Bullet Collision Detection & Physics Library
btDeformableMassSpringForce.h
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1 /*
2  Written by Xuchen Han <xuchenhan2015@u.northwestern.edu>
3 
4  Bullet Continuous Collision Detection and Physics Library
5  Copyright (c) 2019 Google Inc. http://bulletphysics.org
6  This software is provided 'as-is', without any express or implied warranty.
7  In no event will the authors be held liable for any damages arising from the use of this software.
8  Permission is granted to anyone to use this software for any purpose,
9  including commercial applications, and to alter it and redistribute it freely,
10  subject to the following restrictions:
11  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.
12  2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
13  3. This notice may not be removed or altered from any source distribution.
14  */
15 
16 #ifndef BT_MASS_SPRING_H
17 #define BT_MASS_SPRING_H
18 
20 
22 {
23  // If true, the damping force will be in the direction of the spring
24  // If false, the damping force will be in the direction of the velocity
27 
28 public:
31  {
32  }
33  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)
34  {
36  {
38  }
39  }
40 
41  virtual void addScaledForces(btScalar scale, TVStack& force)
42  {
43  addScaledDampingForce(scale, force);
44  addScaledElasticForce(scale, force);
45  }
46 
47  virtual void addScaledExplicitForce(btScalar scale, TVStack& force)
48  {
49  addScaledElasticForce(scale, force);
50  }
51 
52  virtual void addScaledDampingForce(btScalar scale, TVStack& force)
53  {
54  int numNodes = getNumNodes();
55  btAssert(numNodes <= force.size());
56  for (int i = 0; i < m_softBodies.size(); ++i)
57  {
58  const btSoftBody* psb = m_softBodies[i];
59  if (!psb->isActive())
60  {
61  continue;
62  }
63  for (int j = 0; j < psb->m_links.size(); ++j)
64  {
65  const btSoftBody::Link& link = psb->m_links[j];
66  btSoftBody::Node* node1 = link.m_n[0];
67  btSoftBody::Node* node2 = link.m_n[1];
68  size_t id1 = node1->index;
69  size_t id2 = node2->index;
70 
71  // damping force
72  btVector3 v_diff = (node2->m_v - node1->m_v);
73  btVector3 scaled_force = scale * m_dampingStiffness * v_diff;
75  {
76  if ((node2->m_x - node1->m_x).norm() > SIMD_EPSILON)
77  {
78  btVector3 dir = (node2->m_x - node1->m_x).normalized();
79  scaled_force = scale * m_dampingStiffness * v_diff.dot(dir) * dir;
80  }
81  }
82  force[id1] += scaled_force;
83  force[id2] -= scaled_force;
84  }
85  }
86  }
87 
88  virtual void addScaledElasticForce(btScalar scale, TVStack& force)
89  {
90  int numNodes = getNumNodes();
91  btAssert(numNodes <= force.size());
92  for (int i = 0; i < m_softBodies.size(); ++i)
93  {
94  const btSoftBody* psb = m_softBodies[i];
95  if (!psb->isActive())
96  {
97  continue;
98  }
99  for (int j = 0; j < psb->m_links.size(); ++j)
100  {
101  const btSoftBody::Link& link = psb->m_links[j];
102  btSoftBody::Node* node1 = link.m_n[0];
103  btSoftBody::Node* node2 = link.m_n[1];
104  btScalar r = link.m_rl;
105  size_t id1 = node1->index;
106  size_t id2 = node2->index;
107 
108  // elastic force
109  btVector3 dir = (node2->m_q - node1->m_q);
110  btVector3 dir_normalized = (dir.norm() > SIMD_EPSILON) ? dir.normalized() : btVector3(0, 0, 0);
111  btScalar scaled_stiffness = scale * (link.m_bbending ? m_bendingStiffness : m_elasticStiffness);
112  btVector3 scaled_force = scaled_stiffness * (dir - dir_normalized * r);
113  force[id1] += scaled_force;
114  force[id2] -= scaled_force;
115  }
116  }
117  }
118 
