File indexing completed on 2025-08-03 03:49:58

 /*
 * Copyright (c) 2006-2009 Erin Catto http://www.gphysics.com
 *
 * 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 b2TimeStep& step)
 {
     b2Body* bA = m_bodyA;
     b2Body* bB = m_bodyB;
 
     // Compute the effective mass matrix.
     b2Vec2 rA = b2Mul(bA->GetTransform().R, m_localAnchorA - bA->GetLocalCenter());
     b2Vec2 rB = b2Mul(bB->GetTransform().R, m_localAnchorB - bB->GetLocalCenter());
 
     // 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]
 
     qreal mA = bA->m_invMass, mB = bB->m_invMass;
     qreal iA = bA->m_invI, iB = bB->m_invI;
 
     b2Mat22 K1;
     K1.col1.x = mA + mB;    K1.col2.x = 0.0f;
     K1.col1.y = 0.0f;       K1.col2.y = mA + mB;
 
     b2Mat22 K2;
     K2.col1.x =  iA * rA.y * rA.y;  K2.col2.x = -iA * rA.x * rA.y;
     K2.col1.y = -iA * rA.x * rA.y;  K2.col2.y =  iA * rA.x * rA.x;
 
     b2Mat22 K3;
     K3.col1.x =  iB * rB.y * rB.y;  K3.col2.x = -iB * rB.x * rB.y;
     K3.col1.y = -iB * rB.x * rB.y;  K3.col2.y =  iB * rB.x * rB.x;
 
     b2Mat22 K = K1 + K2 + K3;
     m_linearMass = K.GetInverse();
 
     m_angularMass = iA + iB;
     if (m_angularMass > 0.0f)
     {
         m_angularMass = 1.0f / m_angularMass;
     }
 
     if (step.warmStarting)
     {
         // Scale impulses to support a variable time step.
         m_linearImpulse *= step.dtRatio;
         m_angularImpulse *= step.dtRatio;
 
         b2Vec2 P(m_linearImpulse.x, m_linearImpulse.y);
 
         bA->m_linearVelocity -= mA * P;
         bA->m_angularVelocity -= iA * (b2Cross(rA, P) + m_angularImpulse);
 
         bB->m_linearVelocity += mB * P;
         bB->m_angularVelocity += iB * (b2Cross(rB, P) + m_angularImpulse);
     }
     else
     {
         m_linearImpulse.SetZero();
         m_angularImpulse = 0.0f;
     }
 }
 
 void b2FrictionJoint::SolveVelocityConstraints(const b2TimeStep& step)
 {
     B2_NOT_USED(step);
 
     b2Body* bA = m_bodyA;
     b2Body* bB = m_bodyB;
 
     b2Vec2 vA = bA->m_linearVelocity;
     qreal wA = bA->m_angularVelocity;
     b2Vec2 vB = bB->m_linearVelocity;
     qreal wB = bB->m_angularVelocity;
 
     qreal mA = bA->m_invMass, mB = bB->m_invMass;
     qreal iA = bA->m_invI, iB = bB->m_invI;
 
     b2Vec2 rA = b2Mul(bA->GetTransform().R, m_localAnchorA - bA->GetLocalCenter());
     b2Vec2 rB = b2Mul(bB->GetTransform().R, m_localAnchorB - bB->GetLocalCenter());
 
     // Solve angular friction
     {
         qreal Cdot = wB - wA;
         qreal impulse = -m_angularMass * Cdot;
 
         qreal oldImpulse = m_angularImpulse;
         qreal maxImpulse = step.dt * 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, rB) - vA - b2Cross(wA, rA);
 
         b2Vec2 impulse = -b2Mul(m_linearMass, Cdot);
         b2Vec2 oldImpulse = m_linearImpulse;
         m_linearImpulse += impulse;
 
         qreal maxImpulse = step.dt * m_maxForce;
 
         if (m_linearImpulse.LengthSquared() > maxImpulse * maxImpulse)
         {
             m_linearImpulse.Normalize();
             m_linearImpulse *= maxImpulse;
         }
 
         impulse = m_linearImpulse - oldImpulse;
 
         vA -= mA * impulse;
         wA -= iA * b2Cross(rA, impulse);
 
         vB += mB * impulse;
         wB += iB * b2Cross(rB, impulse);
     }
 
     bA->m_linearVelocity = vA;
     bA->m_angularVelocity = wA;
     bB->m_linearVelocity = vB;
     bB->m_angularVelocity = wB;
 }
 
 bool b2FrictionJoint::SolvePositionConstraints(qreal baumgarte)
 {
     B2_NOT_USED(baumgarte);
 
     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(qreal inv_dt) const
 {
     return inv_dt * m_linearImpulse;
 }
 
 qreal b2FrictionJoint::GetReactionTorque(qreal inv_dt) const
 {
     return inv_dt * m_angularImpulse;
 }
 
 void b2FrictionJoint::SetMaxForce(qreal force)
 {
     b2Assert(b2IsValid(force) && force >= 0.0f);
     m_maxForce = force;
 }
 
 qreal b2FrictionJoint::GetMaxForce() const
 {
     return m_maxForce;
 }
 
 void b2FrictionJoint::SetMaxTorque(qreal torque)
 {
     b2Assert(b2IsValid(torque) && torque >= 0.0f);
     m_maxTorque = torque;
 }
 
 qreal b2FrictionJoint::GetMaxTorque() const
 {
     return m_maxTorque;
 }