Graphics Reference
In-Depth Information
Motion
When you see a sequence of related images in rapid succession, they blend
together and create the perception that objects in the images are moving. This
need not involve a computer: Cartoons drawn in a flip-book and analog film pro-
jection (see Figure 35.1) both create the illusion of motion this way. The individual
images are called
frames
and the entire sequence is called an
animation.
Beware
that both of these terms have additional meanings in computer graphics; for exam-
ple, a coordinate transform is a “reference frame” and an “animation” can refer to
either the rendered images or the input data describing one object's motion.
This chapter presents some fundamental methods for describing the motion of
objects in a 3D world over time. These mainly involve either interpolating between
key positions (see Figure 35.2) or simulating dynamics according to the laws of
physics (see Figure 35.3). Note that the laws of physics as in a virtual world need
not be those of the real world.
Most character animation that you have observed was driven by key positions,
with those positions created either by an artist or via motion capture of an actor
(see Figure 35.4). Those processes are not particularly demanding from a com-
putational perspective, but producing the animations is expensive and relatively
slow because of the time and skill that they require from the artists in the process.
In contrast, dynamics is computationally challenging but requires comparatively
little input from an artist. This is a classic example of leveraging a computer to
multiply a human's efforts dramatically. It is natural that as animation algorithms
have become more sophisticated and computer hardware has become both more
efficient and less expensive, the broad trend has been to increase the amount of
animation produced by dynamics.
The artistry and algorithms of animations are subjects that have filled many
texts, and thus even a survey would strain the bounds of a single chapter. This
chapter focuses on the rendering and computational aspects of
physically based
animation. In particular, it emphasizes
concepts
in the interpolation and rendering
sections and
mathematical detail
in the dynamics section. Dynamics is primarily
Figure 35.1: An animation is a
sequence of frames.
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