119  virtual void addScaledDampingForceDifferential(btScalar scale, const TVStack& dv, TVStack& df)
120  {
121  // implicit damping force differential
122  for (int i = 0; i < m_softBodies.size(); ++i)
123  {
124  btSoftBody* psb = m_softBodies[i];
125  if (!psb->isActive())
126  {
127  continue;
128  }
129  btScalar scaled_k_damp = m_dampingStiffness * scale;
130  for (int j = 0; j < psb->m_links.size(); ++j)
131  {
132  const btSoftBody::Link& link = psb->m_links[j];
133  btSoftBody::Node* node1 = link.m_n[0];
134  btSoftBody::Node* node2 = link.m_n[1];
135  size_t id1 = node1->index;
136  size_t id2 = node2->index;
137 
138  btVector3 local_scaled_df = scaled_k_damp * (dv[id2] - dv[id1]);
140  {
141  if ((node2->m_x - node1->m_x).norm() > SIMD_EPSILON)
142  {
143  btVector3 dir = (node2->m_x - node1->m_x).normalized();
144  local_scaled_df = scaled_k_damp * (dv[id2] - dv[id1]).dot(dir) * dir;
145  }
146  }
147  df[id1] += local_scaled_df;
148  df[id2] -= local_scaled_df;
149  }
150  }
151  }
152 
154  {
155  // implicit damping force differential
156  for (int i = 0; i < m_softBodies.size(); ++i)
157  {
158  btSoftBody* psb = m_softBodies[i];
159  if (!psb->isActive())
160  {
161  continue;
162  }
163  btScalar scaled_k_damp = m_dampingStiffness * scale;
164  for (int j = 0; j < psb->m_links.size(); ++j)
165  {
166  const btSoftBody::Link& link = psb->m_links[j];
167  btSoftBody::Node* node1 = link.m_n[0];
168  btSoftBody::Node* node2 = link.m_n[1];
169  size_t id1 = node1->index;
170  size_t id2 = node2->index;
172  {
173  if ((node2->m_x - node1->m_x).norm() > SIMD_EPSILON)
174  {
175  btVector3 dir = (node2->m_x - node1->m_x).normalized();
176  for (int d = 0; d < 3; ++d)
177  {
178  if (node1->m_im > 0)
179  diagA[id1][d] -= scaled_k_damp * dir[d] * dir[d];
180  if (node2->m_im > 0)
181  diagA[id2][d] -= scaled_k_damp * dir[d] * dir[d];
182  }
183  }
184  }
185  else
186  {
187  for (int d = 0; d < 3; ++d)
188  {
189  if (node1->m_im > 0)
190  diagA[id1][d] -= scaled_k_damp;
191  if (node2->m_im > 0)
192  diagA[id2][d] -= scaled_k_damp;
193  }
194  }
195  }
196  }
197  }
198 
199  virtual double totalElasticEnergy(btScalar dt)
200  {
201  double energy = 0;
202  for (int i = 0; i < m_softBodies.size(); ++i)
203  {
204  const btSoftBody* psb = m_softBodies[i];
205  if (!psb->isActive())
206  {
207  continue;
208  }
209  for (int j = 0; j < psb->m_links.size(); ++j)
210  {
211  const btSoftBody::Link& link = psb->m_links[j];
212  btSoftBody::Node* node1 = link.m_n[0];
213  btSoftBody::Node* node2 = link.m_n[1];
214  btScalar r = link.m_rl;
215 
216  // elastic force
217  btVector3 dir = (node2->m_q - node1->m_q);
218  energy += 0.5 * m_elasticStiffness * (dir.norm() - r) * (dir.norm() - r);
219  }
220  }
221  return energy;
222  }
223 
224  virtual double totalDampingEnergy(btScalar dt)
225  {
226  double energy = 0;
227  int sz = 0;
228  for (int i = 0; i < m_softBodies.size(); ++i)
229  {
230  btSoftBody* psb = m_softBodies[i];
231  if (!psb->isActive())
232  {
233  continue;
234  }
235  for (int j = 0; j < psb->m_nodes.size(); ++j)
236  {
237  sz = btMax(sz, psb->m_nodes[j].index);
238  }
239  }
240  TVStack dampingForce;
241  dampingForce.resize(sz + 1);
242  for (int i = 0; i < dampingForce.size(); ++i)
243  dampingForce[i].setZero();
244  addScaledDampingForce(0.5, dampingForce);
245  for (int i = 0; i < m_softBodies.size(); ++i)
246  {
247  btSoftBody* psb = m_softBodies[i];
248  for (int j = 0; j < psb->m_nodes.size(); ++j)
249  {
250  const btSoftBody::Node& node = psb->m_nodes[j];
251  energy -= dampingForce[node.index].dot(node.m_v) / dt;
252  }
253  }
254  return energy;
255  }
