File indexing completed on 2024-12-29 03:29:26

 /*
 * Copyright (c) 2006-2011 Erin Catto http://www.box2d.org
 *
 * This software is provided 'as-is', without any express or implied
 * warranty.  In no event will the authors be held liable for any damages
 * arising from the use of this software.
 * Permission is granted to anyone to use this software for any purpose,
 * including commercial applications, and to alter it and redistribute it
 * freely, subject to the following restrictions:
 * 1. The origin of this software must not be misrepresented; you must not
 * claim that you wrote the original software. If you use this software
 * in a product, an acknowledgment in the product documentation would be
 * appreciated but is not required.
 * 2. Altered source versions must be plainly marked as such, and must not be
 * misrepresented as being the original software.
 * 3. This notice may not be removed or altered from any source distribution.
 */
 
 #include <Box2D/Dynamics/Joints/b2DistanceJoint.h>
 #include <Box2D/Dynamics/b2Body.h>
 #include <Box2D/Dynamics/b2TimeStep.h>
 
 // 1-D constrained system
 // m (v2 - v1) = lambda
 // v2 + (beta/h) * x1 + gamma * lambda = 0, gamma has units of inverse mass.
 // x2 = x1 + h * v2
 
 // 1-D mass-damper-spring system
 // m (v2 - v1) + h * d * v2 + h * k * 
 
 // C = norm(p2 - p1) - L
 // u = (p2 - p1) / norm(p2 - p1)
 // Cdot = dot(u, v2 + cross(w2, r2) - v1 - cross(w1, r1))
 // J = [-u -cross(r1, u) u cross(r2, u)]
 // K = J * invM * JT
 //   = invMass1 + invI1 * cross(r1, u)^2 + invMass2 + invI2 * cross(r2, u)^2
 
 void b2DistanceJointDef::Initialize(b2Body* b1, b2Body* b2,
                                     const b2Vec2& anchor1, const b2Vec2& anchor2)
 {
     bodyA = b1;
     bodyB = b2;
     localAnchorA = bodyA->GetLocalPoint(anchor1);
     localAnchorB = bodyB->GetLocalPoint(anchor2);
     b2Vec2 d = anchor2 - anchor1;
     length = d.Length();
 }
 
 b2DistanceJoint::b2DistanceJoint(const b2DistanceJointDef* def)
 : b2Joint(def)
 {
     m_localAnchorA = def->localAnchorA;
     m_localAnchorB = def->localAnchorB;
     m_length = def->length;
     m_frequencyHz = def->frequencyHz;
     m_dampingRatio = def->dampingRatio;
     m_impulse = 0.0f;
     m_gamma = 0.0f;
     m_bias = 0.0f;
 }
 
 void b2DistanceJoint::InitVelocityConstraints(const b2SolverData& data)
 {
     m_indexA = m_bodyA->m_islandIndex;
     m_indexB = m_bodyB->m_islandIndex;
     m_localCenterA = m_bodyA->m_sweep.localCenter;
     m_localCenterB = m_bodyB->m_sweep.localCenter;
     m_invMassA = m_bodyA->m_invMass;
     m_invMassB = m_bodyB->m_invMass;
     m_invIA = m_bodyA->m_invI;
     m_invIB = m_bodyB->m_invI;
 
     b2Vec2 cA = data.positions[m_indexA].c;
     float32 aA = data.positions[m_indexA].a;
     b2Vec2 vA = data.velocities[m_indexA].v;
     float32 wA = data.velocities[m_indexA].w;
 
     b2Vec2 cB = data.positions[m_indexB].c;
     float32 aB = data.positions[m_indexB].a;
     b2Vec2 vB = data.velocities[m_indexB].v;
     float32 wB = data.velocities[m_indexB].w;
 
     b2Rot qA(aA), qB(aB);
 
     m_rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
     m_rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
     m_u = cB + m_rB - cA - m_rA;
 
     // Handle singularity.
     float32 length = m_u.Length();
     if (length > b2_linearSlop)
     {
         m_u *= 1.0f / length;
     }
     else
     {
         m_u.Set(0.0f, 0.0f);
     }
 
     float32 crAu = b2Cross(m_rA, m_u);
     float32 crBu = b2Cross(m_rB, m_u);
     float32 invMass = m_invMassA + m_invIA * crAu * crAu + m_invMassB + m_invIB * crBu * crBu;
 
     // Compute the effective mass matrix.
     m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;
 
     if (m_frequencyHz > 0.0f)
     {
         float32 C = length - m_length;
 
         // Frequency
         float32 omega = 2.0f * b2_pi * m_frequencyHz;
 
         // Damping coefficient
         float32 d = 2.0f * m_mass * m_dampingRatio * omega;
 
         // Spring stiffness
         float32 k = m_mass * omega * omega;
 
