27-07-2012, 09:42 AM
Computer Animation in Future Technologies
Computer Animation.pdf (Size: 23.96 KB / Downloads: 85)
Abstract
In this introductory chapter, we try to situate the role of Computer Animation in the
new technologies: digital television, virtual reality, multimedia, cooperative work. We
also overview the main techniques which will be further discussed throughout this
book by international experts.
The high-tech role of computer animation
The term "Computer Animation" suggests that computers bring something new to
the traditional way of animating. Traditional animation is defined as a technique in
which the illusion of movement is created by photographing a series of individual
drawings on successive frames of film. Is this definition, due to John Halas (Halas and
Manwell 1968), still true for Computer Animation ? The definition is essentially
correct if we change the definition of the words photographing, drawings, and
successive frames. A definition of computer animation could be: a technique in which
the illusion of movement is created by displaying on a screen, or recording on a
recording device a series of individual states of a dynamic scene. We use the term
"recording" also for photographing and we consider both a cine-camera and a
videorecorder as a recording device.
A guide to advanced computer animation techniques
Computer animation means the creation of motion through a computer. There are
several ways to do it: performance animation, keyframing, inverse kinematics,
dynamics, task-level animation, behavioral animation. Moreover specific techniques
like cloth animation, hair animation and facial animation offer a big challenge.
Performance animation consists of recording the motion by a specific device for
each frame and using this information to generate the image by computer. For
example, a human walking motion may be recorded and then applied to a computergenerated
3D character. This approach will provide a very good motion, because it
comes directly from reality. However, it has severe limitations as for any new motion,
it is necessary to record the reality again. Roberto Maiocchi (Chapter 2) discusses in
details this type of technique.
A popular alternative to performance animation is the well-known techniques of
keyframing. A brief introduction to the topics is presented in Section
More details
may be found in (Magnenat Thalmann and Thalmann 1990).
The use of inverse-kinematics permits direct specification of end point positions.
Joint angles are automatically determined. This is the key problem, because
independent variables in a synthetic actor are joint angles. Unfortunately, the
transformation of position from Cartesian to joint coordinates generally does not have
a closed-form solution. However, there are a number of special arrangements of the
joint axes for which closed-form solutions have been suggested. Inverse kinematics is
discussed by Boulic and Mas (Chapter 3) and an alternative called inverse kinetics is
proposed.
A brief survey of traditional computer animation methods
For many years, most authors (Hanrahan and Sturman 1985; Parke 1982;
Magnenat-Thalmann and Thalmann 1985; Steketee and Badler 1985; Zeltzer 1985)
distinguished between three types of computer animation methods: image-based keyframe
animation, parametric keyframe animation and procedural animation.
Image-based keyframe animation
Keyframe animation consists of the automatic generation of intermediate frames,
called in-betweens, based on a set of key-frames supplied by the animator. In imagebased
keyframe animation, the in-betweens are obtained by interpolating the keyframe
images themselves. This is an old technique, introduced by Burtnyk and Wein (1971).
When corresponding images have not the same number of vertices, it is necessary to
add extra vertices. A linear interpolation algorithm produces undesirable effects such
as lack of smoothness in motion, discontinuities in the speed of motion and distortions
in rotations. Alternate methods have been proposed by Baecker (1969), Burtnyk and
Wein (1976), Reeves (1981). The method may be extended to three-dimensional
objects. The principle is the same when objects are modeled in wire-frame. However,
the technique is much more complex when objects are facet-based, because a
correspondence between facets and between vertices must be found. Vertices and
facets must be added in order to have the same numbers for both objects. A complete
algorithm has been introduced by Hong et al. (1988).