File indexing completed on 2025-08-03 03:50:00

 /*
 * Copyright (c) 2006-2007 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/b2RevoluteJoint.h>
 #include <Box2D/Dynamics/b2Body.h>
 #include <Box2D/Dynamics/b2TimeStep.h>
 
 // Point-to-point constraint
 // C = p2 - p1
 // 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)
 
 // Motor constraint
 // Cdot = w2 - w1
 // J = [0 0 -1 0 0 1]
 // K = invI1 + invI2
 
 void b2RevoluteJointDef::Initialize(b2Body* b1, b2Body* b2, const b2Vec2& anchor)
 {
     bodyA = b1;
     bodyB = b2;
     localAnchorA = bodyA->GetLocalPoint(anchor);
     localAnchorB = bodyB->GetLocalPoint(anchor);
     referenceAngle = bodyB->GetAngle() - bodyA->GetAngle();
 }
 
 b2RevoluteJoint::b2RevoluteJoint(const b2RevoluteJointDef* def)
 : b2Joint(def)
 {
     m_localAnchor1 = def->localAnchorA;
     m_localAnchor2 = def->localAnchorB;
     m_referenceAngle = def->referenceAngle;
 
     m_impulse.SetZero();
     m_motorImpulse = 0.0f;
 
     m_lowerAngle = def->lowerAngle;
     m_upperAngle = def->upperAngle;
     m_maxMotorTorque = def->maxMotorTorque;
     m_motorSpeed = def->motorSpeed;
     m_enableLimit = def->enableLimit;
     m_enableMotor = def->enableMotor;
     m_limitState = e_inactiveLimit;
 }
 
 void b2RevoluteJoint::InitVelocityConstraints(const b2TimeStep& step)
 {
     b2Body* b1 = m_bodyA;
     b2Body* b2 = m_bodyB;
 
     if (m_enableMotor || m_enableLimit)
     {
         // You cannot create a rotation limit between bodies that
         // both have fixed rotation.
         b2Assert(b1->m_invI > 0.0f || b2->m_invI > 0.0f);
     }
 
     // Compute the effective mass matrix.
     b2Vec2 r1 = b2Mul(b1->GetTransform().R, m_localAnchor1 - b1->GetLocalCenter());
     b2Vec2 r2 = b2Mul(b2->GetTransform().R, m_localAnchor2 - b2->GetLocalCenter());
 
     // J = [-I -r1_skew I r2_skew]
     //     [ 0       -1 0       1]
     // r_skew = [-ry; rx]
 
     // Matlab
     // K = [ m1+r1y^2*i1+m2+r2y^2*i2,  -r1y*i1*r1x-r2y*i2*r2x,          -r1y*i1-r2y*i2]
     //     [  -r1y*i1*r1x-r2y*i2*r2x, m1+r1x^2*i1+m2+r2x^2*i2,           r1x*i1+r2x*i2]
     //     [          -r1y*i1-r2y*i2,           r1x*i1+r2x*i2,                   i1+i2]
 
     qreal m1 = b1->m_invMass, m2 = b2->m_invMass;
     qreal i1 = b1->m_invI, i2 = b2->m_invI;
 
     m_mass.col1.x = m1 + m2 + r1.y * r1.y * i1 + r2.y * r2.y * i2;
     m_mass.col2.x = -r1.y * r1.x * i1 - r2.y * r2.x * i2;
     m_mass.col3.x = -r1.y * i1 - r2.y * i2;
     m_mass.col1.y = m_mass.col2.x;
     m_mass.col2.y = m1 + m2 + r1.x * r1.x * i1 + r2.x * r2.x * i2;
     m_mass.col3.y = r1.x * i1 + r2.x * i2;
     m_mass.col1.z = m_mass.col3.x;
     m_mass.col2.z = m_mass.col3.y;
     m_mass.col3.z = i1 + i2;
 
     m_motorMass = i1 + i2;
     if (m_motorMass > 0.0f)
     {
         m_motorMass = 1.0f / m_motorMass;
     }
 
