Computer Graphics Animation Techniques

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Computer GraphicsAnimation Techniques
Slides borrowed from Ronen Barzel http//www.ense
ignement.polytechnique.fr/profs/informatique/Ronen
.Barzel/
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Types of Animation

• Scripted Animation
• Procedural Animation
• Motion capture

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Scripted Animation

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Procedural animation

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Motion capture
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Motion Capture

• capture of motion of (human) actor
• whole body, upper body, face
• Technology optical, magnetic, mechanical
• Employs special sensors (trackers) to record
  motion of human performer
• Problems
• Difficult to accurately measure motion of human
  body clothes may shift, etc
• If object used to generate recorded motion and
  graphical object have different dimensions -gt
  animation will have noticeable flaws.

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Topics in Animation

• Scripted Animation
• Keyframe interpolation, articulated models,
  inverse kinematics, deformations,
• Procedural Animation
• Particle Systems, Flocks, Crowds, Cloth, Fire,
  Smoke, Water,
• Motion capture
• Filtering, editing, retargeting, stitching,

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Overview of CG animation

• Story

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Overview of CG animation

• Art

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Overview of CG animation

• Recording

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Overview of CG animation

• Modeling

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Overview of CG animation

• Layout

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Overview of CG animation

• Animation

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Overview of CG animation

• Shading

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Overview of CG animation

• Lighting

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Overview of CG animation

• Rendering

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Why CG animation?

• One medium of many
• live action, classical animation, clay, puppets,
  
• 3D has aspects of live film
• Camera, cinematography
• CG has aspects of classical animation
• Stylization, fantasy, scripted motion,
• Accessible
• Computer editing.
• Dont need to act, dont need to draw.
• Need to understand acting, need to understand
  images

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Keyframe animation classical

• Animator draws character at extreme poses
• Fill in in-betweens
• Illusion of Life

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Keyframe animation CG

• Model parameters
• Position (x,y,z), joint angles, deformations,
• Key frames data points
• In-between interpolation
• Many ways to interpolate
• linear
• splines (polynomial curves)

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Linear interpolation

• Given points P0 and P1, define curve L(t)
• L(t) (1-t) P0 t P1 t in 0,1
• Weighted average of endpoints.
• Curve is linear segment

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Linear interpolation N points

• Given P0PN, define segment
• Li(s) (1-s) Pi s Pi1 s in 0,1
• Then define piecewise-linear curve
• L(t) Li(s) for tilt t lt ti1
• Where s (t-ti)/(ti1-ti)
• and (lets say) ti i/N

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Spline curves

• To make smooth curves from data points
• Many types of splines, many properties
• Interpolating, approximating,
• Build an order-k polynomial from k1 points

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Spline curve segment

• General form for a segment
• C(t) B0(t)P0 Bk(t)Pk
• Weighted average of k control points
• Basis functions Bi(t), order-k polynomials
• Many choices, many properties
• Most commonly k3 (cubic)
• E.g. Bézier curve

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Speed along the spline

• Uniform parameterization speed varies
• Arc-length parameterization constant speed
• User-specified speed

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Animation Principles

• From
• Principles of Traditional Animation Applied
  to 3D Computer Animation
• John Lasseter, ACM Computer Graphics, 21(4), 1987
• In turn from
• The Illusion of Life

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Squash and stretch

• Determines feeling of weight, flexibility,
  response to pressure.
• Maintain volume

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Timing

• Rate of acceleration conveys weight
• Speed and acceleration of characters movements
  convey emotion

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Anticipation

• Prepare for each movement
• For physical realism
• To direct audiences attention

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Anticipation Overlap
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Follow Through

• Overlapping motion
• Motion doesnt stop suddenly
• Pieces continue at different rates
• One motion starts while previous is finishing,
  keeps animation smooth

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Staging

• Picture is 2D
• Make situation clear
• Audience looking in right place
• Action clear in silhouette

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Action

• Traditional Pose-to-pose or straight ahead
• CG allows layered animation
• First animate core and important parts
• Gradually go back and refine, add others
• In practice, combination of blocked poses and
  layered motion.

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Ease-in and Ease-Out

• Movement doesnt start stop abruptly.
• Also contributes to weight and emotion

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Arcs

• Move in curves, not in straight lines
• www.comet-cartoons.com

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Exaggeration

• Helps make actions clear
• Helps emphasize story points and emotion
• Must balance with non-exaggerated parts

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Secondary action

• Motion that results from some other action
• Needed for interest and realism
• Shouldnt distract from primary motion
• (today often use simulation to compute secondary
  motion of hair, clothing, etc.)

