// get neigbouring particles of P and calculate the according connection probabilities
void MetropolisHastingsSampler::ComputeEndPointProposalDistribution(EndPoint P)
{
Particle *p = P.p;
int ep = P.ep;
float dist,dot;
vnl_vector_fixed<float, 3> R = p->GetPos() + (p->GetDir() * (ep*m_ParticleLength) );
m_ParticleGrid->ComputeNeighbors(R);
m_SimpSamp.clear();
m_SimpSamp.add(m_StopProb,EndPoint(nullptr,0));
for (;;)
{
Particle *p2 = m_ParticleGrid->GetNextNeighbor();
if (p2 == nullptr) break;
if (p!=p2 && p2->label == 0)
{
if (p2->mID == -1)
{
dist = (p2->GetPos() - p2->GetDir() * m_ParticleLength - R).squared_magnitude();
if (dist < m_DistanceThreshold)
{
dot = dot_product(p2->GetDir(),p->GetDir()) * ep;
if (dot > m_CurvatureThreshold)
{
float en = m_EnergyComputer->ComputeInternalEnergyConnection(p,ep,p2,-1);
m_SimpSamp.add(exp(en/m_TractProb),EndPoint(p2,-1));
}
}
}
if (p2->pID == -1)
{
dist = (p2->GetPos() + p2->GetDir() * m_ParticleLength - R).squared_magnitude();
if (dist < m_DistanceThreshold)
{
dot = dot_product(p2->GetDir(),p->GetDir()) * (-ep);
if (dot > m_CurvatureThreshold)
{
float en = m_EnergyComputer->ComputeInternalEnergyConnection(p,ep,p2,+1);
m_SimpSamp.add(exp(en/m_TractProb),EndPoint(p2,+1));
}
}
}
}
}
}
// generate actual proposal (birth, death, shift and connection of particle)
void MetropolisHastingsSampler::MakeProposal()
{
float randnum = m_RandGen->GetVariate();
// Birth Proposal
if (randnum < m_BirthProb)
{
m_BirthTime.Start();
vnl_vector_fixed<float, 3> R;
m_EnergyComputer->DrawRandomPosition(R);
vnl_vector_fixed<float, 3> N = GetRandomDirection();
Particle prop;
prop.GetPos() = R;
prop.GetDir() = N;
float prob = m_Density * m_DeathProb /((m_BirthProb)*(m_ParticleGrid->m_NumParticles+1));
float ex_energy = m_EnergyComputer->ComputeExternalEnergy(R,N,nullptr);
float in_energy = m_EnergyComputer->ComputeInternalEnergy(&prop);
prob *= exp((in_energy/m_InTemp+ex_energy/m_ExTemp)) ;
if (prob > 1 || m_RandGen->GetVariate() < prob)
{
Particle *p = m_ParticleGrid->NewParticle(R);
if (p!=nullptr)
{
p->GetPos() = R;
p->GetDir() = N;
m_AcceptedProposals++;
}
}
m_BirthTime.Stop();
}
// Death Proposal
else if (randnum < m_BirthProb+m_DeathProb)
{
m_DeathTime.Start();
if (m_ParticleGrid->m_NumParticles > 0)
{
int pnum = m_RandGen->GetIntegerVariate()%m_ParticleGrid->m_NumParticles;
Particle *dp = m_ParticleGrid->GetParticle(pnum);
if (dp->pID == -1 && dp->mID == -1)
{
float ex_energy = m_EnergyComputer->ComputeExternalEnergy(dp->GetPos(),dp->GetDir(),dp);
float in_energy = m_EnergyComputer->ComputeInternalEnergy(dp);
float prob = m_ParticleGrid->m_NumParticles * (m_BirthProb) /(m_Density*m_DeathProb); //*SpatProb(dp->R);
prob *= exp(-(in_energy/m_InTemp+ex_energy/m_ExTemp)) ;
if (prob > 1 || m_RandGen->GetVariate() < prob)
{
m_ParticleGrid->RemoveParticle(pnum);
m_AcceptedProposals++;
}
}
}
m_DeathTime.Stop();
}
// Shift Proposal
else if (randnum < m_BirthProb+m_DeathProb+m_ShiftProb)
{
if (m_ParticleGrid->m_NumParticles > 0)
{
m_ShiftTime.Start();
int pnum = m_RandGen->GetIntegerVariate()%m_ParticleGrid->m_NumParticles;
Particle *p = m_ParticleGrid->GetParticle(pnum);
Particle prop_p = *p;
DistortVector(m_Sigma, prop_p.GetPos());
DistortVector(m_Sigma/(2*m_ParticleLength), prop_p.GetDir());
prop_p.GetDir().normalize();
float ex_energy = m_EnergyComputer->ComputeExternalEnergy(prop_p.GetPos(),prop_p.GetDir(),p)
- m_EnergyComputer->ComputeExternalEnergy(p->GetPos(),p->GetDir(),p);
float in_energy = m_EnergyComputer->ComputeInternalEnergy(&prop_p) - m_EnergyComputer->ComputeInternalEnergy(p);
float prob = exp(ex_energy/m_ExTemp+in_energy/m_InTemp);
