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Retargetting Motion to New Characters

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Retargetting Motion to New Characters Michael Gleicher Motion Retargeting By Michael Gleicher * Outline Introduction Constraints and Objectives Examples Discussion ... – PowerPoint PPT presentation

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Title: Retargetting Motion to New Characters


1
Retargetting Motion to New Characters
  • Michael Gleicher

2
Outline
  • Introduction
  • Constraints and Objectives
  • Examples
  • Discussion

3
Motion retargetting is to adapt an animated
motion from one character to another,
independently of how the motion was created.
Example of retargetting
  • Figure with different sizes or proportions feet
    skating and wrong hands position.
  • Constraints essential to the action hands
    position, hands distance apart while carrying,
    feet on the ground.
  • A constrained optimization problem
  • preserve high-level properties
  • Spacetime constraints Make adjustments based on
    all the requirements over frames
  • Using the objective function minimized energy
    consumption.

4
Outline
  • Introduction
  • Constraints and Objectives
  • Examples
  • Discussion

5
Spacetime Constraints consider relationships
among multiple frames, make choices based on
other parts of the motion. 
Spacetime Constraints
Unsatisfied Results
  • Enforce the laws of physics, biomechanical
    limitations
  • Equality and Inequality
  • Difficult to encode mathematically
  • Richer sets are more accurate but may lead to
    difficulty to solve.

Objective function
  • Minimizing the amount of change in the important
    properties

IK solver considers each frame independently
6
Constraints are at particular instants of time
come from restrictions on the character,
the environment, or the motion. 
Define the problem
Modeling parameters
  • Original motion m0(t)
  • New (retargeted) Motion m(t)
  • Difference between the two d(t)
  • qti parameters of motion at time ti Define
    Constraints f(qti) constant
  • Equality Constraints an angle
  • through all the frames.
  • Inequality Motion dependent. Example In 5
    certain frame the joint angle a2 70 (active set
    method to solve)
  • B-Splines Use control points to model the
    angles.

m0(t) m1(t)
Retargetting the motion to a person with longer
arms
  • Objective Minimize?(ai - ai)2 for every frame
    t.
  • Using B-Splines with control points every 2, 4 or
    8 frames (uniform spacing), then interpolate
    other frames, so that for 8 frames spacing, the
    total parameters of 40 frames can be reduced from
    (24080) to (25 10), which increase the
    speed.

7
To solve this problem, parameters are modeled
using B-Splines and weighted by a sensitivity
matrix M.
M Sensitivities in the variable
Solving procedure
  • Start by scaling motion to match scaled
    character, and translation by finding the
    constraint displacements, interpolating and
    smoothing.
  • Solve non-linear constraint problem by solving
    the objective equation g(x)½xMx
  • M sensitivity function (weight) that summing all
    the displacement of a point in each frame
  • x B-Splines model of all the parameters (angles)
  • From a1 to a1 the limb motion is not really
    affected much by the change of the angle, while
    the change from a2 to a2 greatly affect the
    motion (sensitive). Use Matrix M to balance the
    parameters.

This part is illustrated in other two papers
Motion editing with spacetime constraints
(1997) Constraint-based Motion adaption (1996)
8
Outline
  • Introduction
  • Constraints and Objectives
  • Examples
  • Discussion

9
Examples on motion capture data from an optical
motion capture system, while Euler angle
representations are used for the joints.
Swing dancing
Walking
L original C only female adapted Rboth
adapted.
  • A walk adapted to a figure 60 size of the
    original. The smaller one is forced to use the
    original footplant positions. When the
    displacement keys are too distant, overfitting
    causes the wide swings in the yellow foot traces.
    Proper key spacing (blue) results in a motion
    similar to the original (purple).
  • Both two characters are adapted, even if only one
    changes size. Hands of the two must remain
    connected in addition to the footplant
    constraints. In center the female gets lifted by
    the hand-hold when spinning and in left both are
    adapted simultaneously with 11 more parameters
    per key.

10
Outline
  • Introduction
  • Constraints and Objectives
  • Examples
  • Discussion

11
The computing of the adaption to retarget motions
to a new character can be solved as a constrained
optimization problem.
Discussion
Morphing and differing
  • Morphing Use a different scaling amount and
    time-varying translation on each frame.
  • Differing characters with similar dimensions
    making constraints on correspondences between
    original and new features and than use standard
    retargetting again.
  • Geometric constraints and a simple objective
    function are used.
  • Quality of the resulting motions are influenced
    by the complexity of the constraints and
    objective function. Lack of physics constraints
    can lead to unrealistic situations.
  • Improved solvers for the numerical problems and
    techniques to avoid the burden of specification,
    would improve the results for a wider range of
    motions.
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