1/*
2* Copyright (c) 2006-2012 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/b2MotorJoint.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
35void b2MotorJointDef::Initialize(b2Body* bA, b2Body* bB)
36{
37 bodyA = bA;
38 bodyB = bB;
39 b2Vec2 xB = bodyB->GetPosition();
40 linearOffset = bodyA->GetLocalPoint(xB);
41
42 float32 angleA = bodyA->GetAngle();
43 float32 angleB = bodyB->GetAngle();
44 angularOffset = angleB - angleA;
45}
46
47b2MotorJoint::b2MotorJoint(const b2MotorJointDef* def)
48: b2Joint(def)
49{
50 m_linearOffset = def->linearOffset;
51 m_angularOffset = def->angularOffset;
52
53 m_linearImpulse.SetZero();
54 m_angularImpulse = 0.0f;
55
56 m_maxForce = def->maxForce;
57 m_maxTorque = def->maxTorque;
58 m_correctionFactor = def->correctionFactor;
59}
60
61void b2MotorJoint::InitVelocityConstraints(const b2SolverData& data)
62{
63 m_indexA = m_bodyA->m_islandIndex;
64 m_indexB = m_bodyB->m_islandIndex;
65 m_localCenterA = m_bodyA->m_sweep.localCenter;
66 m_localCenterB = m_bodyB->m_sweep.localCenter;
67 m_invMassA = m_bodyA->m_invMass;
68 m_invMassB = m_bodyB->m_invMass;
69 m_invIA = m_bodyA->m_invI;
70 m_invIB = m_bodyB->m_invI;
71
72 b2Vec2 cA = data.positions[m_indexA].c;
73 float32 aA = data.positions[m_indexA].a;
74 b2Vec2 vA = data.velocities[m_indexA].v;
75 float32 wA = data.velocities[m_indexA].w;
76
77 b2Vec2 cB = data.positions[m_indexB].c;
78 float32 aB = data.positions[m_indexB].a;
79 b2Vec2 vB = data.velocities[m_indexB].v;
80 float32 wB = data.velocities[m_indexB].w;
81
82 b2Rot qA(aA), qB(aB);
83
84 // Compute the effective mass matrix.
85 m_rA = b2Mul(qA, -m_localCenterA);
86 m_rB = b2Mul(qB, -m_localCenterB);
87
88 // J = [-I -r1_skew I r2_skew]
89 // [ 0 -1 0 1]
90 // r_skew = [-ry; rx]
91
92 // Matlab
93 // K = [ mA+r1y^2*iA+mB+r2y^2*iB, -r1y*iA*r1x-r2y*iB*r2x, -r1y*iA-r2y*iB]
94 // [ -r1y*iA*r1x-r2y*iB*r2x, mA+r1x^2*iA+mB+r2x^2*iB, r1x*iA+r2x*iB]
95 // [ -r1y*iA-r2y*iB, r1x*iA+r2x*iB, iA+iB]
96
97 float32 mA = m_invMassA, mB = m_invMassB;
98 float32 iA = m_invIA, iB = m_invIB;
99
100 b2Mat22 K;
101 K.ex.x = mA + mB + iA * m_rA.y * m_rA.y + iB * m_rB.y * m_rB.y;
102 K.ex.y = -iA * m_rA.x * m_rA.y - iB * m_rB.x * m_rB.y;
103 K.ey.x = K.ex.y;
104 K.ey.y = mA + mB + iA * m_rA.x * m_rA.x + iB * m_rB.x * m_rB.x;
105
106 m_linearMass = K.GetInverse();
107
108 m_angularMass = iA + iB;
109 if (m_angularMass > 0.0f)
110 {
111 m_angularMass = 1.0f / m_angularMass;
112 }
113
114 m_linearError = cB + m_rB - cA - m_rA - b2Mul(qA, m_linearOffset);
115 m_angularError = aB - aA - m_angularOffset;
116
117 if (data.step.warmStarting)
118 {
119 // Scale impulses to support a variable time step.