if (m_RandGen->GetVariate() < prob)
{
vnl_vector_fixed<float, 3> Rtmp = p->GetPos();
vnl_vector_fixed<float, 3> Ntmp = p->GetDir();
p->GetPos() = prop_p.GetPos();
p->GetDir() = prop_p.GetDir();
if (!m_ParticleGrid->TryUpdateGrid(pnum))
{
p->GetPos() = Rtmp;
p->GetDir() = Ntmp;
}
m_AcceptedProposals++;
}
m_ShiftTime.Stop();
}
}
// Optimal Shift Proposal
else if (randnum < m_BirthProb+m_DeathProb+m_ShiftProb+m_OptShiftProb)
{
if (m_ParticleGrid->m_NumParticles > 0)
{
m_OptShiftTime.Start();
int pnum = m_RandGen->GetIntegerVariate()%m_ParticleGrid->m_NumParticles;
Particle *p = m_ParticleGrid->GetParticle(pnum);
bool no_proposal = false;
Particle prop_p = *p;
if (p->pID != -1 && p->mID != -1)
{
Particle *plus = m_ParticleGrid->GetParticle(p->pID);
int ep_plus = (plus->pID == p->ID)? 1 : -1;
Particle *minus = m_ParticleGrid->GetParticle(p->mID);
int ep_minus = (minus->pID == p->ID)? 1 : -1;
prop_p.GetPos() = (plus->GetPos() + plus->GetDir() * (m_ParticleLength * ep_plus) + minus->GetPos() + minus->GetDir() * (m_ParticleLength * ep_minus));
prop_p.GetPos() *= 0.5;
prop_p.GetDir() = plus->GetPos() - minus->GetPos();
prop_p.GetDir().normalize();
}
else if (p->pID != -1)
{
Particle *plus = m_ParticleGrid->GetParticle(p->pID);
int ep_plus = (plus->pID == p->ID)? 1 : -1;
prop_p.GetPos() = plus->GetPos() + plus->GetDir() * (m_ParticleLength * ep_plus * 2);
prop_p.GetDir() = plus->GetDir();
}
else if (p->mID != -1)
{
Particle *minus = m_ParticleGrid->GetParticle(p->mID);
int ep_minus = (minus->pID == p->ID)? 1 : -1;
prop_p.GetPos() = minus->GetPos() + minus->GetDir() * (m_ParticleLength * ep_minus * 2);
prop_p.GetDir() = minus->GetDir();
}
else
no_proposal = true;
if (!no_proposal)
{
float cos = dot_product(prop_p.GetDir(), p->GetDir());
float p_rev = exp(-((prop_p.GetPos()-p->GetPos()).squared_magnitude() + (1-cos*cos))*m_Gamma)/m_Z;
float ex_energy = m_EnergyComputer->ComputeExternalEnergy(prop_p.GetPos(),prop_p.GetDir(),p)
- m_EnergyComputer->ComputeExternalEnergy(p->GetPos(),p->GetDir(),p);
float in_energy = m_EnergyComputer->ComputeInternalEnergy(&prop_p) - m_EnergyComputer->ComputeInternalEnergy(p);
float prob = exp(ex_energy/m_ExTemp+in_energy/m_InTemp)*m_ShiftProb*p_rev/(m_OptShiftProb+m_ShiftProb*p_rev);
if (m_RandGen->GetVariate() < prob)
{
vnl_vector_fixed<float, 3> Rtmp = p->GetPos();
vnl_vector_fixed<float, 3> Ntmp = p->GetDir();
p->GetPos() = prop_p.GetPos();
p->GetDir() = prop_p.GetDir();
if (!m_ParticleGrid->TryUpdateGrid(pnum))
{
p->GetPos() = Rtmp;
p->GetDir() = Ntmp;
}
m_AcceptedProposals++;
}
}
m_OptShiftTime.Stop();
}
}
// Connection Proposal
else
{
if (m_ParticleGrid->m_NumParticles > 0)
{
m_ConnectionTime.Start();
int pnum = m_RandGen->GetIntegerVariate()%m_ParticleGrid->m_NumParticles;
Particle *p = m_ParticleGrid->GetParticle(pnum);
EndPoint P;
P.p = p;
P.ep = (m_RandGen->GetVariate() > 0.5)? 1 : -1; // direction of the new tract
RemoveAndSaveTrack(P); // remove old tract and save it for later
if (m_BackupTrack.m_Probability != 0)
{
MakeTrackProposal(P); // propose new tract starting from P
float prob = (m_ProposalTrack.m_Energy-m_BackupTrack.m_Energy)/m_InTemp ;
prob = exp(prob)*(m_BackupTrack.m_Probability * pow(m_DelProb,m_ProposalTrack.m_Length))
/(m_ProposalTrack.m_Probability * pow(m_DelProb,m_BackupTrack.m_Length));
if (m_RandGen->GetVariate() < prob)
{
ImplementTrack(m_ProposalTrack); // accept proposed tract
m_AcceptedProposals++;
}
else
{
ImplementTrack(m_BackupTrack); // reject proposed tract and restore old one
}
}
else
ImplementTrack(m_BackupTrack);
m_ConnectionTime.Stop();
}
}
}
