[SOLVED] CS flex CSCI 520 Computer Animation and Simulation

CSCI 520 Computer Animation and Simulation
Keyframe Animation
Jernej Barbic
University of Southern California
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Animation
There is no particular mystery in animationits really very simple, and like anything that is simple, it is about the hardest thing in the world to do.
Bill Tytla at the Walt Disney Studio, June 28, 1937.
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Computer Animation

Models have parameters:
polygon positions,
normals,
spline control points,
joint angles,
camera parameters,
lights,
color, etc.
n parameters define an n-dimensional state space Values of n parameters = point in state space

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Computer Animation
Animation defined by path through state space
To produce animation:
1. start at beginning of state space path
2. set the parameters of your model
3. render the image
4. move to next point along state space path, 5. Goto 2.
Path usually defined by a set of motion curves (one for each parameter)
Animation = specifying state space trajectory
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Animation vs Modeling
Modeling and animation are tightly coupled
Modeling: what are the control knobs and what do they do? Animation: how to vary them to generate desired motions?
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Animation vs Modeling
Building models that are easy to control is a VERY important part of doing animation
Hierarchical modeling can help
Where does modeling end and animation begin? Sometimes a fuzzy distinction

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Basic Animation Techniques
Traditional(framebyframe)
Keyframing
Proceduraltechniques
Behavioraltechniques(e.g.,flocking) Performance-based(motioncapture) Physically-based(dynamics)
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Traditional Animation
Film runs at 24 frames per second (fps) Thats 1440 pictures to draw per minute
1800 fpm for video (30fps)
Productions issues:
Need to stay organized for efficiency and cost reasons Need to render the frames systematically
Source: Wikipedia and Disney
Artistic issues:
How to create the desired look and mood while conveying story?
Artistic vision has to be converted into a sequence of still frames Not enough to get the stills rightmust look right at full speed
Hardtoseethemotiongiventhestills
Hardtoseethemotionatthewrongframerate
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Traditional Animation Process
Storyboard:sequenceofsketcheswithstory
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A Bugs Life [Pixar, 1998]

Traditional Animation Process
Keyframes
Important frames
Motion-based description
Example: beginning of stride, end of stride
Inbetweens:drawremainingframes
Traditionally done by (low-paid) human animators
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Layered Motion
Its often useful to have multiple layers of animation
How to make an object move in front of a background?
Use one layer for background, one for object
Can have multiple animators working simultaneously on different layers, avoid re-drawing and flickering
Transparent acetate allows multiple layers
Draw each separately
Stack them on a copy stand
Transfer onto film by taking a photograph of the stack

Source: Wikipedia
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Principles of Traditional Animation [Lasseter, SIGGRAPH 1987]
Stylistic conventions followed by Disneys animators and others (but this is not the only interesting style, of course)
From experience built up over many years
Squash and stretch use distortions to convey flexibility
Timing speed conveys mass, personality
Anticipation prepare the audience for an action
Followthrough and overlapping action continuity with next action Slow in and out speed of transitions conveys subtleties
Arcs motion is usually curved
Exaggeration emphasize emotional content
Secondary Action motion occurring as a consequence
Appeal audience must enjoy watching it
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Squash and Stretch
[convey rigidity and mass of an object by distorting its shape]
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Squash
and Stretch
[convey rigidity and mass of an object by distorting its shape]
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Slow in and out
[the spacing of the in-between frames to achieve subtlety of timing and movement]
Source: SIGGRAPH
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Anticipation
[the preparation for an action]
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Follow-through
[the termination of an action and establishing its relationship to the next action]
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Secondary Action
[action that results from another action]
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Computer-Assisted Animations
Computerized Cel painting
Digitize the line drawing, color it using seed fill
Eliminates cel painters
Widely used in production (little hand painting any more) e.g. Lion King
Cartoon Inbetweening
Automatically interpolate between two drawings to produce inbetweens (similar to morphing)
Hard to get right
inbetweensoftendontlooknatural
whataretheparameterstointerpolate?Notclear notusedveryoften
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True Computer Animations
Generateimagesbyrenderinga3Dmodel
Varyparameterstoproduceanimation
Bruteforce
Manually set the parameters for every frame
1440n values per minute for n parameters Maintenance problem
Computerkeyframing
Lead animators create important frames
Computers draw inbetweens from 3D(!) Dominant production method
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Interpolation
Hardtointerpolatehand-drawnkeyframes Computers dont help much
Thesituationisdifferentin3Dcomputeranimation:
Each keyframe is a defined by a bunch of parameters (state)
Sequence of keyframes = points in high-dimensional state space
Computerinbetweeninginterpolatesthesepoints
How?Youguessedit:splines
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Keyframing Basics
Despite the name, there arent really keyframes, per se
For each variable, specify its value at the important frames. Not all variables need agree about which frames are important
Hence, key values rather than key frames
Create path for each parameter by interpolating key values
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Keyframing: Issues
Whatshouldthekeyvaluesbe?
Whenshouldthekeyvaluesoccur?
Howcanthekeyvaluesbespecified?
Howarethekeyvaluesinterpolated?
WhatkindsofBADTHINGScanoccurfrom interpolation?
Invalid configurations (pass through objects) Unnatural motions (painful twists/bends)
Jerky motion
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Keyframing: Production Issues
Howtolearnthecraft
apprentice to an animator practice, practice, practice
Pixarstartswithanimators,teachesthem computers and starts with computer folks and teaches them some art
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Interpolation
Splines: non-uniform, C1 is pretty good
Velocity control is needed at the keyframes
Classic example: a ball bouncing under gravity zero vertical velocity at start
high downward velocity just before impact
lower upward velocity after
motion produced by fitting a smooth spline looks unnatural
What kind of spline might we want to use?
Hermite is good
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Problems with Interpolation
Splinesdontalwaysdotherightthing
Classicproblems
Important constraints may break between keyframes
feet sink through the floor
hands pass through walls
3D rotations
Euler angles dont always interpolate in a natural way
Classicsolutions:
More keyframes!
Quaternions help fix rotation problems
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Example: From Toy Story (1995)
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Scene from Toy Story 2
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Some Research Issues
Inversekinematics
How to plot a path through state space Multiple degrees of freedom
Also important in robotics
Summary
TraditionalAnimation KeyframeAnimation ComputerAnimation
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