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

 /*
 * 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/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_localAnchor1 = def->localAnchorA;
     m_localAnchor2 = 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 b2TimeStep& step)
 {
     b2Body* b1 = m_bodyA;
     b2Body* b2 = m_bodyB;
 
     // 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());
     m_u = b2->m_sweep.c + r2 - b1->m_sweep.c - r1;
 
     // Handle singularity.
     qreal length = m_u.Length();
     if (length > b2_linearSlop)
     {
         m_u *= 1.0f / length;
     }
     else
     {
         m_u.Set(0.0f, 0.0f);
     }
 
     qreal cr1u = b2Cross(r1, m_u);
     qreal cr2u = b2Cross(r2, m_u);
     qreal invMass = b1->m_invMass + b1->m_invI * cr1u * cr1u + b2->m_invMass + b2->m_invI * cr2u * cr2u;
 
     m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;
 
     if (m_frequencyHz > 0.0f)
     {
         qreal C = length - m_length;
 
         // Frequency
         qreal omega = 2.0f * b2_pi * m_frequencyHz;
 
         // Damping coefficient
         qreal d = 2.0f * m_mass * m_dampingRatio * omega;
 
         // Spring stiffness
         qreal k = m_mass * omega * omega;
 
         // magic formulas
         m_gamma = step.dt * (d + step.dt * k);
         m_gamma = m_gamma != 0.0f ? 1.0f / m_gamma : 0.0f;
         m_bias = C * step.dt * k * m_gamma;
 
         m_mass = invMass + m_gamma;
         m_mass = m_mass != 0.0f ? 1.0f / m_mass : 0.0f;
     }
 
     if (step.warmStarting)
     {
         // Scale the impulse to support a variable time step.
         m_impulse *= step.dtRatio;
 
         b2Vec2 P = m_impulse * m_u;
         b1->m_linearVelocity -= b1->m_invMass * P;
         b1->m_angularVelocity -= b1->m_invI * b2Cross(r1, P);
         b2->m_linearVelocity += b2->m_invMass * P;
         b2->m_angularVelocity += b2->m_invI * b2Cross(r2, P);
     }
     else
     {
         m_impulse = 0.0f;
     }
 }
 
 void b2DistanceJoint::SolveVelocityConstraints(const b2TimeStep& step)
 {
     B2_NOT_USED(step);
 
     b2Body* b1 = m_bodyA;
     b2Body* b2 = m_bodyB;
 
     b2Vec2 r1 = b2Mul(b1->GetTransform().R, m_localAnchor1 - b1->GetLocalCenter());
     b2Vec2 r2 = b2Mul(b2->GetTransform().R, m_localAnchor2 - b2->GetLocalCenter());
 
     // Cdot = dot(u, v + cross(w, r))
     b2Vec2 v1 = b1->m_linearVelocity + b2Cross(b1->m_angularVelocity, r1);
     b2Vec2 v2 = b2->m_linearVelocity + b2Cross(b2->m_angularVelocity, r2);
     qreal Cdot = b2Dot(m_u, v2 - v1);
 
     qreal impulse = -m_mass * (Cdot + m_bias + m_gamma * m_impulse);
     m_impulse += impulse;
 
     b2Vec2 P = impulse * m_u;
     b1->m_linearVelocity -= b1->m_invMass * P;
     b1->m_angularVelocity -= b1->m_invI * b2Cross(r1, P);
     b2->m_linearVelocity += b2->m_invMass * P;
     b2->m_angularVelocity += b2->m_invI * b2Cross(r2, P);
 }
 
 bool b2DistanceJoint::SolvePositionConstraints(qreal baumgarte)
 {
     B2_NOT_USED(baumgarte);
 
     if (m_frequencyHz > 0.0f)
     {
         // There is no position correction for soft distance constraints.
         return true;
     }
 
     b2Body* b1 = m_bodyA;
     b2Body* b2 = m_bodyB;
 
     b2Vec2 r1 = b2Mul(b1->GetTransform().R, m_localAnchor1 - b1->GetLocalCenter());
     b2Vec2 r2 = b2Mul(b2->GetTransform().R, m_localAnchor2 - b2->GetLocalCenter());
 
     b2Vec2 d = b2->m_sweep.c + r2 - b1->m_sweep.c - r1;
 
     qreal length = d.Normalize();
     qreal C = length - m_length;
     C = b2Clamp(C, -b2_maxLinearCorrection, b2_maxLinearCorrection);
 
     qreal impulse = -m_mass * C;
     m_u = d;
     b2Vec2 P = impulse * m_u;
 
     b1->m_sweep.c -= b1->m_invMass * P;
     b1->m_sweep.a -= b1->m_invI * b2Cross(r1, P);
     b2->m_sweep.c += b2->m_invMass * P;
     b2->m_sweep.a += b2->m_invI * b2Cross(r2, P);
 
     b1->SynchronizeTransform();
     b2->SynchronizeTransform();
 
     return b2Abs(C) < b2_linearSlop;
 }
 
 b2Vec2 b2DistanceJoint::GetAnchorA() const
 {
     return m_bodyA->GetWorldPoint(m_localAnchor1);
 }
 
 b2Vec2 b2DistanceJoint::GetAnchorB() const
 {
     return m_bodyB->GetWorldPoint(m_localAnchor2);
 }
 
 b2Vec2 b2DistanceJoint::GetReactionForce(qreal inv_dt) const
 {
     b2Vec2 F = (inv_dt * m_impulse) * m_u;
     return F;
 }
 
 qreal b2DistanceJoint::GetReactionTorque(qreal inv_dt) const
 {
     B2_NOT_USED(inv_dt);
     return 0.0f;
 }