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//
// btContactProjection.cpp
// BulletSoftBody
//
// Created by Xuchen Han on 7/4/19.
//
#include "btContactProjection.h"
#include "btDeformableRigidDynamicsWorld.h"
#include <algorithm>
void btContactProjection::update(const TVStack& dv, const TVStack& backupVelocity)
{
///solve rigid body constraints
m_world->btMultiBodyDynamicsWorld::solveConstraints(m_world->getSolverInfo());
// loop through constraints to set constrained values
for (auto& it : m_constraints)
{
Friction& friction = m_frictions[it.first];
btAlignedObjectArray<Constraint>& constraints = it.second;
for (int i = 0; i < constraints.size(); ++i)
{
Constraint& constraint = constraints[i];
if (constraint.m_contact == nullptr)
{
// nothing needs to be done for dirichelet constraints
continue;
}
const btSoftBody::RContact* c = constraint.m_contact;
const btSoftBody::sCti& cti = c->m_cti;
btMultiBodyJacobianData jacobianData;
if (cti.m_colObj->hasContactResponse())
{
btVector3 va(0, 0, 0);
btRigidBody* rigidCol = 0;
btMultiBodyLinkCollider* multibodyLinkCol = 0;
btScalar* deltaV;
// grab the velocity of the rigid body
if (cti.m_colObj->getInternalType() == btCollisionObject::CO_RIGID_BODY)
{
rigidCol = (btRigidBody*)btRigidBody::upcast(cti.m_colObj);
va = rigidCol ? (rigidCol->getVelocityInLocalPoint(c->m_c1)) * m_dt : btVector3(0, 0, 0);
}
else if (cti.m_colObj->getInternalType() == btCollisionObject::CO_FEATHERSTONE_LINK)
{
multibodyLinkCol = (btMultiBodyLinkCollider*)btMultiBodyLinkCollider::upcast(cti.m_colObj);
if (multibodyLinkCol)
{
const int ndof = multibodyLinkCol->m_multiBody->getNumDofs() + 6;
jacobianData.m_jacobians.resize(ndof);
jacobianData.m_deltaVelocitiesUnitImpulse.resize(ndof);
btScalar* jac = &jacobianData.m_jacobians[0];
multibodyLinkCol->m_multiBody->fillContactJacobianMultiDof(multibodyLinkCol->m_link, c->m_node->m_x, cti.m_normal, jac, jacobianData.scratch_r, jacobianData.scratch_v, jacobianData.scratch_m);
deltaV = &jacobianData.m_deltaVelocitiesUnitImpulse[0];
multibodyLinkCol->m_multiBody->calcAccelerationDeltasMultiDof(&jacobianData.m_jacobians[0], deltaV, jacobianData.scratch_r, jacobianData.scratch_v);
btScalar vel = 0.0;
for (int j = 0; j < ndof; ++j)
{
vel += multibodyLinkCol->m_multiBody->getVelocityVector()[j] * jac[j];
}
va = cti.m_normal * vel * m_dt;
}
}
const btVector3 vb = c->m_node->m_v * m_dt;
const btVector3 vr = vb - va;
const btScalar dn = btDot(vr, cti.m_normal);
btVector3 impulse = c->m_c0 * vr;
const btVector3 impulse_normal = c->m_c0 *(cti.m_normal * dn);
btVector3 impulse_tangent = impulse - impulse_normal;
if (dn < 0 && impulse_tangent.norm() > SIMD_EPSILON)
{
btScalar impulse_tangent_magnitude = std::min(impulse_normal.norm()*c->m_c3, impulse_tangent.norm());
impulse_tangent_magnitude = 0;
const btVector3 tangent_dir = impulse_tangent.normalized();
impulse_tangent = impulse_tangent_magnitude * tangent_dir;
friction.m_direction = impulse_tangent;
friction.m_dv = -impulse_tangent * c->m_c2/m_dt + (c->m_node->m_v - backupVelocity[m_indices[c->m_node]]).dot(tangent_dir)*tangent_dir;
}
impulse = impulse_normal + impulse_tangent;
// if (1) // in the same CG solve, the set of constraits doesn't change
if (dn <= SIMD_EPSILON)
{
// c0 is the impulse matrix, c3 is 1 - the friction coefficient or 0, c4 is the contact hardness coefficient
// TODO: only contact is considered here, add friction later
// dv = new_impulse + accumulated velocity change in previous CG iterations
// so we have the invariant node->m_v = backupVelocity + dv;
btVector3 dv = -impulse * c->m_c2/m_dt + c->m_node->m_v - backupVelocity[m_indices[c->m_node]];
btScalar dvn = dv.dot(cti.m_normal);
constraint.m_value = dvn;
if (cti.m_colObj->getInternalType() == btCollisionObject::CO_RIGID_BODY)
{
if (rigidCol)
rigidCol->applyImpulse(impulse_normal, c->m_c1);
}
else if (cti.m_colObj->getInternalType() == btCollisionObject::CO_FEATHERSTONE_LINK)
{
if (multibodyLinkCol)
{
double multiplier = 0.5;
multibodyLinkCol->m_multiBody->applyDeltaVeeMultiDof(deltaV, -impulse.length() * multiplier);
}
}
}
}
}
}
}
void btContactProjection::setConstraintDirections()
{
// set Dirichlet constraint
for (int i = 0; i < m_softBodies.size(); ++i)
{
btSoftBody* psb = m_softBodies[i];
for (int j = 0; j < psb->m_nodes.size(); ++j)
{
if (psb->m_nodes[j].m_im == 0)
{
