1 /* 2 * Copyright (c) 2007 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/b2PulleyJoint.h> 20 #include <Box2D/Dynamics/b2Body.h> 21 #include <Box2D/Dynamics/b2TimeStep.h> 22 23 // Pulley: 24 // length1 = norm(p1 - s1) 25 // length2 = norm(p2 - s2) 26 // C0 = (length1 + ratio * length2)_initial 27 // C = C0 - (length1 + ratio * length2) 28 // u1 = (p1 - s1) / norm(p1 - s1) 29 // u2 = (p2 - s2) / norm(p2 - s2) 30 // Cdot = -dot(u1, v1 + cross(w1, r1)) - ratio * dot(u2, v2 + cross(w2, r2)) 31 // J = -[u1 cross(r1, u1) ratio * u2 ratio * cross(r2, u2)] 32 // K = J * invM * JT 33 // = invMass1 + invI1 * cross(r1, u1)^2 + ratio^2 * (invMass2 + invI2 * cross(r2, u2)^2) 34 35 void b2PulleyJointDef::Initialize(b2Body* bA, b2Body* bB, 36 const b2Vec2& groundA, const b2Vec2& groundB, 37 const b2Vec2& anchorA, const b2Vec2& anchorB, 38 float32 r) 39 { 40 bodyA = bA; 41 bodyB = bB; 42 groundAnchorA = groundA; 43 groundAnchorB = groundB; 44 localAnchorA = bodyA->GetLocalPoint(anchorA); 45 localAnchorB = bodyB->GetLocalPoint(anchorB); 46 b2Vec2 dA = anchorA - groundA; 47 lengthA = dA.Length(); 48 b2Vec2 dB = anchorB - groundB; 49 lengthB = dB.Length(); 50 ratio = r; 51 b2Assert(ratio > b2_epsilon); 52 } 53 54 b2PulleyJoint::b2PulleyJoint(const b2PulleyJointDef* def) 55 : b2Joint(def) 56 { 57 m_groundAnchorA = def->groundAnchorA; 58 m_groundAnchorB = def->groundAnchorB; 59 m_localAnchorA = def->localAnchorA; 60 m_localAnchorB = def->localAnchorB; 61 62 m_lengthA = def->lengthA; 63 m_lengthB = def->lengthB; 64 65 b2Assert(def->ratio != 0.0f); 66 m_ratio = def->ratio; 67 68 m_constant = def->lengthA + m_ratio * def->lengthB; 69 70 m_impulse = 0.0f; 71 } 72 73 void b2PulleyJoint::InitVelocityConstraints(const b2SolverData& data) 74 { 75 m_indexA = m_bodyA->m_islandIndex; 76 m_indexB = m_bodyB->m_islandIndex; 77 m_localCenterA = m_bodyA->m_sweep.localCenter; 78 m_localCenterB = m_bodyB->m_sweep.localCenter; 79 m_invMassA = m_bodyA->m_invMass; 80 m_invMassB = m_bodyB->m_invMass; 81 m_invIA = m_bodyA->m_invI; 82 m_invIB = m_bodyB->m_invI; 83 84 b2Vec2 cA = data.positions[m_indexA].c; 85 float32 aA = data.positions[m_indexA].a; 86 b2Vec2 vA = data.velocities[m_indexA].v; 87 float32 wA = data.velocities[m_indexA].w; 88 89 b2Vec2 cB = data.positions[m_indexB].c; 90 float32 aB = data.positions[m_indexB].a; 91 b2Vec2 vB = data.velocities[m_indexB].v; 92 float32 wB = data.velocities[m_indexB].w; 93 94 b2Rot qA(aA), qB(aB); 95 96 m_rA = b2Mul(qA, m_localAnchorA - m_localCenterA); 97 m_rB = b2Mul(qB, m_localAnchorB - m_localCenterB); 98 99 // Get the pulley axes. 100 m_uA = cA + m_rA - m_groundAnchorA; 101 m_uB = cB + m_rB - m_groundAnchorB; 102 103 float32 lengthA = m_uA.Length(); 104 float32 lengthB = m_uB.Length(); 105 106 if (lengthA > 10.0f * b2_linearSlop) 107 { 108 m_uA *= 1.0f / lengthA; 109 } 110 else 111 { 112 m_uA.SetZero(); 113 } 114 115 if (lengthB > 10.0f * b2_linearSlop) 116 { 117 m_uB *= 1.0f / lengthB; 118 } 119 else 120 { 121 