1 /*
2 * Copyright (c) 2006-2011 Erin Catto http://www.box2d.org
3 *
4 * This software is provided 'as-is', without any express or implied
5 * warranty. In no event will the authors be held liable for any damages
6 * arising from the use of this software.
7 * Permission is granted to anyone to use this software for any purpose,
8 * including commercial applications, and to alter it and redistribute it
9 * freely, subject to the following restrictions:
10 * 1. The origin of this software must not be misrepresented; you must not
11 * claim that you wrote the original software. If you use this software
12 * in a product, an acknowledgment in the product documentation would be
13 * appreciated but is not required.
14 * 2. Altered source versions must be plainly marked as such, and must not be
15 * misrepresented as being the original software.
16 * 3. This notice may not be removed or altered from any source distribution.
17 */
18
19 #include <Box2D/Dynamics/Joints/b2FrictionJoint.h>
20 #include <Box2D/Dynamics/b2Body.h>
21 #include <Box2D/Dynamics/b2TimeStep.h>
22
23 // Point-to-point constraint
24 // Cdot = v2 - v1
25 // = v2 + cross(w2, r2) - v1 - cross(w1, r1)
26 // J = [-I -r1_skew I r2_skew ]
27 // Identity used:
28 // w k % (rx i + ry j) = w * (-ry i + rx j)
29
30 // Angle constraint
31 // Cdot = w2 - w1
32 // J = [0 0 -1 0 0 1]
33 // K = invI1 + invI2
34
Initialize(b2Body * bA,b2Body * bB,const b2Vec2 & anchor)35 void b2FrictionJointDef::Initialize(b2Body* bA, b2Body* bB, const b2Vec2& anchor)
36 {
37 bodyA = bA;
38 bodyB = bB;
39 localAnchorA = bodyA->GetLocalPoint(anchor);
40 localAnchorB = bodyB->GetLocalPoint(anchor);
41 }
42
b2FrictionJoint(const b2FrictionJointDef * def)43 b2FrictionJoint::b2FrictionJoint(const b2FrictionJointDef* def)
44 : b2Joint(def)
45 {
46 m_localAnchorA = def->localAnchorA;
47 m_localAnchorB = def->localAnchorB;
48
49 m_linearImpulse.SetZero();
50 m_angularImpulse = 0.0f;
51
52 m_maxForce = def->maxForce;
53 m_maxTorque = def->maxTorque;
54 }
55
InitVelocityConstraints(const b2SolverData & data)56 void b2FrictionJoint::InitVelocityConstraints(const b2SolverData& data)
57 {
58 m_indexA = m_bodyA->m_islandIndex;
59 m_indexB = m_bodyB->m_islandIndex;
60 m_localCenterA = m_bodyA->m_sweep.localCenter;
61 m_localCenterB = m_bodyB->m_sweep.localCenter;
62 m_invMassA = m_bodyA->m_invMass;
63 m_invMassB = m_bodyB->m_invMass;
64 m_invIA = m_bodyA->m_invI;
65 m_invIB = m_bodyB->m_invI;
66
67 float32 aA = data.positions[m_indexA].a;
68 b2Vec2 vA = data.velocities[m_indexA].v;
69 float32 wA = data.velocities[m_indexA].w;
70
71 float32 aB = data.positions[m_indexB].a;
72 b2Vec2 vB = data.velocities[m_indexB].v;
73 float32 wB = data.velocities[m_indexB].w;
74
75 b2Rot qA(aA), qB(aB);
76
77 // Compute the effective mass matrix.
78 m_rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
79 m_rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
80
81 // J = [-I -r1_skew I r2_skew]
82 // [ 0 -1 0 1]
83 // r_skew = [-ry; rx]
84
85 // Matlab
86 // K = [ mA+r1y^2*iA+mB+r2y^2*iB, -r1y*iA*r1x-r2y*iB*r2x, -r1y*iA-r2y*iB]
87 // [ -r1y*iA*r1x-r2y*iB*r2x, mA+r1x^2*iA+mB+r2x^2*iB, r1x*iA+r2x*iB]
88 // [ -r1y*iA-r2y*iB, r1x*iA+r2x*iB, iA+iB]
89
90 float32 mA = m_invMassA, mB = m_invMassB;
91 float32 iA = m_invIA, iB = m_invIB;
92
93 b2Mat22 K;
94 K.ex.x = mA + mB + iA * m_rA.y * m_rA.y + iB * m_rB.y * m_rB.y;
95 K.ex.y = -iA * m_rA.x * m_rA.y - iB * m_rB.x * m_rB.y;
96 K.ey.x = K.ex.y;
97 K.ey.y = mA + mB + iA * m_rA.x * m_rA.x + iB * m_rB.x * m_rB.x;
98
99 m_linearMass = K.GetInverse();
100
101 m_angularMass = iA + iB;
102 if (m_angularMass > 0.0f)
103 {
104 m_angularMass = 1.0f / m_angularMass;
105 }
106
107 if (data.step.warmStarting)
108 {
109 // Scale impulses to support a variable time step.
