Computer animation is the
process used for generating animated images by using computer graphics. The
more general term computer generated imagery encompasses both static scenes and
dynamic images, while computer animation only refers to moving images.
Modern computer animation usually
uses 3D computer graphics, although 2D computer graphics are still used for
stylistic, low bandwidth, and faster real-time renderings. Sometimes the target
of the animation is the computer itself, but sometimes the target is another
medium, such as film.
Computer animation is essentially a
digital successor to the stop motion techniques used in traditional animation
with 3D models and frame-by-frame animation of 2D illustrations. Computer
generated animations are more controllable than other more physically based
processes, such as constructing miniatures for effects shots or hiring extras
for crowd scenes, and because it allows the creation of images that would not
be feasible using any other technology. It can also allow a single graphic
artist to produce such content without the use of actors, expensive set pieces,
or props.
To create the illusion of movement,
an image is displayed on the computer screen and repeatedly replaced by a new
image that is similar to it, but advanced slightly in time (usually at a rate
of 24 or 30 frames/second). This technique is identical to how the illusion of
movement is achieved with television and motion pictures.
For 3D animations, objects (models)
are built on the computer monitor (modeled) and 3D figures are rigged with a
virtual skeleton. For 2D figure animations, separate objects (illustrations)
and separate transparent layers are used, with or without a virtual skeleton.
Then the limbs, eyes, mouth, clothes, etc. of the figure are moved by the
animator on key frames. The differences in appearance between key frames are
automatically calculated by the computer in a process known as tweening or morphing.
Finally, the animation is rendered.
For 3D animations, all frames must
be rendered after modeling is complete. For 2D vector animations, the rendering
process is the key frame illustration process, while tweened frames are
rendered as needed. For pre-recorded presentations, the rendered frames are
transferred to a different format or
medium such as film or digital video. The frames may also be rendered in real
time as they are presented to the end-user audience. Low bandwidth animations
transmitted via the internet (e.g. 2D Flash, X3D) often use software on the
end-users computer to render in real time as an alternative to streaming or
pre-loaded high bandwidth animations.
Explanation
To trick the eye and brain
into thinking they are seeing a smoothly moving object, the pictures should be
drawn at around 12 frames per second (frame/s) or faster (a frame is one
complete image). With rates above 75-120 frames/s no improvement in realism or
smoothness is perceivable due to the way the eye and brain process images. At
rates below 12 frame/s most people can detect jerkiness associated with the
drawing of new images which detracts from the illusion of realistic movement.
Conventional hand-drawn cartoon animation often uses 15 frames/s in order to
save on the number of drawings needed, but this is usually accepted because of
the stylized nature of cartoons. Because it produces more realistic imagery computer
animation demands higher frame rates to reinforce this realism.
Movie film seen in theaters in the
United States runs at 24 frames per second, which is sufficient to create the
illusion of continuous movement. For high resolution, adapters are used.
Methods of animating virtual characters
In most 3D computer animation
systems, an animator creates a simplified representation of a character's
anatomy, analogous to a skeleton or stick figure. The position of each segment
of the skeletal model is defined by animation variables, or Avars. In human and
animal characters, many parts of the skeletal model correspond to actual bones,
but skeletal animation is also used to animate other things, such as facial
features (though other methods for facial animation exist). The character
"Woody" in Toy Story, for example, uses 700 Avars, including 100
Avars in the face. The computer does not usually render the skeletal model
directly (it is invisible), but uses the skeletal model to compute the exact
position and orientation of the character, which is eventually rendered into an
image. Thus by changing the values of Avars over time, the animator creates
motion by making the character move from frame to frame.
There are several methods for
generating the Avar values to obtain realistic motion. Traditionally, animators
manipulate the Avars directly. Rather than set Avars for every frame, they
usually set Avars at strategic points (frames) in time and let the computer
interpolate or 'tween' between them, a process called keyframing. Keyframing
puts control in the hands of the animator, and has roots in hand-drawn
traditional animation.
In contrast, a newer method called
motion capture makes use of live action. When computer animation is driven by
motion capture, a real performer acts out the scene as if they were the
character to be animated. His or her motion is recorded to a computer using
video cameras and markers, and that performance is then applied to the animated
character.
Each method has its advantages, and
as of 2007, games and films are using either or both of these methods in
productions. Keyframe animation can produce motions that would be difficult or
impossible to act out, while motion capture can reproduce the subtleties of a
particular actor. For example, in the 2006 film Pirates of the Caribbean: Dead
Man's Chest, actor Bill Nighy provided the performance for the character Davy
Jones. Even though Nighy himself doesn't appear in the film, the movie
benefited from his performance by recording the nuances of his body language,
posture, facial expressions, etc. Thus motion capture is appropriate in situations
where believable, realistic behavior and action is required, but the types of
characters required exceed what can be done through conventional costuming.
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