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 X 0 : initial position of widget X(t): position of widget at time t C´: rotational component of transform from studio to camara coordinates Y 0 : initial position of character feature Y(t): position of character feature at time t Y ´(t): character feature’s position after edit X R 0, X R (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) = Y 0 + C´(X(t) - X 0 ) Y R ´(t) = C´X R (t)(X R 0 ) -1 Y R 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) = Y 0 + K(t)C´(X(t) - X 0 )

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) = Y 0 + K(t)C´(X(t) - X 0 )

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.