btAlignedObjectArray<Constraint> c;
c.push_back(Constraint(btVector3(1,0,0)));
c.push_back(Constraint(btVector3(0,1,0)));
c.push_back(Constraint(btVector3(0,0,1)));
m_constraints[&(psb->m_nodes[j])] = c;
}
}
}
for (int i = 0; i < m_softBodies.size(); ++i)
{
btSoftBody* psb = m_softBodies[i];
btMultiBodyJacobianData jacobianData;
for (int j = 0; j < psb->m_rcontacts.size(); ++j)
{
const btSoftBody::RContact& c = psb->m_rcontacts[j];
// skip anchor points
if (c.m_node->m_im == 0)
{
continue;
}
const btSoftBody::sCti& cti = c.m_cti;
if (cti.m_colObj->hasContactResponse())
{
btVector3 va(0, 0, 0);
btRigidBody* rigidCol = 0;
btMultiBodyLinkCollider* multibodyLinkCol = 0;
btScalar* deltaV;
// grab the velocity of the rigid body
if (cti.m_colObj->getInternalType() == btCollisionObject::CO_RIGID_BODY)
{
rigidCol = (btRigidBody*)btRigidBody::upcast(cti.m_colObj);
va = rigidCol ? (rigidCol->getVelocityInLocalPoint(c.m_c1)) * m_dt : btVector3(0, 0, 0);
}
else if (cti.m_colObj->getInternalType() == btCollisionObject::CO_FEATHERSTONE_LINK)
{
multibodyLinkCol = (btMultiBodyLinkCollider*)btMultiBodyLinkCollider::upcast(cti.m_colObj);
if (multibodyLinkCol)
{
const int ndof = multibodyLinkCol->m_multiBody->getNumDofs() + 6;
jacobianData.m_jacobians.resize(ndof);
jacobianData.m_deltaVelocitiesUnitImpulse.resize(ndof);
btScalar* jac = &jacobianData.m_jacobians[0];
multibodyLinkCol->m_multiBody->fillContactJacobianMultiDof(multibodyLinkCol->m_link, c.m_node->m_x, cti.m_normal, jac, jacobianData.scratch_r, jacobianData.scratch_v, jacobianData.scratch_m);
deltaV = &jacobianData.m_deltaVelocitiesUnitImpulse[0];
multibodyLinkCol->m_multiBody->calcAccelerationDeltasMultiDof(&jacobianData.m_jacobians[0], deltaV, jacobianData.scratch_r, jacobianData.scratch_v);
btScalar vel = 0.0;
for (int j = 0; j < ndof; ++j)
{
vel += multibodyLinkCol->m_multiBody->getVelocityVector()[j] * jac[j];
}
va = cti.m_normal * vel * m_dt;
}
}
const btVector3 vb = c.m_node->m_v * m_dt;
const btVector3 vr = vb - va;
const btScalar dn = btDot(vr, cti.m_normal);
if (dn < SIMD_EPSILON)
{
if (m_constraints.find(c.m_node) == m_constraints.end())
{
btAlignedObjectArray<Constraint> constraints;
constraints.push_back(Constraint(c));
m_constraints[c.m_node] = constraints;
m_frictions[c.m_node] = Friction();
}
else
{
m_constraints[c.m_node].push_back(Constraint(c));
}
continue;
}
}
}
}
// for particles with more than three constrained directions, prune constrained directions so that there are at most three constrained directions
const int dim = 3;
for (auto& it : m_constraints)
{
btAlignedObjectArray<Constraint>& c = it.second;
if (c.size() > dim)
{
btAlignedObjectArray<Constraint> prunedConstraints;
// always keep the first constrained direction
prunedConstraints.push_back(c[0]);
// find the direction most orthogonal to the first direction and keep it
size_t selected = 1;
btScalar min_dotProductAbs = std::abs(prunedConstraints[0].m_direction.dot(c[selected].m_direction));
for (int j = 2; j < c.size(); ++j)
{
btScalar dotProductAbs =std::abs(prunedConstraints[0].m_direction.dot(c[j].m_direction));
if (dotProductAbs < min_dotProductAbs)
{
selected = j;
min_dotProductAbs = dotProductAbs;
}
}
if (std::abs(std::abs(min_dotProductAbs)-1) < SIMD_EPSILON)
{
it.second = prunedConstraints;
continue;
}
prunedConstraints.push_back(c[selected]);
// find the direction most orthogonal to the previous two directions and keep it
size_t selected2 = (selected == 1) ? 2 : 1;
btVector3 normal = btCross(prunedConstraints[0].m_direction, prunedConstraints[1].m_direction);
normal.normalize();
btScalar max_dotProductAbs = std::abs(normal.dot(c[selected2].m_direction));
for (int j = 3; j < c.size(); ++j)
{
btScalar dotProductAbs = std::abs(normal.dot(c[j].m_direction));
if (dotProductAbs > min_dotProductAbs)
{
selected2 = j;
max_dotProductAbs = dotProductAbs;
}
}
prunedConstraints.push_back(c[selected2]);
it.second = prunedConstraints;
}
else
{
// prune out collinear constraints
const btVector3& first_dir = c[0].m_direction;
int i = 1;
while (i < c.size())
{
if (std::abs(std::abs(first_dir.dot(c[i].m_direction)) - 1) < 4*SIMD_EPSILON)
c.removeAtIndex(i);
else
++i;
}
if (c.size() == 3)
{
if (std::abs(std::abs(c[1].m_direction.dot(c[2].m_direction)) - 1) < 4*SIMD_EPSILON)
c.removeAtIndex(2);
}
}
}
}
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