Computer animation, for many people, is synonymous with big-screen events
such as
Star Wars,
Toy Story,
and
Titanic
. But not all, or arguably even most, computer
animation is done in Hollywood. It is not unusual for Saturday morning
cartoons to be entirely computer generated. Computer games take advantage of
state-of-the-art computer graphics techniques. Real-time performance-driven
computer animation has appeared at SIGGRAPH
1
and on
Sesame Street.
Desktop
computer animation is now possible at a reasonable cost. Computer animation on
the Web is routine. Digital simulators for training pilots, SWAT teams, and
nuclear reactor operators are commonplace. The distinguishing characteristics of
these various venues are the cost, the image quality desired, and the amount and
type of interaction allowed. This book does not address the issues concerned with
a particular venue, but it does present the algorithms and techniques used to do
animation in all of them.
In computer animation, any value that can be changed can be animated. An
object’s position and orientation are obvious candidates for animation, but all of
1. SIGGRAPH is the Association for Computing Machinery’s (ACM’s) Special Interest Group on Computer Graphics.
The Essay on Computer Animation
... for the reality is responsible for the development of computer animation. Animation as a form of art exists already almost ... opportunity to depict imagination with the help of computer animation. The modeler contains the power of creation, while ... used to accelerate traditional animation processes, modern computer animations require still faster and more powerful computers to exploit new techniques ...
The ACM is the main professional group for computer scientists.
C H A P T E R
1
Introduction
Team LRN
2
1: Introduction
the following can be animated as well: the object’s shape, its shading parameters,
its texture coordinates, the light source parameters, and the camera parameters.
To set the context for computer animation, it is important to understand its
heritage, its history, and certain relevant concepts. 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 Perception
Images 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 everchanging
world; it is designed to notice and interpret movement.
When animation is created for later viewing it is typically recorded on film or
video as a series of still images that when displayed in rapid sequence are perceived
by an observer as a single moving image. This is possible because the eye-brain
complex assembles the sequence of still images and interprets it as continuous
movement. A single image presented to a viewer for a short time will leave an
imprint of itself—the
positive afterimage
—in the visual system for a short time
after it is removed (dramatically demonstrated by looking into the flash produced
by a flash camera).
This phenomenon is attributed to what has come to be called
persistence of vision
. When a person is presented with a sequence of closely related
still images at a fast enough rate, persistence of vision induces the sensation of continuous
imagery.
2
The afterimages of the individual stills fill in the gaps between
the images. In both film and video, a sequence of images is recorded that can be
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played back 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 image
is said to
flicker
. In this case, the animation appears as a rapid sequence of still
images to the eye-brain. Depending on conditions such as room lighting and
viewing distance, the rate at which single images must be played back in order to
maintain the persistence of vision varies. This rate is referred to as the
flicker rate
.
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.
2. Recently in the literature, it has been argued that persistence of vision is not the mechanism responsible for successfully
viewing film and video as continuous imagery. I avoid controversy here and, for purposes of this book and consistent
with popular practice, assume that whatever psychophysical mechanism is responsible for persistence of vision is
also responsible for the impression of continuous imagery in film and video.
Team LRN
Perception 3
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 is not involved because no individual images are present.
While persistence of vision addresses the lower limits for establishing the perception
of continuous imagery, there are also upper limits to what the eye can perceive.
The receptors in the eye continually sample light in the environment. The
limitations on motion perception are determined by the reaction time of those
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... motion, in the same way we discern it when an object moves smoothly in the real world. Another way to measure visual persistence ... becomes more complicated. Figure 5. 19: Example of images produced by binocular (stereo) vision with camera separation equivalent to the spacing of ... the United States causes electric lights to flicker at a rate of 120 times per second. Because of human visual latency, ...
