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+/*
+ Written by Xuchen Han <xuchenhan2015@u.northwestern.edu>
+
+ Bullet Continuous Collision Detection and Physics Library
+ Copyright (c) 2019 Google Inc. http://bulletphysics.org
+ This software is provided 'as-is', without any express or implied warranty.
+ In no event will the authors be held liable for any damages arising from the use of this software.
+ Permission is granted to anyone to use this software for any purpose,
+ including commercial applications, and to alter it and redistribute it freely,
+ subject to the following restrictions:
+ 1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
+ 2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
+ 3. This notice may not be removed or altered from any source distribution.
+ */
+
+#ifndef BT_MASS_SPRING_H
+#define BT_MASS_SPRING_H
+
+#include "btDeformableLagrangianForce.h"
+
+class btDeformableMassSpringForce : public btDeformableLagrangianForce
+{
+ // If true, the damping force will be in the direction of the spring
+ // If false, the damping force will be in the direction of the velocity
+ bool m_momentum_conserving;
+ btScalar m_elasticStiffness, m_dampingStiffness, m_bendingStiffness;
+public:
+ typedef btAlignedObjectArray<btVector3> TVStack;
+ btDeformableMassSpringForce() : m_momentum_conserving(false), m_elasticStiffness(1), m_dampingStiffness(0.05)
+ {
+ }
+ btDeformableMassSpringForce(btScalar k, btScalar d, bool conserve_angular = true, double bending_k = -1) : m_momentum_conserving(conserve_angular), m_elasticStiffness(k), m_dampingStiffness(d), m_bendingStiffness(bending_k)
+ {
+ if (m_bendingStiffness < btScalar(0))
+ {
+ m_bendingStiffness = m_elasticStiffness;
+ }
+ }
+
+ virtual void addScaledForces(btScalar scale, TVStack& force)
+ {
+ addScaledDampingForce(scale, force);
+ addScaledElasticForce(scale, force);
+ }
+
+ virtual void addScaledExplicitForce(btScalar scale, TVStack& force)
+ {
+ addScaledElasticForce(scale, force);
+ }
+
+ virtual void addScaledDampingForce(btScalar scale, TVStack& force)
+ {
+ int numNodes = getNumNodes();
+ btAssert(numNodes <= force.size());
+ for (int i = 0; i < m_softBodies.size(); ++i)
+ {
+ const btSoftBody* psb = m_softBodies[i];
+ if (!psb->isActive())
+ {
+ continue;
+ }
+ for (int j = 0; j < psb->m_links.size(); ++j)
+ {
+ const btSoftBody::Link& link = psb->m_links[j];
+ btSoftBody::Node* node1 = link.m_n[0];
+ btSoftBody::Node* node2 = link.m_n[1];
+ size_t id1 = node1->index;
+ size_t id2 = node2->index;
+
+ // damping force
+ btVector3 v_diff = (node2->m_v - node1->m_v);
+ btVector3 scaled_force = scale * m_dampingStiffness * v_diff;
+ if (m_momentum_conserving)
+ {
+ if ((node2->m_x - node1->m_x).norm() > SIMD_EPSILON)
+ {
+ btVector3 dir = (node2->m_x - node1->m_x).normalized();
+ scaled_force = scale * m_dampingStiffness * v_diff.dot(dir) * dir;
+ }
+ }
+ force[id1] += scaled_force;
+ force[id2] -= scaled_force;
+ }
+ }
+ }
+
+ virtual void addScaledElasticForce(btScalar scale, TVStack& force)
+ {
+ int numNodes = getNumNodes();
+ btAssert(numNodes <= force.size());
+ for (int i = 0; i < m_softBodies.size(); ++i)
+ {
+ const btSoftBody* psb = m_softBodies[i];
+ if (!psb->isActive())
+ {
+ continue;
+ }
+ for (int j = 0; j < psb->m_links.size(); ++j)
+ {
+ const btSoftBody::Link& link = psb->m_links[j];
+ btSoftBody::Node* node1 = link.m_n[0];
+ btSoftBody::Node* node2 = link.m_n[1];
+ btScalar r = link.m_rl;
+ size_t id1 = node1->index;
+ size_t id2 = node2->index;
+
+ // elastic force
+ btVector3 dir = (node2->m_q - node1->m_q);
+ btVector3 dir_normalized = (dir.norm() > SIMD_EPSILON) ? dir.normalized() : btVector3(0,0,0);
+ btScalar scaled_stiffness = scale * (link.m_bbending ? m_bendingStiffness : m_elasticStiffness);
+ btVector3 scaled_force = scaled_stiffness * (dir - dir_normalized * r);
