Introduction to Computer Animation
September 2000

David S. Ebert

Main Categories

• Key Framing
  Burtnyk and Wein 1971 - MSGEN

  Animation produced: Hunger by Peter Feldes

  Specify position of object/parts at specific points in time (key frames)

  Use software to interpolate motion between those key positions (tweening or inbetweening).

  Traditional (handdrawn) animation - lead animator defined key frames, inbetweener drew the inbetween frames.

  Can use a variety of methods for interpolation to make the motion smooth, including ease-in and ease-out curves.

  Example of 3 keyframes for an animated ball
  

  Scripting Systems - Animation Languages

  LISP commonly used

  Very powerful, flexible

  ASAS (Reynolds 82)

  MIRA (Magnanet 85)

  Kinematics (forward)

  Specification of motion independent of the underlying forces.

  Direct specification of joint motion - rotation and translation, of each joint

  Kochanek 84, Badler 85

  Adapted from physics - position, velocity, acceleration

  Inverse Kinematics

  Specification of end effector potion or motion only

  Goal Directed Motion

  Goal Directed Motion

  Graphics use adapted from robotics field

  System automatically calcuates the motion of each underlying part.

      This inverse solution involves much more calculation.

  As the number of joints increases, the solution space becomes large and

      the problem becomes undefined.

  May have multiple solutions or no solution.

  Often dealing with nonlinear sets of equations.

  Constraints are added to reduce the solution space.

  Minimization of Maximization techniques are then applied to find a solution.

  Example of moving 2 segments (Image Courtesy of Rick Parent):

  Articulated Figures

  Structure that consists of a series of rigid links and joints.

  Hierarchical transformation applied to allow movement specification at higher levels

  to propogate to lower joints.

  Often Inverse kinematics used for complicated articulated structures.

  Early work at Ohio State:

      Zeltzer 1982 -Kinematics

      Girard 1985 - Inverse Kinematics

  Example of jointed kinematics where each joints rotation is specified: (courtesy of Rick Parent):

  Skeletal / Character Animation

  Zeltzer - 1982 - Kinematics based animation of walk cycle

  Girard 1985 -IK for multi-legged animation

      main computation time for solving equations from Jacobian

  Badler - JACK system ~1987

  Character with skin & Muscle layer

      Chadwick 1989

  Character with complicated skin system and better muscles - Wilhelms 1997

  Rigid Body Dynamics

  Solving F=ma equations, conservation of momentum, etc.

  Elastic collisions

  Hahn's work - 1988

  Barraff's work on accurate collision detection and deformation forces.

  Inverse Dynamics

  Specify goal position, inverse calculation of forces

  Barr's work early 1990's (teleological modeling)

  Flexible Object Dynamics

  3D Field forces applied to object

  Free-Form Deformations common technique (classical) - Sederberg 1986

      Embed model in a 3D lattice, deform lattice, recalculate location of model points

  Mass + Spring model

      model composed of a collection of masses and springs

      Solve spring equation with external forces - e.g., Haumann 1989

  Facial Animation

  Early Work by Waters, Parke, and Badler

  "Simple" muscle models that pull and squeeze the face

  Example of more recent work is the work in Antz by Beth Hofer and the work in Babe

  More accurate physical Model by Keith Waters in the past 3 years.

  Procedural animation model by Ken Perlin can be found at http://www.mrl.nyu.edu/perlin/

      Goal here is to come up with intuitive expression sliders.

  Other Techniques:

  • Particle Systems

  • Procedural Animation

  • Evolving Organics, Natural Phenomena

  • Goal Directed Animation and Behavioral Animation

  • Intelligent Characters & Improvisational Animation