1 COMPUTER ANIMATION Nadia

Most of the phenomena which may be represented on the screen of a workstation are typically time-dependent. The techniques of computer graphics allow the construction of 3D graphical objects using geometric modeling techniques. Moreover, in a 3D space, scenes are viewed using virtual cameras and they may be lit by synthetic light sources. In order to visualize these phenomena at any given time, it is necessary to know the appearance of the scene at this time and then Computer Graphics techniques allow us to build and display the scene according to viewing and lighting parameters. The problems to solve are how to express time dependence in the scene, and how to make it evolve over time. These problems and their various solutions are part of what is usually understood by the term 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 [ 1 ], still true for Computer Animation ? The definition is essentially correct if we change the definition of the words photographing, drawings, and successive frames. A 2 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. We will see in this chapter that there are two ways of considering computer animation and its evolution. The first approach corresponds to an extension of traditional animation methods by the use of the computer. The second approach corresponds to simulation methods based on laws of physics, especially laws of mechanics. For example, traditional methods allow us to create three-dimensional characters with exaggerated movements while simulation methods are used to try to model a human behavior accurately. For example, consider the bouncing ball example as described by Lasseter [ 2 ]. The motion is improved by introducing squash and stretch. When an object is squashed flat and stretches out drastically, it gives the sense that the object is made out of a soft, pliable material. This is a well known …