b2DistanceJoint.cpp

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/*
* Copyright (c) 2006-2011 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/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_localAnchorA = def->localAnchorA;
        m_localAnchorB = 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 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;

        // Handle singularity.
        float32 length = m_u.Length();
        if (length > b2_linearSlop)
        {
                m_u *= 1.0f / length;
        }
        else
        {
                m_u.Set(0.0f, 0.0f);
        }

        float32 crAu = b2Cross(m_rA, m_u);
        float32 crBu = b2Cross(m_rB, m_u);
        float32 invMass = m_invMassA + m_invIA * crAu * crAu + m_invMassB + m_invIB * crBu * crBu;

        // Compute the effective mass matrix.
        m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;

        if (m_frequencyHz > 0.0f)
        {
                float32 C = length - m_length;

                // Frequency
                float32 omega = 2.0f * b2_pi * m_frequencyHz;

                // Damping coefficient
                float32 d = 2.0f * m_mass * m_dampingRatio * omega;

                // Spring stiffness
                float32 k = m_mass * omega * omega;

                // magic formulas
                float32 h = data.step.dt;
                m_gamma = h * (d + h * k);
                m_gamma = m_gamma != 0.0f ? 1.0f / m_gamma : 0.0f;
                m_bias = C * h * k * m_gamma;

                invMass += m_gamma;
                m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;
        }
        else
        {
                m_gamma = 0.0f;
                m_bias = 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 b2DistanceJoint::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 Cdot = b2Dot(m_u, vpB - vpA);

        float32 impulse = -m_mass * (Cdot + m_bias + m_gamma * m_impulse);
        m_impulse += impulse;

        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 b2DistanceJoint::SolvePositionConstraints(const b2SolverData& data)
{
        if (m_frequencyHz > 0.0f)
        {
                // There is no position correction for soft distance constraints.
                return true;
        }

        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_length;
        C = b2Clamp(C, -b2_maxLinearCorrection, 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 b2Abs(C) < b2_linearSlop;
}

b2Vec2 b2DistanceJoint::GetAnchorA() const
{
        return m_bodyA->GetWorldPoint(m_localAnchorA);
}

b2Vec2 b2DistanceJoint::GetAnchorB() const
{
        return m_bodyB->GetWorldPoint(m_localAnchorB);
}

b2Vec2 b2DistanceJoint::GetReactionForce(float32 inv_dt) const
{
        b2Vec2 F = (inv_dt * m_impulse) * m_u;
        return F;
}

float32 b2DistanceJoint::GetReactionTorque(float32 inv_dt) const
{
        B2_NOT_USED(inv_dt);
        return 0.0f;
}

void b2DistanceJoint::Dump()
{
        int32 indexA = m_bodyA->m_islandIndex;
        int32 indexB = m_bodyB->m_islandIndex;

        b2Log("  b2DistanceJointDef 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.length = %.15lef;\n", m_length);
        b2Log("  jd.frequencyHz = %.15lef;\n", m_frequencyHz);
        b2Log("  jd.dampingRatio = %.15lef;\n", m_dampingRatio);
        b2Log("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}