1/*
2* Copyright (c) 2007-2011 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/b2GearJoint.h>
20#include <Box2D/Dynamics/Joints/b2RevoluteJoint.h>
21#include <Box2D/Dynamics/Joints/b2PrismaticJoint.h>
22#include <Box2D/Dynamics/b2Body.h>
23#include <Box2D/Dynamics/b2TimeStep.h>
24
25// Gear Joint:
26// C0 = (coordinate1 + ratio * coordinate2)_initial
27// C = (coordinate1 + ratio * coordinate2) - C0 = 0
28// J = [J1 ratio * J2]
29// K = J * invM * JT
30// = J1 * invM1 * J1T + ratio * ratio * J2 * invM2 * J2T
31//
32// Revolute:
33// coordinate = rotation
34// Cdot = angularVelocity
35// J = [0 0 1]
36// K = J * invM * JT = invI
37//
38// Prismatic:
39// coordinate = dot(p - pg, ug)
40// Cdot = dot(v + cross(w, r), ug)
41// J = [ug cross(r, ug)]
42// K = J * invM * JT = invMass + invI * cross(r, ug)^2
43
44b2GearJoint::b2GearJoint(const b2GearJointDef* def)
45: b2Joint(def)
46{
47 m_joint1 = def->joint1;
48 m_joint2 = def->joint2;
49
50 m_typeA = m_joint1->GetType();
51 m_typeB = m_joint2->GetType();
52
53 b2Assert(m_typeA == e_revoluteJoint || m_typeA == e_prismaticJoint);
54 b2Assert(m_typeB == e_revoluteJoint || m_typeB == e_prismaticJoint);
55
56 float32 coordinateA, coordinateB;
57
58 // TODO_ERIN there might be some problem with the joint edges in b2Joint.
59
60 m_bodyC = m_joint1->GetBodyA();
61 m_bodyA = m_joint1->GetBodyB();
62
63 // Get geometry of joint1
64 b2Transform xfA = m_bodyA->m_xf;
65 float32 aA = m_bodyA->m_sweep.a;
66 b2Transform xfC = m_bodyC->m_xf;
67 float32 aC = m_bodyC->m_sweep.a;
68
69 if (m_typeA == e_revoluteJoint)
70 {
71 b2RevoluteJoint* revolute = (b2RevoluteJoint*)def->joint1;
72 m_localAnchorC = revolute->m_localAnchorA;
73 m_localAnchorA = revolute->m_localAnchorB;
74 m_referenceAngleA = revolute->m_referenceAngle;
75 m_localAxisC.SetZero();
76
77 coordinateA = aA - aC - m_referenceAngleA;
78 }
79 else
80 {
81 b2PrismaticJoint* prismatic = (b2PrismaticJoint*)def->joint1;
82 m_localAnchorC = prismatic->m_localAnchorA;
83 m_localAnchorA = prismatic->m_localAnchorB;
84 m_referenceAngleA = prismatic->m_referenceAngle;
85 m_localAxisC = prismatic->m_localXAxisA;
86
87 b2Vec2 pC = m_localAnchorC;
88 b2Vec2 pA = b2MulT(xfC.q, b2Mul(xfA.q, m_localAnchorA) + (xfA.p - xfC.p));
89 coordinateA = b2Dot(pA - pC, m_localAxisC);
90 }
91
92 m_bodyD = m_joint2->GetBodyA();
93 m_bodyB = m_joint2->GetBodyB();
94
95 // Get geometry of joint2
96 b2Transform xfB = m_bodyB->m_xf;
97 float32 aB = m_bodyB->m_sweep.a;
98 b2Transform xfD = m_bodyD->m_xf;
99 float32 aD = m_bodyD->m_sweep.a;
100
101 if (m_typeB == e_revoluteJoint)
102 {
103 b2RevoluteJoint* revolute = (b2RevoluteJoint*)def->joint2;
104 m_localAnchorD = revolute->m_localAnchorA;
105 m_localAnchorB = revolute->m_localAnchorB;
106 m_referenceAngleB = revolute->m_referenceAngle;
107 m_localAxisD.SetZero();
108
109 coordinateB = aB - aD - m_referenceAngleB;