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Appeal

• Attractive to the eye, strong design,
• Avoid symmetries

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Personality

• Action of character is result of its thoughts
• Must know purpose and mood before animating each
  action.
• No two characters move the same way.

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Animating skinned characters
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Skeleton

• restricted transformations
• no drawn geometry, just bones

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Use of skeleton

• support for direct manipulation
• support for inverse kinematics
• skinning to build models

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Even for non-humans

• www.sharktacos.com

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Even for non-animals

• www.hermoni.com/workshop

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Kinematics

• Considers only motion
• Determined by positions, velocities,
  accelerations
• Forward kinematics
• Low level approach where animator has to
  explicitly specify all motions of every part of
  the animated structure
• Each node in hierarchy inherits movement of all
  nodes above it
• Inverse kinematics
• Requires only the position of the ends of the
  structure
• Functions as black box - controls detailed
  movement of entire structure

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Forward Kinematics

• Animator specifies joint angles
• Computer finds position of end-effector X

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Inverse Kinematics

• Animator specifies end-effector positions
• Computer finds joint angles

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Kinematics Summary

• Forward Kinematics
• Animator has complete freedom over the movements
  of any part of the structure
• Amount of work is function of the complexity of
  the structure, much more expensive when dealing
  with complex structures
• Inverse Kinematics
• Does not leave much scope for the animator to
  inject character into the movements
• Model does not possess the interpretation of a
  human being
• Overall movement of structure depends on the
  formula -gt same footprints, same movements

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What makes this hard?

• Not always a unique solution
• Not always well-behaved
• Nonlinear problem
• Joint limits

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Not always a unique solution

• Disjoint solutions
• Continuum ofsolutions
• No solution

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Not always well-behaved

• Small change in X can cause big change in ?
• Changing ? might not move end towards X

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Deformation

• To deform a model, move its control points.
• The rest is details
• Types of deformation
• Skeleton deformation
• Function-based deformation
• Free-form deformation
• Point cluster deformers
• Shape interpolation, morphing

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Skeleton deformation

• Skeleton (IK) inside the skin

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Skeleton deformation

• Associate each point with nearest link
• When link moves, transform its points.

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Problem collapsing, kinking
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Point weights

• Each point gets affected by several links
• Take weighted average
• Adjust the weights until it looks good

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Function-based deformation

• Define a function over all space
• M R3 ? Transformation (matrix)
• To transform a point P
• evaluate function M at P
• transform P by the result
• P M(P) P

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Undeformed model
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Taper
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Twist
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Vortex
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Bend
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Bend

• Given x0, y0, h, q, rh/q
• Three regions
• Below y0 unaffected
• Above y0h
• translate down by h
• rotate by -q about (x0r, y0)
• Between y0 and y0h
• interpolate translation
• interpolate rotation angle

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Combinations of deformations
Original
Bend
Twist
BendTwist
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Potential problem

• If there arent enough points, model collapses
• Solutions
• adaptively create new points
• build models with enough points where needed

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Free-form deformation (FFD)

• Define a lattice around the model
• Move the points of the lattice
• The model deforms with it

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FFD example
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FFD example
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FFD interpolation

• Different ways to interpolate

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Computing FFD

• find (s,t,u) coordinatesof P in original grid
• interpolate deformedgrid points at (s,t,u)

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Computing FFD coordinates

• Grid
• originQ, orthogonal axesU,V,W, cellsl,m,n
• Grid points
• Point to deform

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Computing FFD interpolation

• Grid points moved to Gijk
• Interpolate using multidimensional Bézier
• Or use piecewise lower-order Bézier segments

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FFD with arbitrary topology
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Skeleton with FFD

• Skeleton moves FFD grid
• FFD moves points

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Point cluster deformers

• Select cluster of points
• Apply an operation directly to some points
• Weights often set by spatial fields

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Point cluster deformers
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Point cluster deformers

• Weights painted on by hand
• (there are more points than shown in the
  wireframe)

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Wires

• Reference curves on model
• Draw target curves

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Shape interpolation

• sculpt several target shapes
• use weighted average
• meshes must have same topology

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Shape interpolation

• used often for mouth shapes
• shapes with different topology