256 
257  virtual void addScaledElasticForceDifferential(btScalar scale, const TVStack& dx, TVStack& df)
258  {
259  // implicit damping force differential
260  for (int i = 0; i < m_softBodies.size(); ++i)
261  {
262  const btSoftBody* psb = m_softBodies[i];
263  if (!psb->isActive())
264  {
265  continue;
266  }
267  for (int j = 0; j < psb->m_links.size(); ++j)
268  {
269  const btSoftBody::Link& link = psb->m_links[j];
270  btSoftBody::Node* node1 = link.m_n[0];
271  btSoftBody::Node* node2 = link.m_n[1];
272  size_t id1 = node1->index;
273  size_t id2 = node2->index;
274  btScalar r = link.m_rl;
275 
276  btVector3 dir = (node1->m_q - node2->m_q);
277  btScalar dir_norm = dir.norm();
278  btVector3 dir_normalized = (dir_norm > SIMD_EPSILON) ? dir.normalized() : btVector3(0, 0, 0);
279  btVector3 dx_diff = dx[id1] - dx[id2];
280  btVector3 scaled_df = btVector3(0, 0, 0);
281  btScalar scaled_k = scale * (link.m_bbending ? m_bendingStiffness : m_elasticStiffness);
282  if (dir_norm > SIMD_EPSILON)
283  {
284  scaled_df -= scaled_k * dir_normalized.dot(dx_diff) * dir_normalized;
285  scaled_df += scaled_k * dir_normalized.dot(dx_diff) * ((dir_norm - r) / dir_norm) * dir_normalized;
286  scaled_df -= scaled_k * ((dir_norm - r) / dir_norm) * dx_diff;
287  }
288 
289  df[id1] += scaled_df;
290  df[id2] -= scaled_df;
291  }
292  }
293  }
294 
296  {
297  return BT_MASSSPRING_FORCE;
298  }
299 };
300 
301 #endif /* btMassSpring_h */
btDeformableLagrangianForceType
const T & btMax(const T &a, const T &b)
Definition: btMinMax.h:27
btScalar dot(const btQuaternion &q1, const btQuaternion &q2)
Calculate the dot product between two quaternions.
Definition: btQuaternion.h:888
float btScalar
The btScalar type abstracts floating point numbers, to easily switch between double and single floati...
Definition: btScalar.h:314
#define SIMD_EPSILON
Definition: btScalar.h:543
#define btAssert(x)
Definition: btScalar.h:153
int size() const
return the number of elements in the array
void resize(int newsize, const T &fillData=T())
btAlignedObjectArray< btSoftBody * > m_softBodies
virtual btDeformableLagrangianForceType getForceType()
virtual void addScaledElasticForceDifferential(btScalar scale, const TVStack &dx, TVStack &df)
btAlignedObjectArray< btVector3 > TVStack
virtual void addScaledForces(btScalar scale, TVStack &force)
virtual void addScaledDampingForce(btScalar scale, TVStack &force)
virtual void addScaledDampingForceDifferential(btScalar scale, const TVStack &dv, TVStack &df)
virtual void addScaledElasticForce(btScalar scale, TVStack &force)
virtual void buildDampingForceDifferentialDiagonal(btScalar scale, TVStack &diagA)
virtual double totalDampingEnergy(btScalar dt)
btDeformableMassSpringForce(btScalar k, btScalar d, bool conserve_angular=true, double bending_k=-1)
virtual double totalElasticEnergy(btScalar dt)
virtual void addScaledExplicitForce(btScalar scale, TVStack &force)
The btSoftBody is an class to simulate cloth and volumetric soft bodies.
Definition: btSoftBody.h:75
tLinkArray m_links
Definition: btSoftBody.h:814
tNodeArray m_nodes
Definition: btSoftBody.h:812
btVector3 can be used to represent 3D points and vectors.
Definition: btVector3.h:82
btScalar dot(const btVector3 &v) const
Return the dot product.
Definition: btVector3.h:229
btScalar norm() const
Return the norm (length) of the vector.
Definition: btVector3.h:263
btVector3 normalized() const
Return a normalized version of this vector.
Definition: btVector3.h:949
btVector3 m_x
Definition: btSoftBody.h:269
btVector3 m_v
Definition: btSoftBody.h:271
btVector3 m_q
Definition: btSoftBody.h:270