer graphics systems. These two approaches are not

completely distinct. Indeed, one of the first systematic examinations of visual

perception is found in the notebooks of Leonardo da Vinci.

The remainder of this chapter provides a partial overview of what is known

about visual perception in people. The emphasis is on aspects of human vision

that are most relevant to computer graphics. The human visual system is ex-

tremely complex in both its operation and its architecture. A chapter such as this

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554 22. Visual Perception

can at best provide a summary of key points, and it is important to avoid over

generalizing from what is presented here. More in-depth treatments of visual per-

ception can be found in Wandell (1995) and Palmer (1999); Gregory (1997) and

Yantis (2000) provide additional useful information. A good computer vision ref-

erence such as Forsyth and Ponce (2002) is also helpful. It is important to note

that despite over 150 years of intensive research, our knowledge of many aspects

of vision is still very limited and imperfect.

22.1 Vision Science

Vision is generally agreed to be the most powerful of the senses in humans.

Vision produces more useful information about the world than does hearing,

Light:

• travels far

• travels fast

• travels in straight lines

• interacts with stuff

• bounces off things

• is produced in nature

• has lots of energy

—Steven Shafer

Figure 22.1. The nature of

light makes vision a power-

ful sense.

touch, smell, or taste. This is a direct consequence of the physics of light (Fig-

ure 22.1). Illumination is pervasive, especially during the day but also at night

due to moonlight, starlight, and artificial sources. Surfaces reflect a substantial

portion of incident illumination and do so in ways that are idiosyncratic to par-

ticular materials and that are dependent on the shape of the surface. The fact

that light (mostly) travels in straight lines through the air allows vision to acquire

information from distant locations.

The study of vision has a long and rich history. Much of what we know

about the eye traces back to the work of philosophers and physicists in the 1600s.

Starting in the mid-1800s, there was an explosion of work by perceptual psy-

chologists exploring the phenomenology of vision and proposing models of how

vision might work. The mid-1900s saw the start of modern neuroscience, which

investigates both the fine-scale workings of individual neurons and the large-scale

architectural organization of the brain and nervous system. A substantial portion

of neuroscience research has focused on vision. More recently, computer science

has contributed to the understanding of visual perception by providing tools for

precisely describing hypothesized models of visual computations and by allow-

ing empirical examination of computer vision programs. The term vision science

was coined to refer to the multidisciplinary study of visual perception involving

perceptual psychology, neuroscience, and computational analysis.

Vision science views the purpose of vision as producing information about

objects, locations, and events in the world from imaged patterns of light reach-

ing the viewer. Psychologists use the term distal stimulus to refer to the physical

world under observation and proximal stimulus to refer to the retinal image.

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In computer vision, the term scene is often used to refer to the external world, while the term

image is used to refer to the projection of the scene onto a sensing plane.