110 }
111 else
112 {
113 b2PrismaticJoint* prismatic = (b2PrismaticJoint*)def->joint2;
114 m_localAnchorD = prismatic->m_localAnchorA;
115 m_localAnchorB = prismatic->m_localAnchorB;
116 m_referenceAngleB = prismatic->m_referenceAngle;
117 m_localAxisD = prismatic->m_localXAxisA;
118
119 b2Vec2 pD = m_localAnchorD;
120 b2Vec2 pB = b2MulT(xfD.q, b2Mul(xfB.q, m_localAnchorB) + (xfB.p - xfD.p));
121 coordinateB = b2Dot(pB - pD, m_localAxisD);
122 }
123
124 m_ratio = def->ratio;
125
126 m_constant = coordinateA + m_ratio * coordinateB;
127
128 m_impulse = 0.0f;
129}
130
131void b2GearJoint::InitVelocityConstraints(const b2SolverData& data)
132{
133 m_indexA = m_bodyA->m_islandIndex;
134 m_indexB = m_bodyB->m_islandIndex;
135 m_indexC = m_bodyC->m_islandIndex;
136 m_indexD = m_bodyD->m_islandIndex;
137 m_lcA = m_bodyA->m_sweep.localCenter;
138 m_lcB = m_bodyB->m_sweep.localCenter;
139 m_lcC = m_bodyC->m_sweep.localCenter;
140 m_lcD = m_bodyD->m_sweep.localCenter;
141 m_mA = m_bodyA->m_invMass;
142 m_mB = m_bodyB->m_invMass;
143 m_mC = m_bodyC->m_invMass;
144 m_mD = m_bodyD->m_invMass;
145 m_iA = m_bodyA->m_invI;
146 m_iB = m_bodyB->m_invI;
147 m_iC = m_bodyC->m_invI;
148 m_iD = m_bodyD->m_invI;
149
150 float32 aA = data.positions[m_indexA].a;
151 b2Vec2 vA = data.velocities[m_indexA].v;
152 float32 wA = data.velocities[m_indexA].w;
153
154 float32 aB = data.positions[m_indexB].a;
155 b2Vec2 vB = data.velocities[m_indexB].v;
156 float32 wB = data.velocities[m_indexB].w;
157
158 float32 aC = data.positions[m_indexC].a;
159 b2Vec2 vC = data.velocities[m_indexC].v;
160 float32 wC = data.velocities[m_indexC].w;
161
162 float32 aD = data.positions[m_indexD].a;
163 b2Vec2 vD = data.velocities[m_indexD].v;
164 float32 wD = data.velocities[m_indexD].w;
165
166 b2Rot qA(aA), qB(aB), qC(aC), qD(aD);
167
168 m_mass = 0.0f;
169
170 if (m_typeA == e_revoluteJoint)
171 {
172 m_JvAC.SetZero();
173 m_JwA = 1.0f;
174 m_JwC = 1.0f;
175 m_mass += m_iA + m_iC;
176 }
177 else
178 {
179 b2Vec2 u = b2Mul(qC, m_localAxisC);
180 b2Vec2 rC = b2Mul(qC, m_localAnchorC - m_lcC);
181 b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_lcA);
182 m_JvAC = u;
183 m_JwC = b2Cross(rC, u);
184 m_JwA = b2Cross(rA, u);
185 m_mass += m_mC + m_mA + m_iC * m_JwC * m_JwC + m_iA * m_JwA * m_JwA;
186 }
187
188 if (m_typeB == e_revoluteJoint)
189 {
190 m_JvBD.SetZero();
191 m_JwB = m_ratio;
192 m_JwD = m_ratio;
193 m_mass += m_ratio * m_ratio * (m_iB + m_iD);
194 }
195 else
196 {
197 b2Vec2 u = b2Mul(qD, m_localAxisD);
198 b2Vec2 rD = b2Mul(qD, m_localAnchorD - m_lcD);
199 b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_lcB);
200 m_JvBD = m_ratio * u;
201 m_JwD = m_ratio * b2Cross(rD, u);
202 m_JwB = m_ratio * b2Cross(rB, u);
203 m_mass += m_ratio * m_ratio * (m_mD + m_mB) + m_iD * m_JwD * m_JwD + m_iB * m_JwB * m_JwB;
204 }
205
206 // Compute effective mass.
