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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

CS4455

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

CS 4455

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

CS 4455

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

CS 4455

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

CS 4455

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

CS 4455

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

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keyframing1
Keyframing

CS 4455

keyframing2
Keyframing

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keyframing3
Keyframing
  • Fine level of control
  • Quality of motion depends on skill of animator

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

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

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keyframing4
The development process:

Adjust trajectory

Playback motion

Parameters:

Locations

Joint angles

Shape -- flexible objects

Material properties

Camera Motion

Lighting

Keyframing

CS 4455

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

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key frames
Key Frames

1

2

3

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key frames1
Key Frames
  • Timeline
  • 1 2 3 3 1

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

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inbetweening1
Inbetweening
  • Frame-Independent (“right”) slow in/out
    • Compute acceleration a

Time of last frame

Time

Tend

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spline driven animation
Spline-driven Animation

y

x,y = Q(u)

for u:[0,1]

  • Equal arc lengths
  • Equal spacing in u

x

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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)

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

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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

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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

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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)

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motion capture1
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

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motion capture2
Motion Capture
  • Goals:
    • Realistic motion
    • Lots of different motions (300-1000)
    • Contact
  • Appropriate game genres
    • Sports
    • Fighting
    • Human characters

CS 4455

applications
Applications

Movies, TV

Video games

Performance animation

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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

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

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

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technology1
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

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technology2
Technology
  • Passive reflection --Acclaim, Motion Analysis
    • Automatically digitized
    • 240Hz
    • Not real-time, Correspondence
    • 3+ markers/body part
    • 2+ cameras for 3D position data

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technology3
Technology
  • Vicon Motion Systems
    • Retroreflective paint on reflectors
    • Lights on camera
    • Very high contrast markers

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technology4
Technology
  • Active light sources -- Optotrak
    • Automatically digitized
    • 256 markers
    • 3500 marker/sec
    • Real-time
    • Specialized cameras

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technology5
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

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technology6
Technology
  • Exoskeleton + angle sensors
    • Analogous
    • Tethered
    • No identification problem
    • Realtime - 500Hz
    • No range limit - Fit
    • Rigid body approximation

CS 4455

technology7
Dataglove

Low accuracy

Focused resolution

Monkey

High accuracy

High data rate

Not realistic motion

No paid actor

Technology

Mechanical motion capture

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technology8
Technology
  • Technology issues
    • Resolution/range of motion
    • Calibration
    • Accuracy
    • Occlusion/Correspondence

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animation issues
Animation Issues
  • Style
  • Scaling
  • Generalization

CS 4455

resolution
Resolution
  • Positioning of camera

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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

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

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calibration1
Calibration
  • Extract best limb lengths
  • Use estimator to compute limb length
  • Minimize or reject outliers

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calibration2
Calibration
  • Example estimator:
    • 508 frames of walking
    • 6 bad frames
    • Collarbone to shoulder:
      • Hand editing: 13.3cm
      • Estimator: 13.2cm
      • Arithmetic mean: 14.1cm

CS 4455

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

CS 4455

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

CS 4455

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

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

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

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generalizating animation1
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

CS 4455

generalizating animation2
Generalizating Animation
  • Motion Editing With Spacetime Constraints
    • Michael Gleicher
    • 1997 Symposium on Interactive 3D Graphics

CS 4455

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

CS 4455

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

CS 4455

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

Active -- Internal source of energy

CS 4455

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

CS 4455

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

CS 4455

generating motion1
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

CS 4455

keyframing5
Keyframing
  • Fine level of control
  • Quality of motion depends on skill of animator

CS 4455

motion capture3
Motion Capture
  • Natural-looking motion
  • Hard to generalize motions
    • Registration is difficult
  • Often seems “weightless” – Bill Kroyer, Rhythm & Hues

CS 4455

simulation broadly defined1
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

CS 4455

when to use what method1
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

CS 4455

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

CS 4455