         // magic formulas
         float32 h = data.step.dt;
         m_gamma = h * (d + h * k);
         m_gamma = m_gamma != 0.0f ? 1.0f / m_gamma : 0.0f;
         m_bias = C * h * k * m_gamma;
 
         invMass += m_gamma;
         m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;
     }
     else
     {
         m_gamma = 0.0f;
         m_bias = 0.0f;
     }
 
     if (data.step.warmStarting)
     {
         // Scale the impulse to support a variable time step.
         m_impulse *= data.step.dtRatio;
 
         b2Vec2 P = m_impulse * m_u;
         vA -= m_invMassA * P;
         wA -= m_invIA * b2Cross(m_rA, P);
         vB += m_invMassB * P;
         wB += m_invIB * b2Cross(m_rB, P);
     }
     else
     {
         m_impulse = 0.0f;
     }
 
     data.velocities[m_indexA].v = vA;
     data.velocities[m_indexA].w = wA;
     data.velocities[m_indexB].v = vB;
     data.velocities[m_indexB].w = wB;
 }
 
 void b2DistanceJoint::SolveVelocityConstraints(const b2SolverData& data)
 {
     b2Vec2 vA = data.velocities[m_indexA].v;
     float32 wA = data.velocities[m_indexA].w;
     b2Vec2 vB = data.velocities[m_indexB].v;
     float32 wB = data.velocities[m_indexB].w;
 
     // Cdot = dot(u, v + cross(w, r))
     b2Vec2 vpA = vA + b2Cross(wA, m_rA);
     b2Vec2 vpB = vB + b2Cross(wB, m_rB);
     float32 Cdot = b2Dot(m_u, vpB - vpA);
 
     float32 impulse = -m_mass * (Cdot + m_bias + m_gamma * m_impulse);
     m_impulse += impulse;
 
     b2Vec2 P = impulse * m_u;
     vA -= m_invMassA * P;
     wA -= m_invIA * b2Cross(m_rA, P);
     vB += m_invMassB * P;
     wB += m_invIB * b2Cross(m_rB, P);
 
     data.velocities[m_indexA].v = vA;
     data.velocities[m_indexA].w = wA;
     data.velocities[m_indexB].v = vB;
     data.velocities[m_indexB].w = wB;
 }
 
 bool b2DistanceJoint::SolvePositionConstraints(const b2SolverData& data)
 {
     if (m_frequencyHz > 0.0f)
     {
         // There is no position correction for soft distance constraints.
         return true;
     }
 
     b2Vec2 cA = data.positions[m_indexA].c;
     float32 aA = data.positions[m_indexA].a;
     b2Vec2 cB = data.positions[m_indexB].c;
     float32 aB = data.positions[m_indexB].a;
 
     b2Rot qA(aA), qB(aB);
 
     b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
     b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
     b2Vec2 u = cB + rB - cA - rA;
 
     float32 length = u.Normalize();
     float32 C = length - m_length;
     C = b2Clamp(C, -b2_maxLinearCorrection, b2_maxLinearCorrection);
 
     float32 impulse = -m_mass * C;
     b2Vec2 P = impulse * u;
 
     cA -= m_invMassA * P;
     aA -= m_invIA * b2Cross(rA, P);
     cB += m_invMassB * P;
     aB += m_invIB * b2Cross(rB, P);
 
     data.positions[m_indexA].c = cA;
     data.positions[m_indexA].a = aA;
     data.positions[m_indexB].c = cB;
     data.positions[m_indexB].a = aB;
 
     return b2Abs(C) < b2_linearSlop;
 }
 
 b2Vec2 b2DistanceJoint::GetAnchorA() const
 {
     return m_bodyA->GetWorldPoint(m_localAnchorA);
 }
 
 b2Vec2 b2DistanceJoint::GetAnchorB() const
 {
     return m_bodyB->GetWorldPoint(m_localAnchorB);
 }
 
 b2Vec2 b2DistanceJoint::GetReactionForce(float32 inv_dt) const
 {
     b2Vec2 F = (inv_dt * m_impulse) * m_u;
     return F;
 }
 
 float32 b2DistanceJoint::GetReactionTorque(float32 inv_dt) const
 {
     B2_NOT_USED(inv_dt);
     return 0.0f;
 }
 
 void b2DistanceJoint::Dump()
 {
     int32 indexA = m_bodyA->m_islandIndex;
     int32 indexB = m_bodyB->m_islandIndex;
 
     b2Log("  b2DistanceJointDef jd;\n");
     b2Log("  jd.bodyA = bodies[%d];\n", indexA);
     b2Log("  jd.bodyB = bodies[%d];\n", indexB);
     b2Log("  jd.collideConnected = bool(%d);\n", m_collideConnected);
     b2Log("  jd.localAnchorA.Set(%.15lef, %.15lef);\n", m_localAnchorA.x, m_localAnchorA.y);
     b2Log("  jd.localAnchorB.Set(%.15lef, %.15lef);\n", m_localAnchorB.x, m_localAnchorB.y);
     b2Log("  jd.length = %.15lef;\n", m_length);
     b2Log("  jd.frequencyHz = %.15lef;\n", m_frequencyHz);
     b2Log("  jd.dampingRatio = %.15lef;\n", m_dampingRatio);
     b2Log("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
 }