207 m_mass = m_mass > 0.0f ? 1.0f / m_mass : 0.0f;
208
209 if (data.step.warmStarting)
210 {
211 vA += (m_mA * m_impulse) * m_JvAC;
212 wA += m_iA * m_impulse * m_JwA;
213 vB += (m_mB * m_impulse) * m_JvBD;
214 wB += m_iB * m_impulse * m_JwB;
215 vC -= (m_mC * m_impulse) * m_JvAC;
216 wC -= m_iC * m_impulse * m_JwC;
217 vD -= (m_mD * m_impulse) * m_JvBD;
218 wD -= m_iD * m_impulse * m_JwD;
219 }
220 else
221 {
222 m_impulse = 0.0f;
223 }
224
225 data.velocities[m_indexA].v = vA;
226 data.velocities[m_indexA].w = wA;
227 data.velocities[m_indexB].v = vB;
228 data.velocities[m_indexB].w = wB;
229 data.velocities[m_indexC].v = vC;
230 data.velocities[m_indexC].w = wC;
231 data.velocities[m_indexD].v = vD;
232 data.velocities[m_indexD].w = wD;
233}
234
235void b2GearJoint::SolveVelocityConstraints(const b2SolverData& data)
236{
237 b2Vec2 vA = data.velocities[m_indexA].v;
238 float32 wA = data.velocities[m_indexA].w;
239 b2Vec2 vB = data.velocities[m_indexB].v;
240 float32 wB = data.velocities[m_indexB].w;
241 b2Vec2 vC = data.velocities[m_indexC].v;
242 float32 wC = data.velocities[m_indexC].w;
243 b2Vec2 vD = data.velocities[m_indexD].v;
244 float32 wD = data.velocities[m_indexD].w;
245
246 float32 Cdot = b2Dot(m_JvAC, vA - vC) + b2Dot(m_JvBD, vB - vD);
247 Cdot += (m_JwA * wA - m_JwC * wC) + (m_JwB * wB - m_JwD * wD);
248
249 float32 impulse = -m_mass * Cdot;
250 m_impulse += impulse;
251
252 vA += (m_mA * impulse) * m_JvAC;
253 wA += m_iA * impulse * m_JwA;
254 vB += (m_mB * impulse) * m_JvBD;
255 wB += m_iB * impulse * m_JwB;
256 vC -= (m_mC * impulse) * m_JvAC;
257 wC -= m_iC * impulse * m_JwC;
258 vD -= (m_mD * impulse) * m_JvBD;
259 wD -= m_iD * impulse * m_JwD;
260
261 data.velocities[m_indexA].v = vA;
262 data.velocities[m_indexA].w = wA;
263 data.velocities[m_indexB].v = vB;
264 data.velocities[m_indexB].w = wB;
265 data.velocities[m_indexC].v = vC;
266 data.velocities[m_indexC].w = wC;
267 data.velocities[m_indexD].v = vD;
268 data.velocities[m_indexD].w = wD;
269}
270
271bool b2GearJoint::SolvePositionConstraints(const b2SolverData& data)
272{
273 b2Vec2 cA = data.positions[m_indexA].c;
274 float32 aA = data.positions[m_indexA].a;
275 b2Vec2 cB = data.positions[m_indexB].c;
276 float32 aB = data.positions[m_indexB].a;
277 b2Vec2 cC = data.positions[m_indexC].c;
278 float32 aC = data.positions[m_indexC].a;
279 b2Vec2 cD = data.positions[m_indexD].c;
280 float32 aD = data.positions[m_indexD].a;
281
282 b2Rot qA(aA), qB(aB), qC(aC), qD(aD);
283
284 float32 linearError = 0.0f;
285
286 float32 coordinateA, coordinateB;
287
288 b2Vec2 JvAC, JvBD;
289 float32 JwA, JwB, JwC, JwD;
290 float32 mass = 0.0f;
291
292 if (m_typeA == e_revoluteJoint)
293 {
294 JvAC.SetZero();
295 JwA = 1.0f;
296 JwC = 1.0f;
297 mass += m_iA + m_iC;
298
299 coordinateA = aA - aC - m_referenceAngleA;
300 }
301 else
302 {
303 b2Vec2 u = b2Mul(qC, m_localAxisC);
304 b2Vec2 rC = b2Mul(qC, m_localAnchorC - m_lcC);
305 b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_lcA);
306 JvAC = u;
307 JwC = b2Cross(rC, u);
