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Computer Animation

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  1. Computer Animation CS 445/645 Fall 2001

  2. Let’s talk about computer animation • Must generate 30 frames per second of animation (24 fps for film) • Issues to consider: • Is the goal to replace or augment the artist? • What does the artist bring to the project? • Is the scene/plot fixed or responsive to user? • What can we automate?

  3. Computer Animation

  4. Keyframing • Traditional animation technique • Dependent on artist to generate ‘key’ frames • Additional, ‘inbetween’ frames are drawn automatically by computer

  5. Keyframing How are we going to interpolate? From “The computer in the visual arts”, Spalter, 1999

  6. Linear Interpolation Simple, but discontinuous velocity

  7. Nonlinear Interpolation Smooth ball trajectory and continuous velocity, but loss of timing

  8. Easing Adjust the timing of the inbetween frames. Can be automated by adjusting the stepsize of parameter, t.

  9. Style or Accuracy? • Interpolating timecaptures accuracyof velocity • Squash and stretchreplaces motionblur stimuli andadds life-likeintent

  10. Traditional Motivation • Ease-in andease-out is likesquash andstretch • Can weautomate theinbetweens forthese? “The Illusion of Life, Disney Animation” Thomas and Johnson

  11. More squash and stretch

  12. Anticipation and Staging • Don’t surprise theaudience • Direct their attention to what’simportant

  13. Follow Through • Audience likes to see resolution of action • Discontinuities are unsettling

  14. Combined

  15. Secondary Motion • Characters should exist in a real environment • Extra movements should not detract

  16. Interpolation • Many parameters can be interpolated to generate animation • Simple interpolation techniques can only generate simple inbetweens • More complicated inbetweening will require a more complicated model of animated object and simulation

  17. Interpolation • Strengths • Animator has exacting control (Woody’s face) • Weaknesses • Interpolation hooks must be simple and direct • Remember the problems with Euler angle interp? • Time consuming and skill intensive • Difficult to reuse and adjust

  18. Movies • Cartoon Physics • Homer3D

  19. Examples • Sports video games • Madden Football • Many movie characters • Phantom Menace • Cartoons

  20. Motion Capture Strengths • Exactly captures the motions of the actor • Michael Jordan’s video game character will capture his style • Easy to capture data

  21. Motion Capture Weaknesses • Noise, noise, noise! • Magnetic system inteference • Visual system occlusions • Mechanical system mass • Tethered (wireless is available now)

  22. Motion Capture Weaknesses • Aligning motion data with CG character • Limb lengths • Idealized perfect joints • Reusing motion data • Difficult to scale in size (must also scale in time) • Changing one part of motion

  23. Motion Capture Weaknesses • Blending segments • Motion clips are short (due to range and tethers) • Dynamic motion generation requires blending at run time • Difficult to manage smooth transition

  24. Movies • Animating Aliens • Retargeting Motion – Siggraph ‘98 • Siggraph ‘99 Papers Tape

  25. Examples • Inanimate video game objects • GT Racer cars • Soapbox about why this is so cool • Special effects • Explosions, water, secondary motion • Phantom Menace CG droids after they were cut in half

  26. Procedural Animation • Very general term for a technique that puts more complex algorithms behind the scenes • Technique attempts to consolidate artistic efforts in algorithms and heuristics • Allows for optimization and physical simulation

  27. Procedural Animation Strengths • Animation can be generated ‘on the fly’ • Dynamic response to user • Write-once, use-often • Algorithms provide accuracy and exhaustive search that animators cannot

  28. Procedural Animation Weaknesses • We’re not great at boiling human skill down to algorithms • How do we move when juggling? • Difficult to generate • Expensive to compute • Difficult to force system to generate a particular solution • Bicycles will fall down

  29. Movies • Humming Along • Siggraph 2000