@techreport{TR-IC-97-12,
  number = {IC-97-12},
  author = {Liesenfeld, Rogério L. W. and Stolfi, Jorge},
  title = {Realistic Simulation of Viscoelastic Bodies},
  month = {September},
  year = {1997}, 
  institution = {Institute of Computing, University of Campinas},
  note = {In English, 22 pages.
    \par\selectlanguage{english}\textbf{Abstract}
      We describe an animation system that simulates the dynamics of
      viscoelastic bodies subject to equality and inequality constraints. 
      We show how Lagrange's method can be used to derive the equations of
      motion of such bodies from general formulas for the elastic and
      kinetic energy, the viscous power loss, and mechanical constraints, in
      terms of generalized coordinates.
    \par
      We also describe a convenient two-parameter non-linear model for the
      elastic forces, that agrees with Hooke's law for small deformations,
      but does not allow the material to be compressed to zero or negative
      volume.  In particular, we derive the equations of motion for elastic
      bodies modeled by tetrahedral finite elements with affine deformations.  
      Finally, we show
      how collisions between such bodies can be efficiently and accurately
      detected by combining Hermite interpolation of the non-penetration
      constraints with Lin and Manocha's bounding box tests.
  }
}