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