+ force[id1] += scaled_force;
+ force[id2] -= scaled_force;
+ }
+ }
+ }
+
+ virtual void addScaledDampingForceDifferential(btScalar scale, const TVStack& dv, TVStack& df)
+ {
+ // implicit damping force differential
+ for (int i = 0; i < m_softBodies.size(); ++i)
+ {
+ btSoftBody* psb = m_softBodies[i];
+ if (!psb->isActive())
+ {
+ continue;
+ }
+ btScalar scaled_k_damp = m_dampingStiffness * scale;
+ for (int j = 0; j < psb->m_links.size(); ++j)
+ {
+ const btSoftBody::Link& link = psb->m_links[j];
+ btSoftBody::Node* node1 = link.m_n[0];
+ btSoftBody::Node* node2 = link.m_n[1];
+ size_t id1 = node1->index;
+ size_t id2 = node2->index;
+
+ btVector3 local_scaled_df = scaled_k_damp * (dv[id2] - dv[id1]);
+ if (m_momentum_conserving)
+ {
+ if ((node2->m_x - node1->m_x).norm() > SIMD_EPSILON)
+ {
+ btVector3 dir = (node2->m_x - node1->m_x).normalized();
+ local_scaled_df= scaled_k_damp * (dv[id2] - dv[id1]).dot(dir) * dir;
+ }
+ }
+ df[id1] += local_scaled_df;
+ df[id2] -= local_scaled_df;
+ }
+ }
+ }
+
+ virtual double totalElasticEnergy(btScalar dt)
+ {
+ double energy = 0;
+ for (int i = 0; i < m_softBodies.size(); ++i)
+ {
+ const btSoftBody* psb = m_softBodies[i];
+ if (!psb->isActive())
+ {
+ continue;
+ }
+ for (int j = 0; j < psb->m_links.size(); ++j)
+ {
+ const btSoftBody::Link& link = psb->m_links[j];
+ btSoftBody::Node* node1 = link.m_n[0];
+ btSoftBody::Node* node2 = link.m_n[1];
+ btScalar r = link.m_rl;
+
+ // elastic force
+ btVector3 dir = (node2->m_q - node1->m_q);
+ energy += 0.5 * m_elasticStiffness * (dir.norm() - r) * (dir.norm() -r);
+ }
+ }
+ return energy;
+ }
+
+ virtual double totalDampingEnergy(btScalar dt)
+ {
+ double energy = 0;
+ int sz = 0;
+ for (int i = 0; i < m_softBodies.size(); ++i)
+ {
+ btSoftBody* psb = m_softBodies[i];
+ if (!psb->isActive())
+ {
+ continue;
+ }
+ for (int j = 0; j < psb->m_nodes.size(); ++j)
+ {
+ sz = btMax(sz, psb->m_nodes[j].index);
+ }
+ }
+ TVStack dampingForce;
+ dampingForce.resize(sz+1);
+ for (int i = 0; i < dampingForce.size(); ++i)
+ dampingForce[i].setZero();
+ addScaledDampingForce(0.5, dampingForce);
+ for (int i = 0; i < m_softBodies.size(); ++i)
+ {
+ btSoftBody* psb = m_softBodies[i];
+ for (int j = 0; j < psb->m_nodes.size(); ++j)
+ {
+ const btSoftBody::Node& node = psb->m_nodes[j];
+ energy -= dampingForce[node.index].dot(node.m_v) / dt;
+ }
+ }
+ return energy;
+ }
+
+ virtual void addScaledElasticForceDifferential(btScalar scale, const TVStack& dx, TVStack& df)
+ {
+ // implicit damping force differential
+ for (int i = 0; i < m_softBodies.size(); ++i)
+ {
+ const btSoftBody* psb = m_softBodies[i];
+ if (!psb->isActive())
+ {
+ continue;
+ }
+ for (int j = 0; j < psb->m_links.size(); ++j)
+ {
+ const btSoftBody::Link& link = psb->m_links[j];
+ btSoftBody::Node* node1 = link.m_n[0];
+ btSoftBody::Node* node2 = link.m_n[1];
+ size_t id1 = node1->index;
+ size_t id2 = node2->index;
+ btScalar r = link.m_rl;
+
+ btVector3 dir = (node1->m_q - node2->m_q);
+ btScalar dir_norm = dir.norm();
+ btVector3 dir_normalized = (dir_norm > SIMD_EPSILON) ? dir.normalized() : btVector3(0,0,0);
+ btVector3 dx_diff = dx[id1] - dx[id2];
+ btVector3 scaled_df = btVector3(0,0,0);
+ btScalar scaled_k = scale * (link.m_bbending ? m_bendingStiffness : m_elasticStiffness);
+ if (dir_norm > SIMD_EPSILON)
+ {
+ scaled_df -= scaled_k * dir_normalized.dot(dx_diff) * dir_normalized;
+ scaled_df += scaled_k * dir_normalized.dot(dx_diff) * ((dir_norm-r)/dir_norm) * dir_normalized;
+ scaled_df -= scaled_k * ((dir_norm-r)/dir_norm) * dx_diff;
+ }
+
+ df[id1] += scaled_df;
+ df[id2] -= scaled_df;
+ }
+ }
+ }
+
+ virtual btDeformableLagrangianForceType getForceType()
+ {
+ return BT_MASSSPRING_FORCE;
+ }
+
+};
+
+#endif /* btMassSpring_h */
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