m_uB.SetZero(); 122 } 123 124 // Compute effective mass. 125 float32 ruA = b2Cross(m_rA, m_uA); 126 float32 ruB = b2Cross(m_rB, m_uB); 127 128 float32 mA = m_invMassA + m_invIA * ruA * ruA; 129 float32 mB = m_invMassB + m_invIB * ruB * ruB; 130 131 m_mass = mA + m_ratio * m_ratio * mB; 132 133 if (m_mass > 0.0f) 134 { 135 m_mass = 1.0f / m_mass; 136 } 137 138 if (data.step.warmStarting) 139 { 140 // Scale impulses to support variable time steps. 141 m_impulse *= data.step.dtRatio; 142 143 // Warm starting. 144 b2Vec2 PA = -(m_impulse) * m_uA; 145 b2Vec2 PB = (-m_ratio * m_impulse) * m_uB; 146 147 vA += m_invMassA * PA; 148 wA += m_invIA * b2Cross(m_rA, PA); 149 vB += m_invMassB * PB; 150 wB += m_invIB * b2Cross(m_rB, PB); 151 } 152 else 153 { 154 m_impulse = 0.0f; 155 } 156 157 data.velocities[m_indexA].v = vA; 158 data.velocities[m_indexA].w = wA; 159 data.velocities[m_indexB].v = vB; 160 data.velocities[m_indexB].w = wB; 161 } 162 163 void b2PulleyJoint::SolveVelocityConstraints(const b2SolverData& data) 164 { 165 b2Vec2 vA = data.velocities[m_indexA].v; 166 float32 wA = data.velocities[m_indexA].w; 167 b2Vec2 vB = data.velocities[m_indexB].v; 168 float32 wB = data.velocities[m_indexB].w; 169 170 b2Vec2 vpA = vA + b2Cross(wA, m_rA); 171 b2Vec2 vpB = vB + b2Cross(wB, m_rB); 172 173 float32 Cdot = -b2Dot(m_uA, vpA) - m_ratio * b2Dot(m_uB, vpB); 174 float32 impulse = -m_mass * Cdot; 175 m_impulse += impulse; 176 177 b2Vec2 PA = -impulse * m_uA; 178 b2Vec2 PB = -m_ratio * impulse * m_uB; 179 vA += m_invMassA * PA; 180 wA += m_invIA * b2Cross(m_rA, PA); 181 vB += m_invMassB * PB; 182 wB += m_invIB * b2Cross(m_rB, PB); 183 184 data.velocities[m_indexA].v = vA; 185 data.velocities[m_indexA].w = wA; 186 data.velocities[m_indexB].v = vB; 187 data.velocities[m_indexB].w = wB; 188 } 189 190 bool b2PulleyJoint::SolvePositionConstraints(const b2SolverData& data) 191 { 192 b2Vec2 cA = data.positions[m_indexA].c; 193 float32 aA = data.positions[m_indexA].a; 194 b2Vec2 cB = data.positions[m_indexB].c; 195 float32 aB = data.positions[m_indexB].a; 196 197 b2Rot qA(aA), qB(aB); 198 199 b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_localCenterA); 200 b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_localCenterB); 201 202 // Get the pulley axes. 203 b2Vec2 uA = cA + rA - m_groundAnchorA; 204 b2Vec2 uB = cB + rB - m_groundAnchorB; 205 206 float32 lengthA = uA.Length(); 207 float32 lengthB = uB.Length(); 208 209 if (lengthA > 10.0f * b2_linearSlop) 210 { 211 uA *= 1.0f / lengthA; 212 } 213 else 214 { 215 uA.SetZero(); 216 } 217 218 if (lengthB > 10.0f * b2_linearSlop) 219 { 220 uB *= 1.0f / lengthB; 221 } 222 else 223 { 224 uB.SetZero(); 225 } 226 227 // Compute effective mass. 228 float32 ruA = b2Cross(rA, uA); 229 float32 ruB = b2Cross(rB, uB); 230 231 float32 mA = m_invMassA + m_invIA * ruA * ruA; 232 float32 mB = m_invMassB + m_invIB * ruB * ruB; 233 234 float32 mass = mA + m_ratio * m_ratio * mB; 235 236 if (mass > 0.0f) 237 { 238 mass = 1.0f / mass; 239 } 240 241 float32 C = m_constant - lengthA - m_ratio * lengthB; 242 float32 linearError = b2Abs(C); 243 244 float32 impulse = -mass * C; 245 246 b2Vec2 PA = -impulse * uA; 247 b2Vec2 PB = -m_ratio * impulse * uB; 248 249 cA += m_invMassA * PA; 250 aA += m_invIA * b2Cross(rA, PA); 251 cB += m_invMassB * PB; 252 aB += m_invIB * b2Cross(rB, PB); 253 254 data.positions[m_indexA].c = cA; 255 data.positions[m_indexA].a = aA; 256 data.positions[m_indexB].c = cB; 257 data.positions[m_indexB].a = aB; 258 259 return linearError < b2_linearSlop; 260 } 261 262 b2Vec2 b2PulleyJoint::GetAnchorA() const 263 { 264 return m_bodyA->GetWorldPoint(m_localAnchorA); 265 } 266 267 b2Vec2 b2PulleyJoint::GetAnchorB() const 268 { 269 return m_bodyB->GetWorldPoint(m_localAnchorB); 270 } 271 272 b2Vec2 b2PulleyJoint::GetReactionForce(float32 inv_dt) const 273 { 274 b2Vec2 P = m_impulse * m_uB; 275 return inv_dt * P; 276 } 277 278 float32 b2PulleyJoint::GetReactionTorque(float32 inv_dt) const 279 { 280 B2_NOT_USED(inv_dt); 281 return 0.0f; 282 } 283 284 b2Vec2 b2PulleyJoint::GetGroundAnchorA() const 285 { 286 return m_groundAnchorA; 287 } 288 289 b2Vec2 b2PulleyJoint::GetGroundAnchorB() const 290 { 291 return m_groundAnchorB; 292 } 293 294 float32 b2PulleyJoint::GetLengthA() const 295 { 296 return m_lengthA; 297 } 298 299 float32 b2PulleyJoint::GetLengthB() const 300 { 301 return m_lengthB; 302 } 303 304 float32 b2PulleyJoint::GetRatio() const 305 { 306 return m_ratio; 307 } 308 309 float32 b2PulleyJoint::GetCurrentLengthA() const 310 { 311 b2Vec2 p = m_bodyA->GetWorldPoint(m_localAnchorA); 312 b2Vec2 s = m_groundAnchorA; 313 b2Vec2 d = p - s; 314 return d.Length(); 315 } 316 317 float32 b2PulleyJoint::GetCurrentLengthB() const 318 { 319 b2Vec2 p = m_bodyB->GetWorldPoint(m_localAnchorB); 320 b2Vec2 s = m_groundAnchorB; 321 b2Vec2 d = p - s; 322 return d.Length(); 323 } 324 325 void b2PulleyJoint::Dump() 326 { 327 int32 indexA = m_bodyA->m_islandIndex; 328 int32 indexB = m_bodyB->m_islandIndex; 329 330 b2Log(" b2PulleyJointDef jd;\n"); 331 b2Log(" jd.bodyA = bodies[%d];\n", indexA); 332 b2Log(" jd.bodyB = bodies[%d];\n", indexB); 333 b2Log(" jd.collideConnected = bool(%d);\n", m_collideConnected); 334 b2Log(" jd.groundAnchorA.Set(%.15lef, %.15lef);\n", m_groundAnchorA.x, m_groundAnchorA.y); 335 b2Log(" jd.groundAnchorB.Set(%.15lef, %.15lef);\n", m_groundAnchorB.x, m_groundAnchorB.y); 336 b2Log(" jd.localAnchorA.Set(%.15lef, %.15lef);\n", m_localAnchorA.x, m_localAnchorA.y); 337 b2Log(" jd.localAnchorB.Set(%.15lef, %.15lef);\n", m_localAnchorB.x, m_localAnchorB.y); 338 b2Log(" jd.lengthA = %.15lef;\n", m_lengthA); 339 b2Log(" jd.lengthB = %.15lef;\n", m_lengthB); 340 b2Log(" jd.ratio = %.15lef;\n", m_ratio); 341 b2Log(" joints[%d] = m_world->CreateJoint(&jd);\n", m_index); 342 } 343 344 void b2PulleyJoint::ShiftOrigin(const b2Vec2& newOrigin) 345 { 346 m_groundAnchorA -= newOrigin; 347 m_groundAnchorB -= newOrigin; 348 } 349
