Layered Acting for Character Animation

By Mira Dontcheva

Gary Yngve

Zoran Popović

presented by Danny House

SIGGRAPH 2003

Editing Motion

• Keyframing: positions and tangents

• Physical Simulation: initial conditions

• Motion Graphs: splicing

• Texturing

Layered Editing

Start with Broad Outline

Top Down Construction

Dimensions for Layering

• Accuracy

• Scale

• Articulation

Layers of Articulated Movement

• Torso

• Limbs to Fingers

• Head to Face to Eyes

• Secondary Motion

Interaction via Acting

• Correspondence of Movement and Time

• Not Quite Puppetry

• Motion Capture

Imitation

• Imitating the User

• Imitating the Animated Character

System Overview

• Hardware

• Space and Time Interaction

• Character Interaction

• Synthesis and Editing With Layers

Hardware

• 8 camera optical motion capture

• wireless mouse

• wall display screen

• various animation widgets

Space and Time Interaction

• Camera View and Studio Correspondence

• Distance to Object Affects Gain on Controls

• Ground Calibration

• Speed and Interval of Playback

• Anticipation Guides– Predictive Ghost Renderings– Projections of Trajectory Streamers

Character Interaction

• User Features

– position and orientation of widgets

• Character Features

• Correspondence Between Character and

User Features

Character Features

• Must be prespecified in the character model.

• May include– joints– points on the body– DOFs– IK handles

Correspondence Between Character and User Features

• Explicit selection by User

• Implicit selection via acting

• Absolute, trajectory-relative, or additive

Synthesis and Editing with Layers

• Select One or More Character Features

• Map them to a Widget

• Act with the Widget while Watching

Playback

• Repeat

Definitions for Feature Mapping

• X0: initial position of widget

• X(t): position of widget at time t• C´: rotational component of transform from studio

to camara coordinates

• Y0: initial position of character feature

• Y(t): position of character feature at time t• Y ´(t): character feature’s position after edit

• XR0, XR(t), etc, are the corresponding rotations

Absolute Feature Mapping

• Used for top level rough sketch of motion

• Good for contact with ground– the system supports snapping to the ground

• Useful for body parts that interact with in the scene

• Could be useful with physical props

Equations of Absolute Feature Mapping

• Y ´(t) = Y0 + C´(X(t) - X0)

• Y R´(t) = C´XR(t)(XR0)-1YR

0

Trajectory-Relative Mapping

• Useful for animating a feature relative to an existing parent motion.

• K(t) is the product of the matrices in the parent hierarchy of the feature being edited.

• Y ´(t) = Y0 + K(t)C´(X(t) - X0)

Additive Mapping

• Useful for adding detail or personality while preserving the underlying motion

• Y ´(t) = Y(t) + C´(X(t) - X(t-dt))

Equations of Trajectory-Relative Mapping

• K(t) is the change in the widget’s

translation over [0,t]

• Y ´(t) = Y0 + K(t)C´(X(t) - X0)

Implicit Editing

• The user selects the features to edit by imitating their behavior.

• The system discovers the relevant features using a statistical technique called “Canonical Correlation Analysis” or CCA.

• CCA is applied to each user/character feature pair.• Heuristics are used to choose the best pairs. They

are non-robust but effective in practice.

Physically Based Motion Transformation

• By Popović, Z. and Witkin, A. P.

• SIGGRAPH 99

• Sophisticated retargeting of motion data.

Preparation for Editing

• Character Simplification– manual elimination of many details

• Spacetime Motion Fitting– manual capture of pose and mechanical

constraints, addition of muscles, assumes optimality

Editing

• Tweak the Constraints– change a pose, alter flexibility of a joint,

introduce obstacles, alter gravity

• Motion Reconstruction– automtic

Interactive Manipulation of Rigid Body Simulations

• Popović, J., Seitz, S. M., Erdmann, M., Popović, Z., and Witkin, A. P.

• SIGGRAPH 2000

• Allows goal oriented editing of physical simulations.

• User specifies waypoints and velocities in simulation.

• Will alter initial conditions if permitted.

• Obtains extra DOFs by tweaking surface normals, elasticity.

• Small changes in these parameters are not noticed by audience.