120 m_linearImpulse *= data.step.dtRatio;
121 m_angularImpulse *= data.step.dtRatio;
122
123 b2Vec2 P(m_linearImpulse.x, m_linearImpulse.y);
124 vA -= mA * P;
125 wA -= iA * (b2Cross(m_rA, P) + m_angularImpulse);
126 vB += mB * P;
127 wB += iB * (b2Cross(m_rB, P) + m_angularImpulse);
128 }
129 else
130 {
131 m_linearImpulse.SetZero();
132 m_angularImpulse = 0.0f;
133 }
134
135 data.velocities[m_indexA].v = vA;
136 data.velocities[m_indexA].w = wA;
137 data.velocities[m_indexB].v = vB;
138 data.velocities[m_indexB].w = wB;
139}
140
141void b2MotorJoint::SolveVelocityConstraints(const b2SolverData& data)
142{
143 b2Vec2 vA = data.velocities[m_indexA].v;
144 float32 wA = data.velocities[m_indexA].w;
145 b2Vec2 vB = data.velocities[m_indexB].v;
146 float32 wB = data.velocities[m_indexB].w;
147
148 float32 mA = m_invMassA, mB = m_invMassB;
149 float32 iA = m_invIA, iB = m_invIB;
150
151 float32 h = data.step.dt;
152 float32 inv_h = data.step.inv_dt;
153
154 // Solve angular friction
155 {
156 float32 Cdot = wB - wA + inv_h * m_correctionFactor * m_angularError;
157 float32 impulse = -m_angularMass * Cdot;
158
159 float32 oldImpulse = m_angularImpulse;
160 float32 maxImpulse = h * m_maxTorque;
161 m_angularImpulse = b2Clamp(m_angularImpulse + impulse, -maxImpulse, maxImpulse);
162 impulse = m_angularImpulse - oldImpulse;
163
164 wA -= iA * impulse;
165 wB += iB * impulse;
166 }
167
168 // Solve linear friction
169 {
170 b2Vec2 Cdot = vB + b2Cross(wB, m_rB) - vA - b2Cross(wA, m_rA) + inv_h * m_correctionFactor * m_linearError;
171
172 b2Vec2 impulse = -b2Mul(m_linearMass, Cdot);
173 b2Vec2 oldImpulse = m_linearImpulse;
174 m_linearImpulse += impulse;
175
176 float32 maxImpulse = h * m_maxForce;
177
178 if (m_linearImpulse.LengthSquared() > maxImpulse * maxImpulse)
179 {
180 m_linearImpulse.Normalize();
181 m_linearImpulse *= maxImpulse;
182 }
183
184 impulse = m_linearImpulse - oldImpulse;
185
186 vA -= mA * impulse;
187 wA -= iA * b2Cross(m_rA, impulse);
188
189 vB += mB * impulse;
190 wB += iB * b2Cross(m_rB, impulse);
191 }
192
193 data.velocities[m_indexA].v = vA;
194 data.velocities[m_indexA].w = wA;
195 data.velocities[m_indexB].v = vB;
196 data.velocities[m_indexB].w = wB;
197}
198
199bool b2MotorJoint::SolvePositionConstraints(const b2SolverData& data)
200{
201 B2_NOT_USED(data);
202
203 return true;
204}
205
206b2Vec2 b2MotorJoint::GetAnchorA() const
207{
208 return m_bodyA->GetPosition();
209}
210
211b2Vec2 b2MotorJoint::GetAnchorB() const
212{
213 return m_bodyB->GetPosition();
214}
215
216b2Vec2 b2MotorJoint::GetReactionForce(float32 inv_dt) const
217{
218 return inv_dt * m_linearImpulse;
219}
220
221float32 b2MotorJoint::GetReactionTorque(float32 inv_dt) const
222{
223 return inv_dt * m_angularImpulse;
224}
225
226void b2MotorJoint::SetMaxForce(float32 force)
227{
228 b2Assert(b2IsValid(force) && force >= 0.0f);
229 m_maxForce = force;
230}
231
232float32 b2MotorJoint::GetMaxForce() const
233{
234 return m_maxForce;
235}
236
237void b2MotorJoint::SetMaxTorque(float32 torque)
238{
239 b2Assert(b2IsValid(torque) && torque >= 0.0f);
240 m_maxTorque = torque;
241}
242
243float32 b2MotorJoint::GetMaxTorque() const
244{
245 return m_maxTorque;
246}
247
248void b2MotorJoint::SetCorrectionFactor(float32 factor)
249{
250 b2Assert(b2IsValid(factor) && 0.0f <= factor && factor <= 1.0f);
251 m_correctionFactor = factor;
252}
253
254float32 b2MotorJoint::GetCorrectionFactor() const
255{
256 return m_correctionFactor;
257}
258
259void b2MotorJoint::SetLinearOffset(const b2Vec2& linearOffset)
260{
261 if (linearOffset.x != m_linearOffset.x || linearOffset.y != m_linearOffset.y)
262 {
263 m_bodyA->SetAwake(true);
264 m_bodyB->SetAwake(true);
265 m_linearOffset = linearOffset;
266 }
267}
268
269const b2Vec2& b2MotorJoint::GetLinearOffset() const
270{
271 return m_linearOffset;
272}
273
274void b2MotorJoint::SetAngularOffset(float32 angularOffset)
275{
276 if (angularOffset != m_angularOffset)
277 {
278 m_bodyA->SetAwake(true);
279 m_bodyB->SetAwake(true);
280 m_angularOffset = angularOffset;
281 }
282}
283
284float32 b2MotorJoint::GetAngularOffset() const
285{
286 return m_angularOffset;
287}
288
289void b2MotorJoint::Dump()
290{
291 int32 indexA = m_bodyA->m_islandIndex;
292 int32 indexB = m_bodyB->m_islandIndex;
293
294 b2Log(" b2MotorJointDef jd;\n");
295 b2Log(" jd.bodyA = bodies[%d];\n", indexA);
296 b2Log(" jd.bodyB = bodies[%d];\n", indexB);
297 b2Log(" jd.collideConnected = bool(%d);\n", m_collideConnected);
298 b2Log(" jd.linearOffset.Set(%.15lef, %.15lef);\n", m_linearOffset.x, m_linearOffset.y);
299 b2Log(" jd.angularOffset = %.15lef;\n", m_angularOffset);
300 b2Log(" jd.maxForce = %.15lef;\n", m_maxForce);
301 b2Log(" jd.maxTorque = %.15lef;\n", m_maxTorque);
302 b2Log(" jd.correctionFactor = %.15lef;\n", m_correctionFactor);
303 b2Log(" joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
304}
305