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/b2FrictionJoint.h>
 #include <Box2D/Dynamics/b2Body.h>
 #include <Box2D/Dynamics/b2TimeStep.h>
 
 // Point-to-point constraint
 // Cdot = v2 - v1
 //      = v2 + cross(w2, r2) - v1 - cross(w1, r1)
 // J = [-I -r1_skew I r2_skew ]
 // Identity used:
 // w k % (rx i + ry j) = w * (-ry i + rx j)
 
 // Angle constraint
 // Cdot = w2 - w1
 // J = [0 0 -1 0 0 1]
 // K = invI1 + invI2
 
 void b2FrictionJointDef::Initialize(b2Body* bA, b2Body* bB, const b2Vec2& anchor)
 {
     bodyA = bA;
     bodyB = bB;
     localAnchorA = bodyA->GetLocalPoint(anchor);
     localAnchorB = bodyB->GetLocalPoint(anchor);
 }
 
 b2FrictionJoint::b2FrictionJoint(const b2FrictionJointDef* def)
 : b2Joint(def)
 {
     m_localAnchorA = def->localAnchorA;
     m_localAnchorB = def->localAnchorB;
 
     m_linearImpulse.SetZero();
     m_angularImpulse = 0.0f;
 
     m_maxForce = def->maxForce;
     m_maxTorque = def->maxTorque;
 }
 
 void b2FrictionJoint::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;
 
     float32 aA = data.positions[m_indexA].a;
     b2Vec2 vA = data.velocities[m_indexA].v;
     float32 wA = data.velocities[m_indexA].w;
 
     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);
 
     // Compute the effective mass matrix.
     m_rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
     m_rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
 
     // J = [-I -r1_skew I r2_skew]
     //     [ 0       -1 0       1]
     // r_skew = [-ry; rx]
 
     // Matlab
     // K = [ mA+r1y^2*iA+mB+r2y^2*iB,  -r1y*iA*r1x-r2y*iB*r2x,          -r1y*iA-r2y*iB]
     //     [  -r1y*iA*r1x-r2y*iB*r2x, mA+r1x^2*iA+mB+r2x^2*iB,           r1x*iA+r2x*iB]
     //     [          -r1y*iA-r2y*iB,           r1x*iA+r2x*iB,                   iA+iB]
 
     float32 mA = m_invMassA, mB = m_invMassB;
     float32 iA = m_invIA, iB = m_invIB;
 
     b2Mat22 K;
     K.ex.x = mA + mB + iA * m_rA.y * m_rA.y + iB * m_rB.y * m_rB.y;
     K.ex.y = -iA * m_rA.x * m_rA.y - iB * m_rB.x * m_rB.y;
     K.ey.x = K.ex.y;
     K.ey.y = mA + mB + iA * m_rA.x * m_rA.x + iB * m_rB.x * m_rB.x;
 
     m_linearMass = K.GetInverse();
 
     m_angularMass = iA + iB;
     if (m_angularMass > 0.0f)
     {
         m_angularMass = 1.0f / m_angularMass;
     }
 
     if (data.step.warmStarting)
     {
         // Scale impulses to support a variable time step.
         m_linearImpulse *= data.step.dtRatio;
         m_angularImpulse *= data.step.dtRatio;
 
         b2Vec2 P(m_linearImpulse.x, m_linearImpulse.y);
         vA -= mA * P;
         wA -= iA * (b2Cross(m_rA, P) + m_angularImpulse);
         vB += mB * P;
         wB += iB * (b2Cross(m_rB, P) + m_angularImpulse);
     }
     else
     {
         m_linearImpulse.SetZero();
         m_angularImpulse = 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 b2FrictionJoint::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;
 
     float32 mA = m_invMassA, mB = m_invMassB;
     float32 iA = m_invIA, iB = m_invIB;
 
     float32 h = data.step.dt;
 
     // Solve angular friction
     {
         float32 Cdot = wB - wA;
         float32 impulse = -m_angularMass * Cdot;
 
         float32 oldImpulse = m_angularImpulse;
         float32 maxImpulse = h * m_maxTorque;
         m_angularImpulse = b2Clamp(m_angularImpulse + impulse, -maxImpulse, maxImpulse);
         impulse = m_angularImpulse - oldImpulse;
 
         wA -= iA * impulse;
         wB += iB * impulse;
     }
 
     // Solve linear friction
     {
         b2Vec2 Cdot = vB + b2Cross(wB, m_rB) - vA - b2Cross(wA, m_rA);
 
         b2Vec2 impulse = -b2Mul(m_linearMass, Cdot);
         b2Vec2 oldImpulse = m_linearImpulse;
         m_linearImpulse += impulse;
 
         float32 maxImpulse = h * m_maxForce;
 
         if (m_linearImpulse.LengthSquared() > maxImpulse * maxImpulse)
         {
             m_linearImpulse.Normalize();
             m_linearImpulse *= maxImpulse;
         }
 
         impulse = m_linearImpulse - oldImpulse;
 
         vA -= mA * impulse;
         wA -= iA * b2Cross(m_rA, impulse);
 
         vB += mB * impulse;
         wB += iB * b2Cross(m_rB, impulse);
     }
 
     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 b2FrictionJoint::SolvePositionConstraints(const b2SolverData& data)
 {
     B2_NOT_USED(data);
 
     return true;
 }
 
 b2Vec2 b2FrictionJoint::GetAnchorA() const
 {
     return m_bodyA->GetWorldPoint(m_localAnchorA);
 }
 
 b2Vec2 b2FrictionJoint::GetAnchorB() const
 {
     return m_bodyB->GetWorldPoint(m_localAnchorB);
 }
 
 b2Vec2 b2FrictionJoint::GetReactionForce(float32 inv_dt) const
 {
     return inv_dt * m_linearImpulse;
 }
 
 float32 b2FrictionJoint::GetReactionTorque(float32 inv_dt) const
 {
     return inv_dt * m_angularImpulse;
 }
 
 void b2FrictionJoint::SetMaxForce(float32 force)
 {
     b2Assert(b2IsValid(force) && force >= 0.0f);
     m_maxForce = force;
 }
 
 float32 b2FrictionJoint::GetMaxForce() const
 {
     return m_maxForce;
 }
 
 void b2FrictionJoint::SetMaxTorque(float32 torque)
 {
     b2Assert(b2IsValid(torque) && torque >= 0.0f);
     m_maxTorque = torque;
 }
 
 float32 b2FrictionJoint::GetMaxTorque() const
 {
     return m_maxTorque;
 }
 
 void b2FrictionJoint::Dump()
 {
     int32 indexA = m_bodyA->m_islandIndex;
     int32 indexB = m_bodyB->m_islandIndex;
 
     b2Log("  b2FrictionJointDef 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.maxForce = %.15lef;\n", m_maxForce);
     b2Log("  jd.maxTorque = %.15lef;\n", m_maxTorque);
     b2Log("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
 }