Introduction to Computer Animation

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Introduction toComputer Animation


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Animation

Animation = An illusion of movement created by

sequentially playing still image frames with different

movements at the general rate of 15 - 30 fps

(frames per second)

This animation moves

at 10 frames per

second.

This animation moves at 2 frames

per second. At this rate, the

individual frames should be

discernible


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Morphing process

of transforming one

image into another

via a series of frames

How Animation Works?

The speed of the image changes is called the frame rate. Movie is typically delivered at 24 30 frames per second Computer animations can be effective at 12 to 15 frames

per second


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Animation

Specify directly or indirectly how thing moves in time and space. Creating thisillusion either by hand or with the assistance of computer, is not easy.

Traditionally, animation has been created by drawing images at certain key points

in the action. These images known as key-frames. Later the images between the

key-frames are filled in to complete the sequence, in a process called in-

betweening (Low level technique). For example, to key-frame hitting a baseball,the animator would draw key moments in the sequence such as the impact of thebat on the ball and the follow through of the swing. The remaining images

would be filled in later, perhaps by a different animator. The most basic computer

animation tools assist the process of hand animation by automatically

interpolating between the key-frames of images.


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Basic Concepts of Tweening

Consider the following sequence of key

frames (key 13) of a bouncing ball:

1

2

3

Keyframe 1 Keyframe 3

Keyframe 2


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Using linear interpolation to yield the in-

betweening

1

2

3


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This is not realistic!

Looks like being deflected

by some unknown source;it doesnt match the rest;

angle may be too sharp


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Better to use a spline (curved lines)

formula

1

2

3


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http://www.cadtutor.net/dd/bryce/anim/anim.html

http://www.cadtutor.net/dd/bryce/anim/anim.htmlhttp://www.cadtutor.net/dd/bryce/anim/anim.html


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The most common form of 2D animation is sprite animation. A sprite is a bitmap

image or set of images that are composite over a background, producing the illusion of

motion. They are usually small with respect to the size of the screen. For example, toanimate a rabbit hopping across a meadow ( ), the animator would create a

sequence of images showing the rabbit hopping. This sequence of images would then

be composited one image per frame onto a background image of the meadow.

The disadvantage of this technique is that, slight changes in lighting and depth

cannot be reproduced. Consequently, sprite animation is most often used in interactive

media where rendering speed is more important than realism.

Two-dimensional Animation


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3D animation involves constructing a virtual world in which objects move and

interact. The animator must model, animate, and render the 3D scene. Modeling

involves describing the elements of a scene and placing them appropriately.Animating species how the objects should move in the 3D world. Rendering

converts the description of the objects and their motion into images.

Modeling Requirements: To animate motion, the user needs botha static description of an object and information about how that object

moves. One common way to specify this additional information is to use anarticulated model such as the one shown in the figure. The motion of

the elbow joint in a human model will affect not only the position of

the lower arm but also the position of the hand and fingers. A second type

of model used in animation is a particle system or collection of points.The laws of physics often provide a basis for the motion. Systems that are

modeled well by particle systems include water spray, smoke, and evenflocks of birds. Deformable objects are a third type of model and

include objects that do not have well-defined articulated joints. Water,

hair, clothing, and fish are among the systems that have been successfully

modeled as deformable objects.

Three-dimensional Animation


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Articulated model

While each of these model types can be used to describe

a wide variety of objects, complex systems often require

hybrid models that combine two or more types. This

approach allows each part of the system to be modeled by

the most appropriate technique. The right image shows a

diver entering a swimming pool. The diver is an articulatedmodel, the water in the pool is a deformable model, and

the spray is a particle system.

Deformable model

Particle system

Rendering requirements

Motion blur is a rendering technique that is requiredfor animation. Animations usually display images at 24

or 30 frames per second, and thus a continuous motion

is being sampled. This sampling process causes therapid motion of an object to create unpleasant effects

For example, objects such as wheels may appear to

move in the wrong direction. To solve this problem, afast moving object can be rendered in several of the

positions it had during the period of time represented by

a frame. This technique creates a blurred representation

of the object. While it may seem strange to think that

quality can be improved by blurring, humans perceive a

motion-blurred animation as more realistic.


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Motion Generation

The task of specifying the motion of an animated object to the computer is

surprisingly difficult. Because humans are very skilled at observing motion and

quickly detect motion that is unnatural or implausible. The animator must be able

to specify subtle details of the motion to convey the personality of a characteror the mood of an animation in a compelling fashion.

1. Motion capture

This method is a very popular technique because of the relative

ease with which many human motions can be recorded. Motion

capture, employs special sensors, called trackers, to record themotion of a human performer. The recorded data is then used

to generate the motion for an animation. However, a number of

problems prevent it from becoming an ideal solution.

First, accurately measuring the motion of the human body istricky because trackers attached to skin creates noise in the

recorded data. Furthermore, if the object used to generate the

recorded motion and the object have different dimensions, the

animation may have noticeable flaws. For example, if the actor

were resting his arms on a real table, the arms of the graphical

actor might appear to be suspended in air. Finally, all sensing

technologies have a relatively small field of view, which limitsthe kinds of actions that can be captured.


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2. Key Framing

Key-framing requires animator to outline the motion by specifying key positions

for the objects being animated. In a process known as in-betweening, a computer

interpolates to determine the positions for the intermediate frames. For example,

to key-frame hitting a baseball, the animator would pose a batter atseveral key moments in the sequence, such as the batter's initial stance

, the contact with the ball, and the follow through. The remaining

images would be filled in by the computer.

