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Inertia_Tensor.cc

//    Vamos Automotive Simulator
//  Copyright (C) 2001--2002 Sam Varner
//
//  This program is free software; you can redistribute it and/or modify
//  it under the terms of the GNU General Public License as published by
//  the Free Software Foundation; either version 2 of the License, or
//  (at your option) any later version.
//
//  This program is distributed in the hope that it will be useful,
//  but WITHOUT ANY WARRANTY; without even the implied warranty of
//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//  GNU General Public License for more details.
//
//  You should have received a copy of the GNU General Public License
//  along with this program; if not, write to the Free Software
//  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

#include <vamos/geometry/Inertia_Tensor.h>

// Add a mass to the system at `position'.  The inertia tensor
// components are calculated here.
void Vamos_Geometry::Inertia_Tensor::
add (double mass, const Three_Vector& position)
{
  m_mass += mass;

  // I_xx
  (*this) [0][0] += mass 
      * (position [1] * position [1] + position [2] * position [2]);
  // I_xy
  (*this) [0][1] -= mass * (position [0] * position [1]);
  // I_xz
  (*this) [0][2] -= mass * (position [0] * position [2]);
  
  // I_yy
  (*this) [1][1] += mass 
      * (position [2] * position [2] + position [0] * position [0]);
  // I_yz
  (*this) [1][2] -= mass * (position [1] * position [2]);
  
  // I_zz
  (*this) [2][2] += mass 
      * (position [0] * position [0] + position [1] * position [1]);      
}

// Zero the components of the inertia tensor and also the mass.
void Vamos_Geometry::Inertia_Tensor::
zero ()
{
  Three_Matrix::zero ();
  m_mass = 0.0;
}

// Calculate the inverse of the inertia tensor.  If the tensor is
// singular, Bad_Inertia_Tensor is thrown.  update() must be called
// after all add()s have been performed.
void Vamos_Geometry::Inertia_Tensor::
update ()
{
  // Fill in the symmetric components of the inertia tensor.
  (*this) [1][0] = (*this) [0][1];
  (*this) [2][0] = (*this) [0][2];
  (*this) [2][1] = (*this) [1][2];

   try
      {
        m_inverse = invert ();
      }
  catch (Vamos_Geometry::Singular_Matrix)
      {
        // If the inertia tensor is singular, throw an exception that tells
        // that the body is ill-formed.
        throw Bad_Inertia_Tensor ();
      }
}

// Return the moment of inertia for a force applied at `position' in
// the direction `force_direction'.  `force_direction' need not be a
// unit vector.
double Vamos_Geometry::Inertia_Tensor::
inertia (const Three_Vector& position, const Three_Vector& force_direction)
  const
{
  // The axis of rotation for a force applied in the direction of normal.
  // The maxnitude of axis is the rotational inertia about that axis.
  Three_Vector axis = m_inverse * (position.cross (force_direction.unit ()));

  return m_mass / 
      (1.0 
       + m_mass 
       * (axis.cross (position).project (force_direction.unit ())).abs ());
}

// Return the moment of inertia for TORQUE applied to the center of
// mass.  TORQUE need not be a unit vector.
double Vamos_Geometry::Inertia_Tensor::
inertia (const Three_Vector& torque) const
{
  return (torque.unit() * (*this)).abs ();
}

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