b2RopeJoint.cpp

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/*
* Copyright (c) 2007-2011 Erin Catto http://www.box2d.org
*
* 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 b2SolverData& data)
{
        m_indexA = m_bodyA->m_islandIndex;
        m_indexB = m_bodyB->m_islandIndex;
        m_localCenterA = m_bodyA->m_sweep.localCenter;
        m_localCenterB = m_bodyB->m_sweep.localCenter;
        m_invMassA = m_bodyA->m_invMass;
        m_invMassB = m_bodyB->m_invMass;
        m_invIA = m_bodyA->m_invI;
        m_invIB = m_bodyB->m_invI;

        b2Vec2 cA = data.positions[m_indexA].c;
        float32 aA = data.positions[m_indexA].a;
        b2Vec2 vA = data.velocities[m_indexA].v;
        float32 wA = data.velocities[m_indexA].w;

        b2Vec2 cB = data.positions[m_indexB].c;
        float32 aB = data.positions[m_indexB].a;
        b2Vec2 vB = data.velocities[m_indexB].v;
        float32 wB = data.velocities[m_indexB].w;

        b2Rot qA(aA), qB(aB);

        m_rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
        m_rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
        m_u = cB + m_rB - cA - m_rA;

        m_length = m_u.Length();

        float32 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.
        float32 crA = b2Cross(m_rA, m_u);
        float32 crB = b2Cross(m_rB, m_u);
        float32 invMass = m_invMassA + m_invIA * crA * crA + m_invMassB + m_invIB * crB * crB;

        m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;

        if (data.step.warmStarting)
        {
                // Scale the impulse to support a variable time step.
                m_impulse *= data.step.dtRatio;

                b2Vec2 P = m_impulse * m_u;
                vA -= m_invMassA * P;
                wA -= m_invIA * b2Cross(m_rA, P);
                vB += m_invMassB * P;
                wB += m_invIB * b2Cross(m_rB, P);
        }
        else
        {
                m_impulse = 0.0f;
        }

        data.velocities[m_indexA].v = vA;
        data.velocities[m_indexA].w = wA;
        data.velocities[m_indexB].v = vB;
        data.velocities[m_indexB].w = wB;
}

void b2RopeJoint::SolveVelocityConstraints(const b2SolverData& data)
{
        b2Vec2 vA = data.velocities[m_indexA].v;
        float32 wA = data.velocities[m_indexA].w;
        b2Vec2 vB = data.velocities[m_indexB].v;
        float32 wB = data.velocities[m_indexB].w;

        // Cdot = dot(u, v + cross(w, r))
        b2Vec2 vpA = vA + b2Cross(wA, m_rA);
        b2Vec2 vpB = vB + b2Cross(wB, m_rB);
        float32 C = m_length - m_maxLength;
        float32 Cdot = b2Dot(m_u, vpB - vpA);

        // Predictive constraint.
        if (C < 0.0f)
        {
                Cdot += data.step.inv_dt * C;
        }

        float32 impulse = -m_mass * Cdot;
        float32 oldImpulse = m_impulse;
        m_impulse = b2Min(0.0f, m_impulse + impulse);
        impulse = m_impulse - oldImpulse;

        b2Vec2 P = impulse * m_u;
        vA -= m_invMassA * P;
        wA -= m_invIA * b2Cross(m_rA, P);
        vB += m_invMassB * P;
        wB += m_invIB * b2Cross(m_rB, P);

        data.velocities[m_indexA].v = vA;
        data.velocities[m_indexA].w = wA;
        data.velocities[m_indexB].v = vB;
        data.velocities[m_indexB].w = wB;
}

bool b2RopeJoint::SolvePositionConstraints(const b2SolverData& data)
{
        b2Vec2 cA = data.positions[m_indexA].c;
        float32 aA = data.positions[m_indexA].a;
        b2Vec2 cB = data.positions[m_indexB].c;
        float32 aB = data.positions[m_indexB].a;

        b2Rot qA(aA), qB(aB);

        b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
        b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
        b2Vec2 u = cB + rB - cA - rA;

        float32 length = u.Normalize();
        float32 C = length - m_maxLength;

        C = b2Clamp(C, 0.0f, b2_maxLinearCorrection);

        float32 impulse = -m_mass * C;
        b2Vec2 P = impulse * u;

        cA -= m_invMassA * P;
        aA -= m_invIA * b2Cross(rA, P);
        cB += m_invMassB * P;
        aB += m_invIB * b2Cross(rB, P);

        data.positions[m_indexA].c = cA;
        data.positions[m_indexA].a = aA;
        data.positions[m_indexB].c = cB;
        data.positions[m_indexB].a = aB;

        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(float32 inv_dt) const
{
        b2Vec2 F = (inv_dt * m_impulse) * m_u;
        return F;
}

float32 b2RopeJoint::GetReactionTorque(float32 inv_dt) const
{
        B2_NOT_USED(inv_dt);
        return 0.0f;
}

float32 b2RopeJoint::GetMaxLength() const
{
        return m_maxLength;
}

b2LimitState b2RopeJoint::GetLimitState() const
{
        return m_state;
}

void b2RopeJoint::Dump()
{
        int32 indexA = m_bodyA->m_islandIndex;
        int32 indexB = m_bodyB->m_islandIndex;

        b2Log("  b2RopeJointDef jd;\n");
        b2Log("  jd.bodyA = bodies[%d];\n", indexA);
        b2Log("  jd.bodyB = bodies[%d];\n", indexB);
        b2Log("  jd.collideConnected = bool(%d);\n", m_collideConnected);
        b2Log("  jd.localAnchorA.Set(%.15lef, %.15lef);\n", m_localAnchorA.x, m_localAnchorA.y);
        b2Log("  jd.localAnchorB.Set(%.15lef, %.15lef);\n", m_localAnchorB.x, m_localAnchorB.y);
        b2Log("  jd.maxLength = %.15lef;\n", m_maxLength);
        b2Log("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}