Introduction - Computer Animation: Algorithms and Techniques - page 2

techniques based on interpolation are concentrated in the early chapters ( Chapters 3-5 ). The second
is data-driven animation in which live motion is digitized and then mapped onto graphical objects.
The primary technology for data-driven animation is referred to as motion capture and is the topic
of Chapter 6 . The third is procedural animation , in which there is a computational model that is
used to control the motion. Usually, this is in the form of setting initial conditions for some type
of physical or behavioral simulation. Procedural animation is concentrated in the later chapters
( Chapters 7-11 ).
To set the context for computer animation, it is important to understand its heritage, its history, and
certain relevant concepts. The rest of this chapter discusses motion perception, the technical evolution
of animation, animation production, and notable works in computer animation. It provides a grounding
in computer animation as a field of endeavor.
1.1 Motion perception
A picture can quickly convey a large amount of information because the human visual system is a
sophisticated information processor. It follows, then, that moving images have the potential to convey
even more information in a short time. Indeed, the human visual system has evolved to provide for
survival in an ever-changing world; it is designed to notice and interpret movement.
It is widely recognized that a series of images, when displayed in rapid succession, are perceived by
an observer as a single moving image. This is possible because the eye-brain complex has the ability,
under sufficient viewing conditions and within certain playback rates, to create a sensation of contin-
uous imagery from such a sequence of still images. A commonly held view is that this experience is due
to persistence of vision , whereby the eye retains a visual imprint of an image for a brief instant once the
stimulus is removed. It is argued that these imprints, called positive afterimages of the individual stills,
fill in the gaps between the images to produce the perception of a continuously changing image. Peter
Roget (of Roget's Thesaurus fame) presented the idea of impressions of light being retained on the
retina in 1824 [ 35 ] . But persistence of vision is not the same as perception of motion. Rotating a white
light source fast enough will create the impression of a stationary white ring. Although this effect can be
attributed to persistence of vision, the result is static. The sequential illumination of a group of lights
typical of a movie theater marquee produces the illusion of a lighted object circling the signage. Motion
is perceived, yet persistence of vision does not appear to be involved because no individual images are
present. Recently, the causality of the (physiological) persistence of vision mechanism has been called
into question and the perception of motion has been attributed to a (psychological) mechanism known
as the phi phenomenon (as is the case in the movie marquee example given above). A related phenom-
enon, for example the apparent motion of a disk traveling between two flickering disks, is referred to as
beta movement [ 1 ] [ 2 ] [ 13 ][ 39 ] .
Whatever the underlying mechanism is, the result is that in both film and video, a sequence of
images can be displayed at rates fast enough to fool the eye into interpreting it as continuous imagery.
When the perception of continuous imagery fails to be created, the display is said to flicker . In this case,
the animation appears as a rapid sequence of still images to the eye-brain complex. Depending on con-
ditions such as room lighting and viewing distance, the rate at which individual images must be played
back in order to maintain the perception of continuous imagery varies. This rate is referred to as the
critical flicker frequency [ 8 ].