110 m_linearImpulse *= data.step.dtRatio;
111 m_angularImpulse *= data.step.dtRatio;
112
113 b2Vec2 P(m_linearImpulse.x, m_linearImpulse.y);
114 vA -= mA * P;
115 wA -= iA * (b2Cross(m_rA, P) + m_angularImpulse);
116 vB += mB * P;
117 wB += iB * (b2Cross(m_rB, P) + m_angularImpulse);
118 }
119 else
120 {
121 m_linearImpulse.SetZero();
122 m_angularImpulse = 0.0f;
123 }
124
125 data.velocities[m_indexA].v = vA;
126 data.velocities[m_indexA].w = wA;
127 data.velocities[m_indexB].v = vB;
128 data.velocities[m_indexB].w = wB;
129 }
130
SolveVelocityConstraints(const b2SolverData & data)131 void b2FrictionJoint::SolveVelocityConstraints(const b2SolverData& data)
132 {
133 b2Vec2 vA = data.velocities[m_indexA].v;
134 float32 wA = data.velocities[m_indexA].w;
135 b2Vec2 vB = data.velocities[m_indexB].v;
136 float32 wB = data.velocities[m_indexB].w;
137
138 float32 mA = m_invMassA, mB = m_invMassB;
139 float32 iA = m_invIA, iB = m_invIB;
140
141 float32 h = data.step.dt;
142
143 // Solve angular friction
144 {
145 float32 Cdot = wB - wA;
146 float32 impulse = -m_angularMass * Cdot;
147
148 float32 oldImpulse = m_angularImpulse;
149 float32 maxImpulse = h * m_maxTorque;
150 m_angularImpulse = b2Clamp(m_angularImpulse + impulse, -maxImpulse, maxImpulse);
151 impulse = m_angularImpulse - oldImpulse;
152
153 wA -= iA * impulse;
154 wB += iB * impulse;
155 }
156
157 // Solve linear friction
158 {
159 b2Vec2 Cdot = vB + b2Cross(wB, m_rB) - vA - b2Cross(wA, m_rA);
160
161 b2Vec2 impulse = -b2Mul(m_linearMass, Cdot);
162 b2Vec2 oldImpulse = m_linearImpulse;
163 m_linearImpulse += impulse;
164
165 float32 maxImpulse = h * m_maxForce;
166
167 if (m_linearImpulse.LengthSquared() > maxImpulse * maxImpulse)
168 {
169 m_linearImpulse.Normalize();
170 m_linearImpulse *= maxImpulse;
171 }
172
173 impulse = m_linearImpulse - oldImpulse;
174
175 vA -= mA * impulse;
176 wA -= iA * b2Cross(m_rA, impulse);
177
178 vB += mB * impulse;
179 wB += iB * b2Cross(m_rB, impulse);
180 }
181
182 data.velocities[m_indexA].v = vA;
183 data.velocities[m_indexA].w = wA;
184 data.velocities[m_indexB].v = vB;
185 data.velocities[m_indexB].w = wB;
186 }
187
SolvePositionConstraints(const b2SolverData & data)188 bool b2FrictionJoint::SolvePositionConstraints(const b2SolverData& data)
189 {
190 B2_NOT_USED(data);
191
192 return true;
193 }
194
GetAnchorA() const195 b2Vec2 b2FrictionJoint::GetAnchorA() const
196 {
197 return m_bodyA->GetWorldPoint(m_localAnchorA);
198 }
199
GetAnchorB() const200 b2Vec2 b2FrictionJoint::GetAnchorB() const
201 {
202 return m_bodyB->GetWorldPoint(m_localAnchorB);
203 }
204
GetReactionForce(float32 inv_dt) const205 b2Vec2 b2FrictionJoint::GetReactionForce(float32 inv_dt) const
206 {
207 return inv_dt * m_linearImpulse;
208 }
209
GetReactionTorque(float32 inv_dt) const210 float32 b2FrictionJoint::GetReactionTorque(float32 inv_dt) const
211 {
212 return inv_dt * m_angularImpulse;
213 }
214
SetMaxForce(float32 force)215 void b2FrictionJoint::SetMaxForce(float32 force)
216 {
217 b2Assert(b2IsValid(force) && force >= 0.0f);
218 m_maxForce = force;
219 }
220
GetMaxForce() const221 float32 b2FrictionJoint::GetMaxForce() const
222 {
223 return m_maxForce;
224 }
225
SetMaxTorque(float32 torque)226 void b2FrictionJoint::SetMaxTorque(float32 torque)
227 {
228 b2Assert(b2IsValid(torque) && torque >= 0.0f);
229 m_maxTorque = torque;
230 }
231
GetMaxTorque() const232 float32 b2FrictionJoint::GetMaxTorque() const
233 {
234 return m_maxTorque;
235 }
236
Dump()237 void b2FrictionJoint::Dump()
238 {
239 int32 indexA = m_bodyA->m_islandIndex;
240 int32 indexB = m_bodyB->m_islandIndex;
241
242 b2Log(" b2FrictionJointDef jd;\n");
243 b2Log(" jd.bodyA = bodies[%d];\n", indexA);
244 b2Log(" jd.bodyB = bodies[%d];\n", indexB);
245 b2Log(" jd.collideConnected = bool(%d);\n", m_collideConnected);
246 b2Log(" jd.localAnchorA.Set(%.15lef, %.15lef);\n", m_localAnchorA.x, m_localAnchorA.y);
247 b2Log(" jd.localAnchorB.Set(%.15lef, %.15lef);\n", m_localAnchorB.x, m_localAnchorB.y);
248 b2Log(" jd.maxForce = %.15lef;\n", m_maxForce);
249 b2Log(" jd.maxTorque = %.15lef;\n", m_maxTorque);
250 b2Log(" joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
251 }
252