The interpolation algorithm is an important factor in the appearance of the

final motion. The simplest form of interpolation, linear interpolation, often results

in motion that appears jerky because the velocities of the moving objects arediscontinuous. To correct this problem better interpolation technique, such as

splines, is used to produce smoothly interpolated curves.

The specification of key-frames can be made easier with techniques such as

inverse kinematics. This technique aids in the placement of articulated models by

allowing the animator to specify the position of one object and have the positions

of the objects above it in the articulated hierarchy computed automatically. For

example, if the hand of an animated character must be in particular locations, aninverse kinematics algorithm could determine the elbow and shoulder

angles. Although these techniques make animation easier, key-framed animation

nevertheless requires that the animator intimately understand how the animatedobject should behave and have the talent to express that behavior in key-frames.


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Milestones of the animation industry in the 20th Century

http://www.awn.com/mag/issue4.10/4.10pages/cohenmilestones.php3

http://www.fact-index.com/a/an/animation.html#History%20of%20Animation

Brief History of NYIT Computer Graphics Labhttp://www-2.cs.cmu.edu/~ph/nyit/masson/nyit.html

Timeline from Brown Animation class

http://www.cs.brown.edu/courses/cs229/animTimeline.html

In-betweening

http://alpha.luc.ac.be/~lucp1112/research/CA2001/results.html

Rick Parent

http://www.cis.ohio-state.edu/~parent/book/Intr.html

http://www.cis.ohio-state.edu/~parent/book/outline.html

Americas Story

http://www.americaslibrary.gov/cgi-bin/page.cgi/sh/animation/blcktn_2

ResourcesFor more information about the implementation andtheory behind these techniques, we recommend thefollowing sites:
http://www.awn.com/mag/issue4.10/4.10pages/cohenmilestones.php3http://www.fact-index.com/a/an/animation.htmlhttp://www-2.cs.cmu.edu/~ph/nyit/masson/nyit.htmlhttp://www.cs.brown.edu/courses/cs229/animTimeline.htmlhttp://alpha.luc.ac.be/~lucp1112/research/CA2001/results.htmlhttp://www.cis.ohio-state.edu/~parent/book/Intr.htmlhttp://www.cis.ohio-state.edu/~parent/book/outline.htmlhttp://www.americaslibrary.gov/cgi-bin/page.cgi/sh/animation/blcktn_2http://www.americaslibrary.gov/cgi-bin/page.cgi/sh/animation/blcktn_2http://www.americaslibrary.gov/cgi-bin/page.cgi/sh/animation/blcktn_2http://www.americaslibrary.gov/cgi-bin/page.cgi/sh/animation/blcktn_2http://www.americaslibrary.gov/cgi-bin/page.cgi/sh/animation/blcktn_2http://www.cis.ohio-state.edu/~parent/book/outline.htmlhttp://www.cis.ohio-state.edu/~parent/book/outline.htmlhttp://www.cis.ohio-state.edu/~parent/book/outline.htmlhttp://www.cis.ohio-state.edu/~parent/book/Intr.htmlhttp://www.cis.ohio-state.edu/~parent/book/Intr.htmlhttp://www.cis.ohio-state.edu/~parent/book/Intr.htmlhttp://alpha.luc.ac.be/~lucp1112/research/CA2001/results.htmlhttp://alpha.luc.ac.be/~lucp1112/research/CA2001/results.htmlhttp://alpha.luc.ac.be/~lucp1112/research/CA2001/results.htmlhttp://alpha.luc.ac.be/~lucp1112/research/CA2001/results.htmlhttp://alpha.luc.ac.be/~lucp1112/research/CA2001/results.htmlhttp://www.cs.brown.edu/courses/cs229/animTimeline.htmlhttp://www.cs.brown.edu/courses/cs229/animTimeline.htmlhttp://www.cs.brown.edu/courses/cs229/animTimeline.htmlhttp://www-2.cs.cmu.edu/~ph/nyit/masson/nyit.htmlhttp://www-2.cs.cmu.edu/~ph/nyit/masson/nyit.htmlhttp://www-2.cs.cmu.edu/~ph/nyit/masson/nyit.htmlhttp://www-2.cs.cmu.edu/~ph/nyit/masson/nyit.htmlhttp://www.fact-index.com/a/an/animation.htmlhttp://www.fact-index.com/a/an/animation.htmlhttp://www.fact-index.com/a/an/animation.htmlhttp://www.fact-index.com/a/an/animation.htmlhttp://www.fact-index.com/a/an/animation.htmlhttp://www.fact-index.com/a/an/animation.htmlhttp://www.fact-index.com/a/an/animation.htmlhttp://www.awn.com/mag/issue4.10/4.10pages/cohenmilestones.php3http://www.awn.com/mag/issue4.10/4.10pages/cohenmilestones.php3http://www.awn.com/mag/issue4.10/4.10pages/cohenmilestones.php3http://www.awn.com/mag/issue4.10/4.10pages/cohenmilestones.php3http://www.awn.com/mag/issue4.10/4.10pages/cohenmilestones.php3http://www.awn.com/mag/issue4.10/4.10pages/cohenmilestones.php3

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Introduction to Computer Animation