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/b2RopeJoint.h>
20#include <Box2D/Dynamics/b2Body.h>
21#include <Box2D/Dynamics/b2TimeStep.h>
22
23
24// Limit:
25// C = norm(pB - pA) - L
26// u = (pB - pA) / norm(pB - pA)
27// Cdot = dot(u, vB + cross(wB, rB) - vA - cross(wA, rA))
28// J = [-u -cross(rA, u) u cross(rB, u)]
29// K = J * invM * JT
30// = invMassA + invIA * cross(rA, u)^2 + invMassB + invIB * cross(rB, u)^2
31
32b2RopeJoint::b2RopeJoint(const b2RopeJointDef* def)
33: b2Joint(def)
34{
35 m_localAnchorA = def->localAnchorA;
36 m_localAnchorB = def->localAnchorB;
37
38 m_maxLength = def->maxLength;
39
40 m_mass = 0.0f;
41 m_impulse = 0.0f;
42 m_state = e_inactiveLimit;
43 m_length = 0.0f;
44}
45
46void b2RopeJoint::InitVelocityConstraints(const b2SolverData& data)
47{
48 m_indexA = m_bodyA->m_islandIndex;
49 m_indexB = m_bodyB->m_islandIndex;
50 m_localCenterA = m_bodyA->m_sweep.localCenter;
51 m_localCenterB = m_bodyB->m_sweep.localCenter;
52 m_invMassA = m_bodyA->m_invMass;
53 m_invMassB = m_bodyB->m_invMass;
54 m_invIA = m_bodyA->m_invI;
55 m_invIB = m_bodyB->m_invI;
56
57 b2Vec2 cA = data.positions[m_indexA].c;
58 float32 aA = data.positions[m_indexA].a;
59 b2Vec2 vA = data.velocities[m_indexA].v;
60 float32 wA = data.velocities[m_indexA].w;
61
62 b2Vec2 cB = data.positions[m_indexB].c;
63 float32 aB = data.positions[m_indexB].a;
64 b2Vec2 vB = data.velocities[m_indexB].v;
65 float32 wB = data.velocities[m_indexB].w;
66
67 b2Rot qA(aA), qB(aB);
68
69 m_rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
70 m_rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
71 m_u = cB + m_rB - cA - m_rA;
72
73 m_length = m_u.Length();
74
75 float32 C = m_length - m_maxLength;
76 if (C > 0.0f)
77 {
78 m_state = e_atUpperLimit;
79 }
80 else
81 {
82 m_state = e_inactiveLimit;
83 }
84
85 if (m_length > b2_linearSlop)
86 {
87 m_u *= 1.0f / m_length;
88 }
89 else
90 {
91 m_u.SetZero();
92 m_mass = 0.0f;
93 m_impulse = 0.0f;
94 return;
95 }
96
97 // Compute effective mass.
98 float32 crA = b2Cross(m_rA, m_u);
99 float32 crB = b2Cross(m_rB, m_u);
100 float32 invMass = m_invMassA + m_invIA * crA * crA + m_invMassB + m_invIB * crB * crB;
101
102 m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;
103
104 if (data.step.warmStarting)
105 {
106 // Scale the impulse to support a variable time step.
107 m_impulse *= data.step.dtRatio;
108
109 b2Vec2 P = m_impulse * m_u;
110 vA -= m_invMassA * P;
111 wA -= m_invIA * b2Cross(m_rA, P);
112 vB += m_invMassB * P;
113 wB += m_invIB * b2Cross(m_rB, P);
114 }
115 else
116 {
117 m_impulse = 0.0f;
118 }
119
120 data.velocities[m_indexA].v = vA;
121 data.velocities[m_indexA].w = wA;
122 data.velocities[m_indexB].v = vB;
123 data.velocities[m_indexB].w = wB;
124}
125
126void b2RopeJoint::SolveVelocityConstraints(const b2SolverData& data)
127{
128 b2Vec2 vA = data.velocities[m_indexA].v;
129 float32 wA = data.velocities[m_indexA].w;
130 b2Vec2 vB = data.velocities[m_indexB].v;
131 float32 wB = data.velocities[m_indexB].w;
132
133 // Cdot = dot(u, v + cross(w, r))
134 b2Vec2 vpA = vA + b2Cross(wA, m_rA);
135 b2Vec2 vpB = vB + b2Cross(wB, m_rB);
136 float32 C = m_length - m_maxLength;
137 float32 Cdot = b2Dot(m_u, vpB - vpA);
138
139 // Predictive constraint.
