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 animation…it’s 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
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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) – That’s 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 right–must look right at full speed
• Hardto“see”themotiongiventhestills
• Hardto“see”themotionatthewrongframerate
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Traditional Animation Process
• Storyboard:sequenceofsketcheswithstory
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A Bug’s 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
It’s 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
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Source: Wikipedia
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Principles of Traditional Animation [Lasseter, SIGGRAPH 1987]
• Stylistic conventions followed by Disney’s 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
• inbetweensoftendon’tlooknatural
• 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 don’t 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 aren’t 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
• Splinesdon’talwaysdotherightthing
• Classicproblems
– Important constraints may break between keyframes
– feet sink through the floor
– hands pass through walls
– 3D rotations
• Euler angles don’t 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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