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Animation Low-Level behaviors Overview Keyframing Motion Capture Simulation Low-Level Behaviors Keyframing Motion Capture Simulation Generating Motion What matters? Quality of motion appropriate for rendering style and frame rate Controllable from UI Controllable from AI

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animation

Animation

Low-Level behaviors Overview

Keyframing

Motion Capture

Simulation

IAT 410

low level behaviors
Low-Level Behaviors
  • Keyframing
  • Motion Capture
  • Simulation

IAT 410

generating motion
Generating Motion
  • What matters?
    • Quality of motion appropriate for rendering style and frame rate
    • Controllable from UI
    • Controllable from AI
    • Personality of the animated character

IAT 410

keyframing
Keyframing
  • Fine level of control
  • Quality of motion depends on skill of animator

IAT 410

motion capture
Motion Capture
  • Natural-looking motion
  • Hard to generalize motions
    • Registration is difficult
    • “Weightless” according to professional animators

IAT 410

simulation broadly defined
Simulation (Broadly Defined)
  • Physics is hard to simulate
  • Pseudo-physics is somewhat hard
  • Control is very hard
  • Gives Generalization + Interactivity

Desired

Behavior

Forces and Torques

Model

Numerical Integrator

User/AI

Control

State

Graphics

IAT 410

when to use what method
When to Use What Method?
  • Keyframing
    • Sprites and other simple animations
    • Non-human characters
    • Coarse collision detection
  • Motion Capture
    • Human figures
    • Subtle motions, long motions
  • Simulation
    • Passive simulations
    • When interactivity w/ motion is important

IAT 410

keyframing10
Keyframing

IAT 410

keyframing11
Keyframing

IAT 410

keyframing13
Keyframing
  • Fine level of control
  • Quality of motion depends on skill of animator

IAT 410

hand drawn animation 2d
Hand Drawn Animation -- 2D
  • Sketches
  • Pencil tests
  • Inking
  • Coloring
  • Digitize to sprites

IAT 410

computer animation 2d or 3d
Computer Animation: 2D or 3D
  • Sketches
  • Models and materials
  • Key configurations
  • Playback of motion or render to sprites

IAT 410

keyframing16
The development process:

Adjust trajectory

Playback motion

Parameters:

Locations

Joint angles

Shape -- flexible objects

Material properties

Camera Motion

Lighting

Keyframing

IAT 410

keyframing interpolation
Keyframing Interpolation
  • Inbetweening

v

1,2,3 4 5 6 7,8,9…

Linear

time

v

1,2,3 4 5 6 7,8,9…

Slow in, Slow out

time

IAT 410

key frames
Key Frames

1

2

3

IAT 410

key frames20
Key Frames
  • Timeline
  • 1 2 3 3 1

IAT 410

inbetweening
Inbetweening
  • Frame dependent (“wrong”) slow-in/out
    • Iterate once per frame
    • This is a variant on the Infinite Impulse Response (IIR) filter:

IAT 410

inbetweening22
Inbetweening
  • Frame-Independent (“right”) slow in/out
    • Compute acceleration a

Time of last frame

Time

Tend

IAT 410

spline driven animation
Spline-driven Animation

y

x,y = Q(u)

for u:[0,1]

  • Equal arc lengths
  • Equal spacing in u

x

IAT 410

reparameterize arc length
Reparameterize Arc Length
  • S= A(u) = arc length
  • Reparam:
  • Find:

Bisection search for a value

of u where A(u) = S with a

numerical evaluation of A(u)

(Details in Watt & Watt)

IAT 410

keyframing constraints
Keyframing -- Constraints
  • Joint limits
  • Position limits
  • Inverse kinematics

IAT 410

kinematics
Kinematics
  • The study of motion without regard to the forces that cause it

Draw graphics

Specify fewer Degrees Of

Freedom (DOF)

More intuitive control of DOF

Pull on hand

Glue feet to ground

IAT 410

what makes ik hard
What makes IK Hard?
  • Many DOF -- non-linear transcendental equations
  • Redundancies
    • Choose a solution that is “closest” to the current configuration
    • Move outermost links the most
    • Energy minimization
    • Minimum time

