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

 /*
 * 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/b2WeldJoint.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)
 
 // Angle constraint
 // C = angle2 - angle1 - referenceAngle
 // Cdot = w2 - w1
 // J = [0 0 -1 0 0 1]
 // K = invI1 + invI2
 
 void b2WeldJointDef::Initialize(b2Body* bA, b2Body* bB, const b2Vec2& anchor)
 {
     bodyA = bA;
     bodyB = bB;
     localAnchorA = bodyA->GetLocalPoint(anchor);
     localAnchorB = bodyB->GetLocalPoint(anchor);
     referenceAngle = bodyB->GetAngle() - bodyA->GetAngle();
 }
 
 b2WeldJoint::b2WeldJoint(const b2WeldJointDef* def)
 : b2Joint(def)
 {
     m_localAnchorA = def->localAnchorA;
     m_localAnchorB = def->localAnchorB;
     m_referenceAngle = def->referenceAngle;
 
     m_impulse.SetZero();
 }
 
 void b2WeldJoint::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;
 
     m_mass.col1.x = mA + mB + rA.y * rA.y * iA + rB.y * rB.y * iB;
     m_mass.col2.x = -rA.y * rA.x * iA - rB.y * rB.x * iB;
     m_mass.col3.x = -rA.y * iA - rB.y * iB;
     m_mass.col1.y = m_mass.col2.x;
     m_mass.col2.y = mA + mB + rA.x * rA.x * iA + rB.x * rB.x * iB;
     m_mass.col3.y = rA.x * iA + rB.x * iB;
     m_mass.col1.z = m_mass.col3.x;
     m_mass.col2.z = m_mass.col3.y;
     m_mass.col3.z = iA + iB;
 
     if (step.warmStarting)
     {
         // Scale impulses to support a variable time step.
         m_impulse *= step.dtRatio;
 
         b2Vec2 P(m_impulse.x, m_impulse.y);
 
         bA->m_linearVelocity -= mA * P;
         bA->m_angularVelocity -= iA * (b2Cross(rA, P) + m_impulse.z);
 
         bB->m_linearVelocity += mB * P;
         bB->m_angularVelocity += iB * (b2Cross(rB, P) + m_impulse.z);
     }
     else
     {
         m_impulse.SetZero();
     }
 }
 
 void b2WeldJoint::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 point-to-point constraint
     b2Vec2 Cdot1 = vB + b2Cross(wB, rB) - vA - b2Cross(wA, rA);
     qreal Cdot2 = wB - wA;
     b2Vec3 Cdot(Cdot1.x, Cdot1.y, Cdot2);
 
     b2Vec3 impulse = m_mass.Solve33(-Cdot);
     m_impulse += impulse;
 
     b2Vec2 P(impulse.x, impulse.y);
 
     vA -= mA * P;
     wA -= iA * (b2Cross(rA, P) + impulse.z);
 
     vB += mB * P;
     wB += iB * (b2Cross(rB, P) + impulse.z);
 
     bA->m_linearVelocity = vA;
     bA->m_angularVelocity = wA;
     bB->m_linearVelocity = vB;
     bB->m_angularVelocity = wB;
 }
 
 bool b2WeldJoint::SolvePositionConstraints(qreal baumgarte)
 {
     B2_NOT_USED(baumgarte);
 
     b2Body* bA = m_bodyA;
     b2Body* bB = m_bodyB;
 
     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());
 
     b2Vec2 C1 =  bB->m_sweep.c + rB - bA->m_sweep.c - rA;
     qreal C2 = bB->m_sweep.a - bA->m_sweep.a - m_referenceAngle;
 
     // Handle large detachment.
     const qreal k_allowedStretch = 10.0f * b2_linearSlop;
     qreal positionError = C1.Length();
     qreal angularError = b2Abs(C2);
     if (positionError > k_allowedStretch)
     {
         iA *= 1.0f;
         iB *= 1.0f;
     }
 
     m_mass.col1.x = mA + mB + rA.y * rA.y * iA + rB.y * rB.y * iB;
     m_mass.col2.x = -rA.y * rA.x * iA - rB.y * rB.x * iB;
     m_mass.col3.x = -rA.y * iA - rB.y * iB;
     m_mass.col1.y = m_mass.col2.x;
     m_mass.col2.y = mA + mB + rA.x * rA.x * iA + rB.x * rB.x * iB;
     m_mass.col3.y = rA.x * iA + rB.x * iB;
     m_mass.col1.z = m_mass.col3.x;
     m_mass.col2.z = m_mass.col3.y;
     m_mass.col3.z = iA + iB;
 
     b2Vec3 C(C1.x, C1.y, C2);
 
     b2Vec3 impulse = m_mass.Solve33(-C);
 
     b2Vec2 P(impulse.x, impulse.y);
 
     bA->m_sweep.c -= mA * P;
     bA->m_sweep.a -= iA * (b2Cross(rA, P) + impulse.z);
 
     bB->m_sweep.c += mB * P;
     bB->m_sweep.a += iB * (b2Cross(rB, P) + impulse.z);
 
     bA->SynchronizeTransform();
     bB->SynchronizeTransform();
 
     return positionError <= b2_linearSlop && angularError <= b2_angularSlop;
 }
 
 b2Vec2 b2WeldJoint::GetAnchorA() const
 {
     return m_bodyA->GetWorldPoint(m_localAnchorA);
 }
 
 b2Vec2 b2WeldJoint::GetAnchorB() const
 {
     return m_bodyB->GetWorldPoint(m_localAnchorB);
 }
 
 b2Vec2 b2WeldJoint::GetReactionForce(qreal inv_dt) const
 {
     b2Vec2 P(m_impulse.x, m_impulse.y);
     return inv_dt * P;
 }
 
 qreal b2WeldJoint::GetReactionTorque(qreal inv_dt) const
 {
     return inv_dt * m_impulse.z;
 }