TVector MassSpringCurve<Real,TVector>::Acceleration (int i, Real time,
const TVector* positions, const TVector* velocities)
{
// Compute spring forces on position X[i]. The positions are not
// necessarily m_akPosition since the RK4 solver in ParticleSystem
// evaluates the acceleration function at intermediate positions. The end
// points of the curve of masses must be handled separately since each
// has only one spring attached to it.
TVector acceleration = ExternalAcceleration(i, time, positions,
velocities);
TVector diff, force;
Real ratio;
if (i > 0)
{
int iM1 = i-1;
diff = positions[iM1] - positions[i];
ratio = mLengths[iM1]/diff.Length();
force = mConstants[iM1]*((Real)1 - ratio)*diff;
acceleration += mInvMasses[i]*force;
}
int iP1 = i+1;
if (iP1 < mNumParticles)
{
diff = positions[iP1] - positions[i];
ratio = mLengths[i]/diff.Length();
force = mConstants[i]*((Real)1 - ratio)*diff;
acceleration += mInvMasses[i]*force;
}
return acceleration;
}
TVector MassSpringArbitrary<Real,TVector>::Acceleration (int i, Real time,
const TVector* positions, const TVector* velocities)
{
// Compute spring forces on position X[i]. The positions are not
// necessarily m_akPosition since the RK4 solver in ParticleSystem
// evaluates the acceleration function at intermediate positions.
TVector acceleration = ExternalAcceleration(i, time, positions,
velocities);
TVector diff, force;
Real ratio;
std::set<int>::iterator iter = mAdjacent[i].begin();
std::set<int>::iterator end = mAdjacent[i].end();
for (/**/; iter != end; ++iter)
{
// Process a spring connected to particle i.
const Spring& spring = mSprings[*iter];
if (i != spring.Particle0)
{
diff = positions[spring.Particle0] - positions[i];
}
else
{
diff = positions[spring.Particle1] - positions[i];
}
ratio = spring.Length/diff.Length();
force = spring.Constant*((Real)1 - ratio)*diff;
acceleration += mInvMasses[i]*force;
}
return acceleration;
}
