ANIMATION
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.
RASTER ANIMATIONS
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
COMPUTER-ANIMATION LANGUAGES
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)
MORPHING
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.
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