     if (m_enableMotor == false)
     {
         m_motorImpulse = 0.0f;
     }
 
     if (m_enableLimit)
     {
         qreal jointAngle = b2->m_sweep.a - b1->m_sweep.a - m_referenceAngle;
         if (b2Abs(m_upperAngle - m_lowerAngle) < 2.0f * b2_angularSlop)
         {
             m_limitState = e_equalLimits;
         }
         else if (jointAngle <= m_lowerAngle)
         {
             if (m_limitState != e_atLowerLimit)
             {
                 m_impulse.z = 0.0f;
             }
             m_limitState = e_atLowerLimit;
         }
         else if (jointAngle >= m_upperAngle)
         {
             if (m_limitState != e_atUpperLimit)
             {
                 m_impulse.z = 0.0f;
             }
             m_limitState = e_atUpperLimit;
         }
         else
         {
             m_limitState = e_inactiveLimit;
             m_impulse.z = 0.0f;
         }
     }
     else
     {
         m_limitState = e_inactiveLimit;
     }
 
     if (step.warmStarting)
     {
         // Scale impulses to support a variable time step.
         m_impulse *= step.dtRatio;
         m_motorImpulse *= step.dtRatio;
 
         b2Vec2 P(m_impulse.x, m_impulse.y);
 
         b1->m_linearVelocity -= m1 * P;
         b1->m_angularVelocity -= i1 * (b2Cross(r1, P) + m_motorImpulse + m_impulse.z);
 
         b2->m_linearVelocity += m2 * P;
         b2->m_angularVelocity += i2 * (b2Cross(r2, P) + m_motorImpulse + m_impulse.z);
     }
     else
     {
         m_impulse.SetZero();
         m_motorImpulse = 0.0f;
     }
 }
 
 void b2RevoluteJoint::SolveVelocityConstraints(const b2TimeStep& step)
 {
     b2Body* b1 = m_bodyA;
     b2Body* b2 = m_bodyB;
 
     b2Vec2 v1 = b1->m_linearVelocity;
     qreal w1 = b1->m_angularVelocity;
     b2Vec2 v2 = b2->m_linearVelocity;
     qreal w2 = b2->m_angularVelocity;
 
     qreal m1 = b1->m_invMass, m2 = b2->m_invMass;
     qreal i1 = b1->m_invI, i2 = b2->m_invI;
 
     // Solve motor constraint.
     if (m_enableMotor && m_limitState != e_equalLimits)
     {
         qreal Cdot = w2 - w1 - m_motorSpeed;
         qreal impulse = m_motorMass * (-Cdot);
         qreal oldImpulse = m_motorImpulse;
         qreal maxImpulse = step.dt * m_maxMotorTorque;
         m_motorImpulse = b2Clamp(m_motorImpulse + impulse, -maxImpulse, maxImpulse);
         impulse = m_motorImpulse - oldImpulse;
 
         w1 -= i1 * impulse;
         w2 += i2 * impulse;
     }
 
     // Solve limit constraint.
     if (m_enableLimit && m_limitState != e_inactiveLimit)
     {
         b2Vec2 r1 = b2Mul(b1->GetTransform().R, m_localAnchor1 - b1->GetLocalCenter());
         b2Vec2 r2 = b2Mul(b2->GetTransform().R, m_localAnchor2 - b2->GetLocalCenter());
 
         // Solve point-to-point constraint
         b2Vec2 Cdot1 = v2 + b2Cross(w2, r2) - v1 - b2Cross(w1, r1);
         qreal Cdot2 = w2 - w1;
         b2Vec3 Cdot(Cdot1.x, Cdot1.y, Cdot2);
 
         b2Vec3 impulse = m_mass.Solve33(-Cdot);
 
         if (m_limitState == e_equalLimits)
         {
             m_impulse += impulse;
         }
         else if (m_limitState == e_atLowerLimit)
         {
             qreal newImpulse = m_impulse.z + impulse.z;
             if (newImpulse < 0.0f)
             {
                 b2Vec2 reduced = m_mass.Solve22(-Cdot1);
                 impulse.x = reduced.x;
                 impulse.y = reduced.y;
                 impulse.z = -m_impulse.z;
                 m_impulse.x += reduced.x;
                 m_impulse.y += reduced.y;
                 m_impulse.z = 0.0f;
             }
         }
         else if (m_limitState == e_atUpperLimit)
         {
             qreal newImpulse = m_impulse.z + impulse.z;
             if (newImpulse > 0.0f)
             {
                 b2Vec2 reduced = m_mass.Solve22(-Cdot1);
                 impulse.x = reduced.x;
                 impulse.y = reduced.y;
                 impulse.z = -m_impulse.z;
                 m_impulse.x += reduced.x;
                 m_impulse.y += reduced.y;
                 m_impulse.z = 0.0f;
             }
         }
 
         b2Vec2 P(impulse.x, impulse.y);
 
         v1 -= m1 * P;
         w1 -= i1 * (b2Cross(r1, P) + impulse.z);
 
         v2 += m2 * P;
         w2 += i2 * (b2Cross(r2, P) + impulse.z);
     }
     else
     {
         b2Vec2 r1 = b2Mul(b1->GetTransform().R, m_localAnchor1 - b1->GetLocalCenter());
         b2Vec2 r2 = b2Mul(b2->GetTransform().R, m_localAnchor2 - b2->GetLocalCenter());
 
