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

 /*
 * Copyright (c) 2007-2010 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/b2RopeJoint.h>
 #include <Box2D/Dynamics/b2Body.h>
 #include <Box2D/Dynamics/b2TimeStep.h>
 
 
 // Limit:
 // C = norm(pB - pA) - L
 // u = (pB - pA) / norm(pB - pA)
 // Cdot = dot(u, vB + cross(wB, rB) - vA - cross(wA, rA))
 // J = [-u -cross(rA, u) u cross(rB, u)]
 // K = J * invM * JT
 //   = invMassA + invIA * cross(rA, u)^2 + invMassB + invIB * cross(rB, u)^2
 
 b2RopeJoint::b2RopeJoint(const b2RopeJointDef* def)
 : b2Joint(def)
 {
     m_localAnchorA = def->localAnchorA;
     m_localAnchorB = def->localAnchorB;
 
     m_maxLength = def->maxLength;
 
     m_mass = 0.0f;
     m_impulse = 0.0f;
     m_state = e_inactiveLimit;
     m_length = 0.0f;
 }
 
 void b2RopeJoint::InitVelocityConstraints(const b2TimeStep& step)
 {
     b2Body* bA = m_bodyA;
     b2Body* bB = m_bodyB;
 
     m_rA = b2Mul(bA->GetTransform().R, m_localAnchorA - bA->GetLocalCenter());
     m_rB = b2Mul(bB->GetTransform().R, m_localAnchorB - bB->GetLocalCenter());
 
     // Rope axis
     m_u = bB->m_sweep.c + m_rB - bA->m_sweep.c - m_rA;
 
     m_length = m_u.Length();
 
     qreal C = m_length - m_maxLength;
     if (C > 0.0f)
     {
         m_state = e_atUpperLimit;
     }
     else
     {
         m_state = e_inactiveLimit;
     }
 
     if (m_length > b2_linearSlop)
     {
         m_u *= 1.0f / m_length;
     }
     else
     {
         m_u.SetZero();
         m_mass = 0.0f;
         m_impulse = 0.0f;
         return;
     }
 
     // Compute effective mass.
     qreal crA = b2Cross(m_rA, m_u);
     qreal crB = b2Cross(m_rB, m_u);
     qreal invMass = bA->m_invMass + bA->m_invI * crA * crA + bB->m_invMass + bB->m_invI * crB * crB;
 
     m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;
 
     if (step.warmStarting)
     {
         // Scale the impulse to support a variable time step.
         m_impulse *= step.dtRatio;
 
         b2Vec2 P = m_impulse * m_u;
         bA->m_linearVelocity -= bA->m_invMass * P;
         bA->m_angularVelocity -= bA->m_invI * b2Cross(m_rA, P);
         bB->m_linearVelocity += bB->m_invMass * P;
         bB->m_angularVelocity += bB->m_invI * b2Cross(m_rB, P);
     }
     else
     {
         m_impulse = 0.0f;
     }
 }
 
 void b2RopeJoint::SolveVelocityConstraints(const b2TimeStep& step)
 {
     B2_NOT_USED(step);
 
     b2Body* bA = m_bodyA;
     b2Body* bB = m_bodyB;
 
     // Cdot = dot(u, v + cross(w, r))
     b2Vec2 vA = bA->m_linearVelocity + b2Cross(bA->m_angularVelocity, m_rA);
     b2Vec2 vB = bB->m_linearVelocity + b2Cross(bB->m_angularVelocity, m_rB);
     qreal C = m_length - m_maxLength;
     qreal Cdot = b2Dot(m_u, vB - vA);
 
     // Predictive constraint.
     if (C < 0.0f)
     {
         Cdot += step.inv_dt * C;
     }
 
     qreal impulse = -m_mass * Cdot;
     qreal oldImpulse = m_impulse;
     m_impulse = b2Min(0.0f, m_impulse + impulse);
     impulse = m_impulse - oldImpulse;
 
     b2Vec2 P = impulse * m_u;
     bA->m_linearVelocity -= bA->m_invMass * P;
     bA->m_angularVelocity -= bA->m_invI * b2Cross(m_rA, P);
     bB->m_linearVelocity += bB->m_invMass * P;
     bB->m_angularVelocity += bB->m_invI * b2Cross(m_rB, P);
 }
 
 bool b2RopeJoint::SolvePositionConstraints(qreal baumgarte)
 {
     B2_NOT_USED(baumgarte);
 
     b2Body* bA = m_bodyA;
     b2Body* bB = m_bodyB;
 
     b2Vec2 rA = b2Mul(bA->GetTransform().R, m_localAnchorA - bA->GetLocalCenter());
     b2Vec2 rB = b2Mul(bB->GetTransform().R, m_localAnchorB - bB->GetLocalCenter());
 
     b2Vec2 u = bB->m_sweep.c + rB - bA->m_sweep.c - rA;
 
     qreal length = u.Normalize();
     qreal C = length - m_maxLength;
 
     C = b2Clamp(C, 0.0f, b2_maxLinearCorrection);
 
     qreal impulse = -m_mass * C;
     b2Vec2 P = impulse * u;
 
     bA->m_sweep.c -= bA->m_invMass * P;
     bA->m_sweep.a -= bA->m_invI * b2Cross(rA, P);
     bB->m_sweep.c += bB->m_invMass * P;
     bB->m_sweep.a += bB->m_invI * b2Cross(rB, P);
 
     bA->SynchronizeTransform();
     bB->SynchronizeTransform();
 
     return length - m_maxLength < b2_linearSlop;
 }
 
 b2Vec2 b2RopeJoint::GetAnchorA() const
 {
     return m_bodyA->GetWorldPoint(m_localAnchorA);
 }
 
 b2Vec2 b2RopeJoint::GetAnchorB() const
 {
     return m_bodyB->GetWorldPoint(m_localAnchorB);
 }
 
 b2Vec2 b2RopeJoint::GetReactionForce(qreal inv_dt) const
 {
     b2Vec2 F = (inv_dt * m_impulse) * m_u;
     return F;
 }
 
 qreal b2RopeJoint::GetReactionTorque(qreal inv_dt) const
 {
     B2_NOT_USED(inv_dt);
     return 0.0f;
 }
 
 qreal b2RopeJoint::GetMaxLength() const
 {
     return m_maxLength;
 }
 
 b2LimitState b2RopeJoint::GetLimitState() const
 {
     return m_state;
 }