retargetting motion to new characters

Retargetting Motion to New Characters

Michael Gleicher

Motion Retargeting By Michael Gleicher 2

Outline

Introduction Constraints and Objectives Examples Discussion


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.


Outline



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.

IK solver considers each frame independently

Objective function

Minimizing the amount of change in the important properties

t t

tdtmtmmg 220 )())()(()(


Constraints are at particular instants of time; come from restrictions on the character, the environment, or the motion.

Define the problemModeling 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 α2= 70 (active set method to solve)

B-Splines: Use control points to model the angles.

α1α2

α1’α2’

Retargetting the motion to a person with longer armsm0(t) m1(t)

Objective: Minimize∑(αi - αi’)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 (2×40=80) to (2×5 = 10), which increase the speed.

[0,180]1


To solve this problem, parameters are modeled using B-Splines and weighted by a sensitivity matrix M.

M: Sensitivities in the variableSolving 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 α1 to α1’ the limb motion is not

really affected much by the change of the angle, while the change from α2 to α2’ 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)

α1

α2

α1’

α1’

m0(t) m1(t)

m0(t) m1(t)


Outline



Examples on motion capture data from an optical motion capture system, while Euler angle representations are used for the joints.

Swing dancingWalking

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.

L: original; C: only female adapted; R:both adapted.


Outline



The computing of the adaption to retarget motions to a new character can be solved as a constrained optimization problem.

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.

DiscussionMorphing and differing

retargetting motion to new characters

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