         // Solve point-to-point constraint
         b2Vec2 Cdot = v2 + b2Cross(w2, r2) - v1 - b2Cross(w1, r1);
         b2Vec2 impulse = m_mass.Solve22(-Cdot);
 
         m_impulse.x += impulse.x;
         m_impulse.y += impulse.y;
 
         v1 -= m1 * impulse;
         w1 -= i1 * b2Cross(r1, impulse);
 
         v2 += m2 * impulse;
         w2 += i2 * b2Cross(r2, impulse);
     }
 
     b1->m_linearVelocity = v1;
     b1->m_angularVelocity = w1;
     b2->m_linearVelocity = v2;
     b2->m_angularVelocity = w2;
 }
 
 bool b2RevoluteJoint::SolvePositionConstraints(qreal baumgarte)
 {
     // TODO_ERIN block solve with limit.
 
     B2_NOT_USED(baumgarte);
 
     b2Body* b1 = m_bodyA;
     b2Body* b2 = m_bodyB;
 
     qreal angularError = 0.0f;
     qreal positionError = 0.0f;
 
     // Solve angular limit constraint.
     if (m_enableLimit && m_limitState != e_inactiveLimit)
     {
         qreal angle = b2->m_sweep.a - b1->m_sweep.a - m_referenceAngle;
         qreal limitImpulse = 0.0f;
 
         if (m_limitState == e_equalLimits)
         {
             // Prevent large angular corrections
             qreal C = b2Clamp(angle - m_lowerAngle, -b2_maxAngularCorrection, b2_maxAngularCorrection);
             limitImpulse = -m_motorMass * C;
             angularError = b2Abs(C);
         }
         else if (m_limitState == e_atLowerLimit)
         {
             qreal C = angle - m_lowerAngle;
             angularError = -C;
 
             // Prevent large angular corrections and allow some slop.
             C = b2Clamp(C + b2_angularSlop, -b2_maxAngularCorrection, 0.0f);
             limitImpulse = -m_motorMass * C;
         }
         else if (m_limitState == e_atUpperLimit)
         {
             qreal C = angle - m_upperAngle;
             angularError = C;
 
             // Prevent large angular corrections and allow some slop.
             C = b2Clamp(C - b2_angularSlop, 0.0f, b2_maxAngularCorrection);
             limitImpulse = -m_motorMass * C;
         }
 
         b1->m_sweep.a -= b1->m_invI * limitImpulse;
         b2->m_sweep.a += b2->m_invI * limitImpulse;
 
         b1->SynchronizeTransform();
         b2->SynchronizeTransform();
     }
 
     // Solve point-to-point constraint.
     {
         b2Vec2 r1 = b2Mul(b1->GetTransform().R, m_localAnchor1 - b1->GetLocalCenter());
         b2Vec2 r2 = b2Mul(b2->GetTransform().R, m_localAnchor2 - b2->GetLocalCenter());
 
         b2Vec2 C = b2->m_sweep.c + r2 - b1->m_sweep.c - r1;
         positionError = C.Length();
 
         qreal invMass1 = b1->m_invMass, invMass2 = b2->m_invMass;
         qreal invI1 = b1->m_invI, invI2 = b2->m_invI;
 
         // Handle large detachment.
         const qreal k_allowedStretch = 10.0f * b2_linearSlop;
         if (C.LengthSquared() > k_allowedStretch * k_allowedStretch)
         {
             // Use a particle solution (no rotation).
             b2Vec2 u = C; u.Normalize();
             qreal m = invMass1 + invMass2;
             if (m > 0.0f)
             {
                 m = 1.0f / m;
             }
             b2Vec2 impulse = m * (-C);
             const qreal k_beta = 0.5f;
             b1->m_sweep.c -= k_beta * invMass1 * impulse;
             b2->m_sweep.c += k_beta * invMass2 * impulse;
 
