DESIGN OF ANIMATION SEQUENCES
In general, an animation sequence is designed with the tollowing steps:
1. Storyboard layout
2. Object definitions
3. Key-frame specifications
4. Generation of in-between frames
This standard approach for animated cartoons is applied to other animation applications as well, although there are many special application that do not follow this sequence.
The Storyboard layout is an outline of the action. It defines the motion sequence as a
set of basic events that are to take place. Depending on the type of animation to be produced, the storyboard could consist of a set of rough sketches or it could be a list of the basic ideas for the motion. An object definition is given for each participant in the action. Objects can bedefined interms of basic shapes, such as polygons or splines. In addition, the associatedmovements for each object are speeded along with the shape.
A keyframe is a detailed drawing of the scene at a certain time in the animation sequence.
Within each key frame, each object is positioned according to the time for that frame. Some key frames are chosen at extreme positions in the action; others are spaced so that the time interval between key frames is not to great. More key frames are specified for intricate motions than for simple, slowly varing motions.
In-between frames are the intermediate frames between the key frames.
The nurnber of in-betweens needed is determined by the media to be used to display theanimation. Film requires 24 frames per second, and graphics terminals are refreshedat the rate of 30 to 60 frames per second. Typically, time intervals for themotion are set up so that there arr from three to five in-betweens for each pair ofkey frames.
GENERAL COMPUTER-ANIMATION FUNCTIONS
Some steps in the development of an animation sequence are well-suited to computer solution. These include object manipulations and rendering, camera motions, and the generation of in-betweens. Animation packages, such as Wavefront,
One function available in animation packages is provided to store and manage the object database. Object shapes and associated parameters are stored and updated in the database. Other object functions include those for motion generation and those for object rendering. Motions can be generated according to specified constraints using two-dimensional or three-dimensional transformations.
On raster systems, we can generate real-time animation in limited applications using raster operations. Sequences of raster operations can be executed to produce real-time animation of either two-dimensional or three-dimensional objects, as long as we restrict the animation to motions in the projection plane. Then no viewing or visible- surface algorithms need be invoked.The animation is then accomplished by changing the color-table values so that the object is "on"at successively positions along the animation path as the preceding position is set-to the background intensity
Design and control of animation sequences are handled with a set of animationroutines. A general-purpose language, such as C, Lisp, Pascal, or FORTRAN, is often used to program the animation functions, but several specialized animation languages have been developed. Animation functions include
a graphics editor, a key-frame generator,
an in-between generator,
and standard graphics routines.
The graphics editor allows us to design and modify object shapes, using spline surfaces, constructive solid-geometry methods, or other representation schemes.
A typical task in an animation specification is scene description. This includes the positioning of objects and light sources, defining the photometric parameters (light-source intensities and surface-illumination properties), and setting the camera parameters (position, orientation, and lens characteristics).
Another standard function is action specification. This involves the layout of motion paths for the objects and camera. And we need the usual graphics routines: viewing and perspective transformations, geometric transformations to generate object movements as a function of accelerations or kinematics path specifications, visible-surface identification, and the surface-rendering operations.
KEY FRAME SYSTEMS
We generate each set of in-betweens from the specification of two (or more) keyframes. Motion paths can be given with a kinematic a s a set of spline curves, or the motions can be physicdly bnscd by specifying the for acting on the objects to be animated.For complex scenes, we can separate the frames into individual components or objects called cels celluloid transparencies)
Transformation of object shapes from one form to another is called morphing,which is a shortened form of metamorphosis. Morphing methods can he applied to any motion or transition involving a change in shape.Givcn two key frames for an object transformation, we first adjust the object specification in one of the frames so that the number of polygon edges (or the number of vertices) is the same for the two frames.
We can state rules for equalizing key frames in terms of either the number of edges or the number of vertices to be added to a key frame. Suppose we equalize the edge count, and parameters Lk and Lk+1 denote the number of line segments in two consecutive frames. We then define
MOTION SPECIFICATIONS There are several ways in which the motions of objects can be specified in an animation system.
Direct Motion SpecificationThe most straightforward method for defining a motion sequence is direct specification of the motion parameters. Here, we explicitly give the rotation angles and translation vectors. Then the geometric transformation matrices are applied to transform coordinate positions. Alternatively, we could use an approximating equation to specify certain kinds of motions.
where A is the initial amplitude, w is the angular frequence, 0, is the phase angle,and k is the damping constant. These methods can be used for simple user-programmedanimation sequences.
Goal-Directed Systems At the opposite extreme, we can specify the motions that are to take place in general terms that abstractly describe the actions. These systems are referred to as goal directed because they determine specific motion parameters given the goalsof the animation.