IAT 410

ik difficulties
IK Difficulties
  • Singularities
    • Equations are ill-conditioned near singularities
    • High state-space velocities for low Cartesian velocities
  • Goal of “Natural Looking” motion
    • Minimize jerk (3rd derivative)

IAT 410

motion capture33
Motion Capture
  • What do we need to know?
    • X, Y, Z
    • Roll, Pitch, Yaw
  • Errors cause
    • Joints to come apart
    • Links grow/shrink
    • Bad contact points
  • Sampling Rate and Accuracy

IAT 410

motion capture34
Motion Capture
  • Goals:
    • Realistic motion
    • Lots of different motions (300-1000)
    • Contact
  • Appropriate game genres
    • Sports
    • Fighting
    • Human characters

IAT 410

applications
Applications

Movies, TV

Video games

Performance animation

IAT 410

plan out shoots carefully
Plan out Shoots Carefully
  • Know needed actions (80-100 takes/day)
    • Bridges between actions
    • Speed of actions
    • Starting/ending positions
  • Hire the right actor
    • Watch for idiosyncrasies in motion
    • Good match in proportions

IAT 410

sensor placement
Sensor Placement
  • Place markers carefully
    • Capture enough information
    • Watch for marker movement
  • Check data part way through shoot
  • Videotape everything!

IAT 410

technology
Technology
  • Numerous technologies
  • Record energy transfer
    • Light
    • Electromagnetism
    • Mechanical skeletons

IAT 410

technology42
Technology
  • Passive reflection – Peak Performance Tech
    • Hand or semi-automatically digitized
    • Video
    • Time consuming
  • Issues
    • No glossy or reflective materials
    • Tight clothing
    • Marker occlusion by props
    • High frames/sec

IAT 410

technology43
Technology
  • Passive reflection --Acclaim, Motion Analysis
    • Automatically digitized
    • 240Hz
    • Not real-time, Correspondence
    • 3+ markers/body part
    • 2+ cameras for 3D position data

IAT 410

technology44
Technology
  • Vicon Motion Systems
    • Retroreflective paint on reflectors
    • Lights on camera
    • Very high contrast markers

IAT 410

technology45
Technology
  • Active light sources -- Optotrak
    • Automatically digitized
    • 256 markers
    • 3500 marker/sec
    • Real-time
    • Specialized cameras

IAT 410

technology46
Technology
  • Electromagnetic Transducers
    • Ascension Flock of Birds, etc
    • Polhemus Fastrak, etc
  • Limited range/resolution
    • Tethered (cables to box)
    • Metal in environment (treadmill, Rebar!)
    • No identification problem
    • 6DOF Realtime
    • 30-144 Hz 13-18 markers

IAT 410

technology47
Technology
  • Exoskeleton + angle sensors
    • Analogous
    • Tethered
    • No identification problem
    • Realtime - 500Hz
    • No range limit - Fit
    • Rigid body approximation

IAT 410

technology48
Dataglove

Low accuracy

Focused resolution

Monkey

High accuracy

High data rate

Not realistic motion

No paid actor

Technology

Mechanical motion capture

IAT 410

technology49
Technology
  • Technology issues
    • Resolution/range of motion
    • Calibration
    • Accuracy
    • Occlusion/Correspondence

IAT 410

animation issues
Animation Issues
  • Style
  • Scaling
  • Generalization

IAT 410

resolution
Resolution
  • Positioning of camera

IAT 410

markers calibration
Markers, Calibration
  • Marker Placement
    • Location should move rigidly with joint
    • Stay away from bulging muscles, loose skin
    • Shoulders: Skeletal motion not closely tied to skin motion
  • Calibration
    • Zero position
    • Fine calibration by hand

IAT 410

calibration
Calibration
  • Finding Joint Locations
    • Move markers to joint centers
      • Assume rigid links, rotary joints
  • Shoulder?

