Retargetting Motion to New Characters

1. Retargetting Motion to New Characters Michael Gleicher

2. Outline • Introduction • Constraints and Objectives • Examples • Discussion Motion Retargeting By Michael Gleicher

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. Motion Retargeting By Michael Gleicher

4. Outline • Introduction • Constraints and Objectives • Examples • Discussion Motion Retargeting By Michael Gleicher

5. Spacetime Constraints consider relationships among multiple frames, make choices based on other parts of the motion.  Unsatisfied Results Spacetime Constraints • 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 Motion Retargeting By Michael Gleicher

6. Constraints are at particular instants of time; come from restrictions on the character, the environment, or the motion.  Modeling parameters Define the problem • 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. α2’ α1’ α2 α1 m0(t) m1(t) Retargetting the motion to a person with longer arms • 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. Motion Retargeting By Michael Gleicher

7. To solve this problem, parameters are modeled using B-Splines and weighted by a sensitivity matrix M. Solving procedure M: Sensitivities in the variable • 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) α1 α1’ m0(t) m1(t) α2 α1’ m0(t) m1(t) • 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) Motion Retargeting By Michael Gleicher

8. Outline • Introduction • Constraints and Objectives • Examples • Discussion Motion Retargeting By Michael Gleicher

9. Examples on motion capture data from an optical motion capture system, while Euler angle representations are used for the joints. Walking Swing dancing L: original; C: only female adapted; R:both 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. Motion Retargeting By Michael Gleicher

10. Outline • Introduction • Constraints and Objectives • Examples • Discussion Motion Retargeting By Michael Gleicher

11. The computing of the adaption to retarget motions to a new character can be solved as a constrained optimization problem. Morphing and differing Discussion • 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. Motion Retargeting By Michael Gleicher