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Interactive Motion Editing

This presentation by Troy McMahon explores Interactive Motion Editing, a technique that allows for the adaptation of existing motion capture data to accommodate variations across different characters and environments. By leveraging existing motion clips, animations can be efficiently generated without the need for new motion captures. The approach emphasizes optimizing character movements while satisfying spatial constraints through advanced methods such as inverse kinematics and spline fitting. The application aims to enhance reusability in motion graphics and animation, making it a valuable contribution to character animation technology.

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Interactive Motion Editing

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  1. Interactive Motion Editing Presented by Troy McMahon

  2. Interactive Motion Editing • Adapting existing motions to compensate for variations in characters and environments.

  3. Motivation Reusability: • Interactive Motion Editing allows us to generate new motions using existing motion clips to • This prevents us from having to capture these new motions.

  4. Video 1

  5. Motivation • Allows us to use motion capture date obtained for one character to animate another character of different dimensions. © Lee, Shinn, Siggraph 1999

  6. Related Work • Motion Warping, Siggraph 95 • Michael Gleicher, “Retargeting Motion to New Characters”, Siggraph 98 • S. Lee, G. Wolberg, and S. Y. Shin, “Scattered Data Interpolation With Multilevel B-Splines”, 1995

  7. Overview • Features of the target motion are represented as spacetime constraints. • Modify existing motions to conform to these constraints. • Goal: Satisfy constraints while preserving characteristics of original motion. • This is an optimization problem

  8. Overview of Algorithm • Intra-frame relationship • Use inverse kinematics to satisfy constraints • Inter-frame relationship • Use curve fitting to make motions smoother

  9. Overview of Algorithm • Use inverse kinematics to conform motion to constraints • Use curve fitting to reduce jerkiness • repeat

  10. Inverse Kinematics • Constraints reduce the number of variables (by making some variables dependent on others) • Use inverse kinematics to determine the optimal motion under these constraints “elbow circle” [Korein and Badler 82]

  11. Inverse Kinematics Solver • Used to adjust each frame to conform to constraints • This may introduce jerkiness

  12. Displacement Maps • Spline curves: The displacement of a coordinate as a function of time • Displacement maps: Array of spline curves over common knot sequence

  13. Motion Displacement Mapping • Map a displacement vector, d(t), to the existing motion vector, m(t), to obtain a new motion vector, m’(t), that satisfies the constraints. • m’(t)=m(t)d(t) • d(t) is not known • B-Spline Approximation technique © Lee, Shinn, Siggraph 1999

  14. B-Spline Approximation technique • Hill climbing algorithm • Each iteration: add a curve, di, that brings the displacement closer to d(t) • Curves go from course to fine • For a sufficiently large h, d(t)d1…dh • As hi, d1…dhid(t)

  15. Multilevel Spline Fitting © Lee, Shinn, Siggraph 1999

  16. Hieratical Motion Fitting • mh=(..(mod1) d2)… dh) • mi=(mi-1di) • At each level this algorithm uses the approximation from the previous level to generate a new approximation

  17. Hieratical Motion Fitting • At each level apply inverse kinetics to the motion from the previous level • Compute the displacement for each frame • Use curve fitting to calculate a displacement map • Use the displacement map to generate a new motion

  18. Motion Fitting Algorithm For every constraint in C Do h times

  19. Choosing An Initial Guess • The better the initial guess, the fewer levels you have to compute in order to obtain a good approximation. • Guess obtained by shifting the root in the original motion.

  20. Knot Sequences • The number of knots doubles with each iteration of motion fitting algorithm. • The more knots, the more closely the motion conforms to the constraints © Lee, Shinn, Siggraph 1999

  21. Results © Lee, Shinn, Siggraph 1999

  22. Results • Video 2 • Video 3 • Video 4

  23. Analysis © Lee, Shinn, Siggraph 1999

  24. Limitations • Does not take into consideration physics of the motions. • Resulting actions may not be natural or realistic.

  25. References • Jehee Lee, Sung Yong Shin, “A Hierarchical Approach to Interactive Motion Editing”, Siggraph 99

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