IAT 410

calibration54
Calibration
  • Extract best limb lengths
  • Use estimator to compute limb length
  • Minimize or reject outliers

IAT 410

calibration55
Calibration
  • Example estimator:
    • 508 frames of walking
    • 6 bad frames
    • Collarbone to shoulder:
      • Hand editing: 13.3cm
      • Estimator: 13.2cm
      • Arithmetic mean: 14.1cm

IAT 410

accuracy
Accuracy
  • Marker movement
  • Noise in sensor readings
  • Skew in measurement time
  • Environment restrictions
  • Frame rate
    • High frame rate allows good filtering

IAT 410

camera calibration
Camera Calibration
  • Internal camera parameters
    • Optical distortion of lens
  • External parameters
    • Position and orientation
  • Correlation between multiple cameras

IAT 410

model based techniques
Model-Based Techniques
  • Restricted search space for markers
  • Dynamics (velocity integration)
    • No infinite accelerations
  • Model of behavior
  • Model of bodies of occlusion
    • Objects don’t pass through each other

IAT 410

scaling animation
Scaling Animation
  • Contact
  • Movement style
  • Inverse kinematics

IAT 410

generalizating animation
Generalizating Animation
  • Interpolation Synthesis for Articulated Figure Motion
  • Wiley and Hahn
  • IEEE CG&A v17#6

IAT 410

generalizating animation61
Generalizating Animation
  • Keyframes as constraints in a smooth deformation
    • Create functions by hand that warp the joint angle curves through time
  • Keyframe placing the ball on the racket at impact

Motion Warping

Witkin and Popovic, SIGGRAPH’95

IAT 410

motion warping
Motion Warping
  • For each joint angle curve C(t)
  • Cnew(t) = C(t) * A(t) + B(t)
  • A(t) is usually a smooth, gentle curve

IAT 410

generalizating animation63
Generalizating Animation
  • Motion Editing With Spacetime Constraints
    • Michael Gleicher
    • 1997 Symposium on Interactive 3D Graphics

IAT 410

blending animations
Blending Animations
  • Efficient Generation of Motion Transitions Using Spacetime Constraints
    • Rose, Guenter, Bodenheimer, Cohen
    • Siggraph ’96
    • Uses dynamics to compute plausible paths
    • Blends these paths

IAT 410

simulation
Simulation
  • Modeling the real world with simple physics
    • Realism
    • A set of rules
    • Better interactivity
  • Objects or Characters

IAT 410

passive no muscles or motors
Passive -- No muscles or motors

Active -- Internal source of energy

IAT 410

equations of motion
Equations of Motion
  • Water
  • Explosions
  • Rigid body models

IAT 410

control systems
Control Systems
  • Wide variety of behaviors
  • Transitions between behaviors
  • Controllable by AI or UI
  • Robust

IAT 410

generating motion70
Generating Motion
  • What matters?
    • Quality of motion appropriate for rendering style and frame rate
    • Controllable from UI
    • Controllable from AI
    • Skills of the animated character
    • Personality of the animated character

IAT 410

keyframing71
Keyframing
  • Fine level of control
  • Quality of motion depends on skill of animator

IAT 410

motion capture72
Motion Capture
  • Natural-looking motion
  • Hard to generalize motions
    • Registration is difficult
  • Often seems “weightless” – Bill Kroyer, Rhythm & Hues

IAT 410

simulation broadly defined73
Simulation (Broadly Defined)
  • Physics is hard to simulate
  • Pseudo-physics is somewhat hard
  • Control is very hard
  • Gives Generalization + Interactivity

Desired

Behavior

Forces and Torques

Model

Numerical Integrator

User/AI

Control

State

Graphics

IAT 410

when to use what method74
When to Use What Method?
  • Keyframing
    • Sprites and other simple animations
    • Non-human characters
    • Coarse collision detection
  • Motion Capture
    • Human figures
    • Subtle motions, long motions
  • Simulation
    • Passive simulations
    • When interactivity w/ motion is important

IAT 410

integration of technologies
Integration of Technologies
  • Layering
    • Add hand/finger motion later
    • Facial animation
  • Use keyframing to modify data
    • Fix holes in data
  • Use motion capture to drive simulation

IAT 410

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