308 JwA = b2Cross(rA, u);
309 mass += m_mC + m_mA + m_iC * JwC * JwC + m_iA * JwA * JwA;
310
311 b2Vec2 pC = m_localAnchorC - m_lcC;
312 b2Vec2 pA = b2MulT(qC, rA + (cA - cC));
313 coordinateA = b2Dot(pA - pC, m_localAxisC);
314 }
315
316 if (m_typeB == e_revoluteJoint)
317 {
318 JvBD.SetZero();
319 JwB = m_ratio;
320 JwD = m_ratio;
321 mass += m_ratio * m_ratio * (m_iB + m_iD);
322
323 coordinateB = aB - aD - m_referenceAngleB;
324 }
325 else
326 {
327 b2Vec2 u = b2Mul(qD, m_localAxisD);
328 b2Vec2 rD = b2Mul(qD, m_localAnchorD - m_lcD);
329 b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_lcB);
330 JvBD = m_ratio * u;
331 JwD = m_ratio * b2Cross(rD, u);
332 JwB = m_ratio * b2Cross(rB, u);
333 mass += m_ratio * m_ratio * (m_mD + m_mB) + m_iD * JwD * JwD + m_iB * JwB * JwB;
334
335 b2Vec2 pD = m_localAnchorD - m_lcD;
336 b2Vec2 pB = b2MulT(qD, rB + (cB - cD));
337 coordinateB = b2Dot(pB - pD, m_localAxisD);
338 }
339
340 float32 C = (coordinateA + m_ratio * coordinateB) - m_constant;
341
342 float32 impulse = 0.0f;
343 if (mass > 0.0f)
344 {
345 impulse = -C / mass;
346 }
347
348 cA += m_mA * impulse * JvAC;
349 aA += m_iA * impulse * JwA;
350 cB += m_mB * impulse * JvBD;
351 aB += m_iB * impulse * JwB;
352 cC -= m_mC * impulse * JvAC;
353 aC -= m_iC * impulse * JwC;
354 cD -= m_mD * impulse * JvBD;
355 aD -= m_iD * impulse * JwD;
356
357 data.positions[m_indexA].c = cA;
358 data.positions[m_indexA].a = aA;
359 data.positions[m_indexB].c = cB;
360 data.positions[m_indexB].a = aB;
361 data.positions[m_indexC].c = cC;
362 data.positions[m_indexC].a = aC;
363 data.positions[m_indexD].c = cD;
364 data.positions[m_indexD].a = aD;
365
366 // TODO_ERIN not implemented
367 return linearError < b2_linearSlop;
368}
369
370b2Vec2 b2GearJoint::GetAnchorA() const
371{
372 return m_bodyA->GetWorldPoint(m_localAnchorA);
373}
374
375b2Vec2 b2GearJoint::GetAnchorB() const
376{
377 return m_bodyB->GetWorldPoint(m_localAnchorB);
378}
379
380b2Vec2 b2GearJoint::GetReactionForce(float32 inv_dt) const
381{
382 b2Vec2 P = m_impulse * m_JvAC;
383 return inv_dt * P;
384}
385
386float32 b2GearJoint::GetReactionTorque(float32 inv_dt) const
387{
388 float32 L = m_impulse * m_JwA;
389 return inv_dt * L;
390}
391
392void b2GearJoint::SetRatio(float32 ratio)
393{
394 b2Assert(b2IsValid(ratio));
395 m_ratio = ratio;
396}
397
398float32 b2GearJoint::GetRatio() const
399{
400 return m_ratio;
401}
402
403void b2GearJoint::Dump()
404{
405 int32 indexA = m_bodyA->m_islandIndex;
406 int32 indexB = m_bodyB->m_islandIndex;
407
408 int32 index1 = m_joint1->m_index;
409 int32 index2 = m_joint2->m_index;
410
411 b2Log(" b2GearJointDef jd;\n");
412 b2Log(" jd.bodyA = bodies[%d];\n", indexA);
413 b2Log(" jd.bodyB = bodies[%d];\n", indexB);
414 b2Log(" jd.collideConnected = bool(%d);\n", m_collideConnected);
415 b2Log(" jd.joint1 = joints[%d];\n", index1);
416 b2Log(" jd.joint2 = joints[%d];\n", index2);
417 b2Log(" jd.ratio = %.15lef;\n", m_ratio);
418 b2Log(" joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
419}
420