Interactive Motion Deformation with Prioritized Constraints

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Interactive Motion Deformation with Prioritized
Constraints

• Benoît Le Callennec and Ronan Boulic
• VRLAB-EPFL

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Content

• Constraint-based motion editing
• Why do we need prioritized constraints?
• Overview of the algorithm
• Priority-based Inverse Kinematics
• Shape-constraints
• Readjusting unbalanced postures
• Results
• Future work and conclusion

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1. Constraint-based Motion Editing

• Changing an existing animation to satisfy a set
  of predefined constraints
• Important features
• Retain as much of the original motion as possible
• Preserve high-frequencies
• Fast feedback
• Constraints easy to define and to edit (no
  mathematical equations, only high-level physical
  constraints)

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2. Why Do We Need Prioritized constraints?

• Conflicts between constraints when editing an
  animation
• Weighting strategy
• No simple relationship weights ? residual errors
• No clear hierarchy of constraints
• Priority strategy
• Concept of relative importance between
  constraints
• Intuitive way to solve conflicts

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2. Why Do We Need Prioritized constraints? -
Example

• Right foot should stay on the ground
• Left foot attracted to another location

• Weights

Priorities
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3. Overview of the Algorithm

• Per-frame (plus filtering) method
• For each frame to compute
• Initialize each joint
• Controlled ? previous deformed frame
• Not controlled ? original motion
• Update the constraints
• Shape-constraints, balance constraint, rotational
  constraints
• Apply priority-based Inverse Kinematics
• Solution as close as possible to the previous
  deformed posture
• Filter results when needed

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4. Priority-based Inverse Kinematics

• Based on the resolved-motion rate method
• Linearize the non-linear equations
• Compute the Jacobian matrix J(?)
• Solve the linear system J(?)d? dx
• Repeat this process
• Exploit the redundancy of the problem
• Projection of each Jacobian onto the Null-space
  of the Jacobians of higher priority levels
• A low-priority constraint cannot perturb higher
  priority ones

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5. Shape-Constraints - Definition

• Continuous in space except at specific points
• Defined as a pair of curves
• Trajectory S(u) defined as a Kochanek-Bartels
  spline
• Break the continuity at control points
• Absolute or displacement (relative to a point)
• Time curve T(t) defined as an increasing
  piecewise linear function
• Stop an end-effector at a location for a period
  of time
• Position P of an end-effector at time t? defined
  as
• P S(T(t?))

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5. Shape-Constraints - Example
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6. Readjusting Unbalanced Postures

• Control of the Center of Mass using Inverse
  Kinetics
• Not flexible to directly consider its original
  position
• Consider its position related to a set of
  reference points on the character
• Advantages
• Simple and efficient to compute
• Directly fits in our framework
• CoMs position can have a priority
• Works well for a large variety of deformations
• But not suited for deformations adding dynamics
  contents
• For example, changing the direction of a jump

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7. Results

• Integration into Alias/Maya 5.0 as plug-ins
• A simple karate animation using three
  constraints
• Right foot constrained to stay on the ground
• Left foot constrained to reach different
  locations
• Balance

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7. Results
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7. Results

• Several animations generated by our algorithm and
  rendered into Alias/Maya 5.0

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7. Results
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8. Future Work and Conclusion

• On-line filters?
• Importance of the input motion
• CoMs trajectory based on the input motion
• Many softwares can pre-filter the initial motion
• Footplants definition
• Specific class of constraints
• Unfeasible to define each footplant by hand
• Automatic footplants detection

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Thanks for your attention

• Questions?