sensors and by other mechanical limitations such as blinking and tracking. If an
object moves too quickly with respect to the viewer, then the receptors in the eye
will not be able to respond fast enough for the brain to distinguish sharply
defined, individual detail;
motion blur
results. In sequences of still images, motion
blur results as a combination of the object’s speed and the time interval over which
the scene is sampled. In a still camera, a fast-moving object will not blur if the
shutter speed is fast enough relative to the object’s speed. In computer graphics,
motion blur will never result if the scene is sampled at a precise instant in time; to
compute motion blur, the scene needs to be sampled over an interval of time or
manipulated to appear as though it were. (See Appendix A for a discussion of
motion blur calculations.) If motion blur is not calculated, then images of a fastmoving
object can appear disjointed. The motion becomes jerky, similar to live
action viewed with a strobe light, and is often referred to as
strobing
. In handdrawn
animation, fast-moving objects are typically stretched in the direction of
travel so that the object’s images in adjacent frames overlap, reducing the strobing
effect.
As reflected in the discussion above, there are actually two rates that are of concern.
One is the
playback rate,
the number of images per second displayed in the
viewing process. The other is the
sampling rate,
the number of different images
that occur per second. The playback rate is the rate related to flicker. The sampling
rate determines how jerky the motion appears. For example, a television signal
conforming to the National Television Standards Committee (NTSC) format displays
images at a rate of thirty per second, but in some programs (e.g., some Saturday
morning cartoons) there may be only six different images per second, with
each image repeatedly displayed five times. Often, lip-synch animation is drawn
on twos (every other frame) because drawing it on ones (animating it in every
frame) appears too hectic. Some films display each frame twice to reduce flicker
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effects. On the other hand, because an NTSC television signal is interlaced (which
means the odd-numbered scanlines are played beginning with the first sixtieth of a
second and the even-numbered scanlines are played beginning with the next sixtieth
of a second), smoother motion can be produced by sampling the scene every
sixtieth of a second even though the complete frames are only played back at thirty
frames per second. See Appendix B for details concerning various film and video
formats.
Team LRN
4
1: Introduction
1.2 The Heritage of Animation
In the most general sense,
animation
3
includes “live-action” puppetry such as that
found on
Sesame Street
and the use of mechanical devices to articulate figures such
as in
animatronics
. However, I concentrate here on devices that use a sequence of
individual stills to create the effect of a single moving image, because these devices
have a closer relationship to computer animation.
1.2.1 Early Devices
Persistence of vision
and the ability to interpret a series of stills as a moving image
were actively investigated in the 1800s
[2]
, well before the film camera was
invented. The recognition and subsequent investigation of this effect led to a variety
of devices intended as parlor toys. Perhaps the simplest of these early devices is
the
thaumatrope,
a flat disk with images drawn on both sides and having two
strings connected opposite each other on the rim of the disk (see Figure 1.1).
The
disk could be quickly flipped back and forth by twirling the strings. When flipped
rapidly enough, the two images appear to be superimposed. The classic example
uses the image of a bird on one side and the image of a birdcage on the other; the
rotating disk visually places the bird inside the birdcage. An equally primitive technique
is the
flipbook,
a tablet of paper with an individual drawing on each page.
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When the pages are flipped rapidly, the viewer has the impression of motion.
One of the most well known early animation devices is the
zoetrope,
or wheel of
life. The zoetrope has a short, fat cylinder that rotates on its axis of symmetry.
Around the inside of the cylinder is a sequence of drawings, each one slightly different
from the ones next to it. The cylinder has long vertical slits cut into its side
between each adjacent pair of images so that when it is spun on its axis each slit
allows the eye to see the image on the opposite wall of the cylinder (see Figure 1.2,
Plate 1).
The sequence of slits passing in front of the eye as the cylinder is spun on
its axis presents a sequence of images to the eye, creating the illusion of motion.
Related gizmos that use a rotating mechanism to present a sequence of stills to
the viewer are the
phenakistoscope
and the
praxinoscope
. The phenakistoscope also
uses a series of rotating slots to present a sequence of images to the viewer by positioning
two disks rotating in unison on an axis; one disk has slits, and the other
contains images facing the slits. One sights along the axis of rotation so the slits