140 if (C < 0.0f)
141 {
142 Cdot += data.step.inv_dt * C;
143 }
144
145 float32 impulse = -m_mass * Cdot;
146 float32 oldImpulse = m_impulse;
147 m_impulse = b2Min(0.0f, m_impulse + impulse);
148 impulse = m_impulse - oldImpulse;
149
150 b2Vec2 P = impulse * m_u;
151 vA -= m_invMassA * P;
152 wA -= m_invIA * b2Cross(m_rA, P);
153 vB += m_invMassB * P;
154 wB += m_invIB * b2Cross(m_rB, P);
155
156 data.velocities[m_indexA].v = vA;
157 data.velocities[m_indexA].w = wA;
158 data.velocities[m_indexB].v = vB;
159 data.velocities[m_indexB].w = wB;
160}
161
162bool b2RopeJoint::SolvePositionConstraints(const b2SolverData& data)
163{
164 b2Vec2 cA = data.positions[m_indexA].c;
165 float32 aA = data.positions[m_indexA].a;
166 b2Vec2 cB = data.positions[m_indexB].c;
167 float32 aB = data.positions[m_indexB].a;
168
169 b2Rot qA(aA), qB(aB);
170
171 b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
172 b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
173 b2Vec2 u = cB + rB - cA - rA;
174
175 float32 length = u.Normalize();
176 float32 C = length - m_maxLength;
177
178 C = b2Clamp(C, 0.0f, b2_maxLinearCorrection);
179
180 float32 impulse = -m_mass * C;
181 b2Vec2 P = impulse * u;
182
183 cA -= m_invMassA * P;
184 aA -= m_invIA * b2Cross(rA, P);
185 cB += m_invMassB * P;
186 aB += m_invIB * b2Cross(rB, P);
187
188 data.positions[m_indexA].c = cA;
189 data.positions[m_indexA].a = aA;
190 data.positions[m_indexB].c = cB;
191 data.positions[m_indexB].a = aB;
192
193 return length - m_maxLength < b2_linearSlop;
194}
195
196b2Vec2 b2RopeJoint::GetAnchorA() const
197{
198 return m_bodyA->GetWorldPoint(m_localAnchorA);
199}
200
201b2Vec2 b2RopeJoint::GetAnchorB() const
202{
203 return m_bodyB->GetWorldPoint(m_localAnchorB);
204}
205
206b2Vec2 b2RopeJoint::GetReactionForce(float32 inv_dt) const
207{
208 b2Vec2 F = (inv_dt * m_impulse) * m_u;
209 return F;
210}
211
212float32 b2RopeJoint::GetReactionTorque(float32 inv_dt) const
213{
214 B2_NOT_USED(inv_dt);
215 return 0.0f;
216}
217
218float32 b2RopeJoint::GetMaxLength() const
219{
220 return m_maxLength;
221}
222
223b2LimitState b2RopeJoint::GetLimitState() const
224{
225 return m_state;
226}
227
228void b2RopeJoint::Dump()
229{
230 int32 indexA = m_bodyA->m_islandIndex;
231 int32 indexB = m_bodyB->m_islandIndex;
232
233 b2Log(" b2RopeJointDef jd;\n");
234 b2Log(" jd.bodyA = bodies[%d];\n", indexA);
235 b2Log(" jd.bodyB = bodies[%d];\n", indexB);
236 b2Log(" jd.collideConnected = bool(%d);\n", m_collideConnected);
237 b2Log(" jd.localAnchorA.Set(%.15lef, %.15lef);\n", m_localAnchorA.x, m_localAnchorA.y);
238 b2Log(" jd.localAnchorB.Set(%.15lef, %.15lef);\n", m_localAnchorB.x, m_localAnchorB.y);
239 b2Log(" jd.maxLength = %.15lef;\n", m_maxLength);
240 b2Log(" joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
241}
242