             C = b2->m_sweep.c + r2 - b1->m_sweep.c - r1;
         }
 
         b2Mat22 K1;
         K1.col1.x = invMass1 + invMass2;    K1.col2.x = 0.0f;
         K1.col1.y = 0.0f;                   K1.col2.y = invMass1 + invMass2;
 
         b2Mat22 K2;
         K2.col1.x =  invI1 * r1.y * r1.y;   K2.col2.x = -invI1 * r1.x * r1.y;
         K2.col1.y = -invI1 * r1.x * r1.y;   K2.col2.y =  invI1 * r1.x * r1.x;
 
         b2Mat22 K3;
         K3.col1.x =  invI2 * r2.y * r2.y;   K3.col2.x = -invI2 * r2.x * r2.y;
         K3.col1.y = -invI2 * r2.x * r2.y;   K3.col2.y =  invI2 * r2.x * r2.x;
 
         b2Mat22 K = K1 + K2 + K3;
         b2Vec2 impulse = K.Solve(-C);
 
         b1->m_sweep.c -= b1->m_invMass * impulse;
         b1->m_sweep.a -= b1->m_invI * b2Cross(r1, impulse);
 
         b2->m_sweep.c += b2->m_invMass * impulse;
         b2->m_sweep.a += b2->m_invI * b2Cross(r2, impulse);
 
         b1->SynchronizeTransform();
         b2->SynchronizeTransform();
     }
     
     return positionError <= b2_linearSlop && angularError <= b2_angularSlop;
 }
 
 b2Vec2 b2RevoluteJoint::GetAnchorA() const
 {
     return m_bodyA->GetWorldPoint(m_localAnchor1);
 }
 
 b2Vec2 b2RevoluteJoint::GetAnchorB() const
 {
     return m_bodyB->GetWorldPoint(m_localAnchor2);
 }
 
 b2Vec2 b2RevoluteJoint::GetReactionForce(qreal inv_dt) const
 {
     b2Vec2 P(m_impulse.x, m_impulse.y);
     return inv_dt * P;
 }
 
 qreal b2RevoluteJoint::GetReactionTorque(qreal inv_dt) const
 {
     return inv_dt * m_impulse.z;
 }
 
 qreal b2RevoluteJoint::GetJointAngle() const
 {
     b2Body* b1 = m_bodyA;
     b2Body* b2 = m_bodyB;
     return b2->m_sweep.a - b1->m_sweep.a - m_referenceAngle;
 }
 
 qreal b2RevoluteJoint::GetJointSpeed() const
 {
     b2Body* b1 = m_bodyA;
     b2Body* b2 = m_bodyB;
     return b2->m_angularVelocity - b1->m_angularVelocity;
 }
 
 bool b2RevoluteJoint::IsMotorEnabled() const
 {
     return m_enableMotor;
 }
 
 void b2RevoluteJoint::EnableMotor(bool flag)
 {
     m_bodyA->SetAwake(true);
     m_bodyB->SetAwake(true);
     m_enableMotor = flag;
 }
 
 qreal b2RevoluteJoint::GetMotorTorque(qreal inv_dt) const
 {
     return inv_dt * m_motorImpulse;
 }
 
 void b2RevoluteJoint::SetMotorSpeed(qreal speed)
 {
     m_bodyA->SetAwake(true);
     m_bodyB->SetAwake(true);
     m_motorSpeed = speed;
 }
 
 void b2RevoluteJoint::SetMaxMotorTorque(qreal torque)
 {
     m_bodyA->SetAwake(true);
     m_bodyB->SetAwake(true);
     m_maxMotorTorque = torque;
 }
 
 bool b2RevoluteJoint::IsLimitEnabled() const
 {
     return m_enableLimit;
 }
 
 void b2RevoluteJoint::EnableLimit(bool flag)
 {
     m_bodyA->SetAwake(true);
     m_bodyB->SetAwake(true);
     m_enableLimit = flag;
 }
 
 qreal b2RevoluteJoint::GetLowerLimit() const
 {
     return m_lowerAngle;
 }
 
 qreal b2RevoluteJoint::GetUpperLimit() const
 {
     return m_upperAngle;
 }
 
 void b2RevoluteJoint::SetLimits(qreal lower, qreal upper)
 {
     b2Assert(lower <= upper);
     m_bodyA->SetAwake(true);
     m_bodyB->SetAwake(true);
     m_lowerAngle = lower;
     m_upperAngle = upper;
 }