axmol/Box2D/Dynamics/Joints/b2GearJoint.cpp

/*
* 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/b2GearJoint.h>
#include <Box2D/Dynamics/Joints/b2RevoluteJoint.h>
#include <Box2D/Dynamics/Joints/b2PrismaticJoint.h>
#include <Box2D/Dynamics/b2Body.h>
#include <Box2D/Dynamics/b2TimeStep.h>

// Gear Joint:
// C0 = (coordinate1 + ratio * coordinate2)_initial
// C = (coordinate1 + ratio * coordinate2) - C0 = 0
// J = [J1 ratio * J2]
// K = J * invM * JT
//   = J1 * invM1 * J1T + ratio * ratio * J2 * invM2 * J2T
//
// Revolute:
// coordinate = rotation
// Cdot = angularVelocity
// J = [0 0 1]
// K = J * invM * JT = invI
//
// Prismatic:
// coordinate = dot(p - pg, ug)
// Cdot = dot(v + cross(w, r), ug)
// J = [ug cross(r, ug)]
// K = J * invM * JT = invMass + invI * cross(r, ug)^2

b2GearJoint::b2GearJoint(const b2GearJointDef* def)
: b2Joint(def)
{
    m_joint1 = def->joint1;
    m_joint2 = def->joint2;

    m_typeA = m_joint1->GetType();
    m_typeB = m_joint2->GetType();

    b2Assert(m_typeA == e_revoluteJoint || m_typeA == e_prismaticJoint);
    b2Assert(m_typeB == e_revoluteJoint || m_typeB == e_prismaticJoint);

    float32 coordinateA, coordinateB;

    // TODO_ERIN there might be some problem with the joint edges in b2Joint.

    m_bodyC = m_joint1->GetBodyA();
    m_bodyA = m_joint1->GetBodyB();

    // Get geometry of joint1
    b2Transform xfA = m_bodyA->m_xf;
    float32 aA = m_bodyA->m_sweep.a;
    b2Transform xfC = m_bodyC->m_xf;
    float32 aC = m_bodyC->m_sweep.a;

    if (m_typeA == e_revoluteJoint)
    {
        b2RevoluteJoint* revolute = (b2RevoluteJoint*)def->joint1;
        m_localAnchorC = revolute->m_localAnchorA;
        m_localAnchorA = revolute->m_localAnchorB;
        m_referenceAngleA = revolute->m_referenceAngle;
        m_localAxisC.SetZero();

        coordinateA = aA - aC - m_referenceAngleA;
    }
    else
    {
        b2PrismaticJoint* prismatic = (b2PrismaticJoint*)def->joint1;
        m_localAnchorC = prismatic->m_localAnchorA;
        m_localAnchorA = prismatic->m_localAnchorB;
        m_referenceAngleA = prismatic->m_referenceAngle;
        m_localAxisC = prismatic->m_localXAxisA;

        b2Vec2 pC = m_localAnchorC;
        b2Vec2 pA = b2MulT(xfC.q, b2Mul(xfA.q, m_localAnchorA) + (xfA.p - xfC.p));
        coordinateA = b2Dot(pA - pC, m_localAxisC);
    }

    m_bodyD = m_joint2->GetBodyA();
    m_bodyB = m_joint2->GetBodyB();

    // Get geometry of joint2
    b2Transform xfB = m_bodyB->m_xf;
    float32 aB = m_bodyB->m_sweep.a;
    b2Transform xfD = m_bodyD->m_xf;
    float32 aD = m_bodyD->m_sweep.a;

    if (m_typeB == e_revoluteJoint)
    {
        b2RevoluteJoint* revolute = (b2RevoluteJoint*)def->joint2;
        m_localAnchorD = revolute->m_localAnchorA;
        m_localAnchorB = revolute->m_localAnchorB;
        m_referenceAngleB = revolute->m_referenceAngle;
        m_localAxisD.SetZero();

        coordinateB = aB - aD - m_referenceAngleB;
    }
    else
    {
        b2PrismaticJoint* prismatic = (b2PrismaticJoint*)def->joint2;
        m_localAnchorD = prismatic->m_localAnchorA;
        m_localAnchorB = prismatic->m_localAnchorB;
        m_referenceAngleB = prismatic->m_referenceAngle;
        m_localAxisD = prismatic->m_localXAxisA;

        b2Vec2 pD = m_localAnchorD;
        b2Vec2 pB = b2MulT(xfD.q, b2Mul(xfB.q, m_localAnchorB) + (xfB.p - xfD.p));
        coordinateB = b2Dot(pB - pD, m_localAxisD);
    }

    m_ratio = def->ratio;

    m_constant = coordinateA + m_ratio * coordinateB;

    m_impulse = 0.0f;
}

void b2GearJoint::InitVelocityConstraints(const b2SolverData& data)
{
    m_indexA = m_bodyA->m_islandIndex;
    m_indexB = m_bodyB->m_islandIndex;
    m_indexC = m_bodyC->m_islandIndex;
    m_indexD = m_bodyD->m_islandIndex;
    m_lcA = m_bodyA->m_sweep.localCenter;
    m_lcB = m_bodyB->m_sweep.localCenter;
    m_lcC = m_bodyC->m_sweep.localCenter;
    m_lcD = m_bodyD->m_sweep.localCenter;
    m_mA = m_bodyA->m_invMass;
    m_mB = m_bodyB->m_invMass;
    m_mC = m_bodyC->m_invMass;
    m_mD = m_bodyD->m_invMass;
    m_iA = m_bodyA->m_invI;
    m_iB = m_bodyB->m_invI;
    m_iC = m_bodyC->m_invI;
    m_iD = m_bodyD->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;

    b2Vec2 cC = data.positions[m_indexC].c;
    float32 aC = data.positions[m_indexC].a;
    b2Vec2 vC = data.velocities[m_indexC].v;
    float32 wC = data.velocities[m_indexC].w;

    b2Vec2 cD = data.positions[m_indexD].c;
    float32 aD = data.positions[m_indexD].a;
    b2Vec2 vD = data.velocities[m_indexD].v;
    float32 wD = data.velocities[m_indexD].w;

    b2Rot qA(aA), qB(aB), qC(aC), qD(aD);

    m_mass = 0.0f;

    if (m_typeA == e_revoluteJoint)
    {
        m_JvAC.SetZero();
        m_JwA = 1.0f;
        m_JwC = 1.0f;
        m_mass += m_iA + m_iC;
    }
    else
    {
        b2Vec2 u = b2Mul(qC, m_localAxisC);
        b2Vec2 rC = b2Mul(qC, m_localAnchorC - m_lcC);
        b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_lcA);
        m_JvAC = u;
        m_JwC = b2Cross(rC, u);
        m_JwA = b2Cross(rA, u);
        m_mass += m_mC + m_mA + m_iC * m_JwC * m_JwC + m_iA * m_JwA * m_JwA;
    }

    if (m_typeB == e_revoluteJoint)
    {
        m_JvBD.SetZero();
        m_JwB = m_ratio;
        m_JwD = m_ratio;
        m_mass += m_ratio * m_ratio * (m_iB + m_iD);
    }
    else
    {
        b2Vec2 u = b2Mul(qD, m_localAxisD);
        b2Vec2 rD = b2Mul(qD, m_localAnchorD - m_lcD);
        b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_lcB);
        m_JvBD = m_ratio * u;
        m_JwD = m_ratio * b2Cross(rD, u);
        m_JwB = m_ratio * b2Cross(rB, u);
        m_mass += m_ratio * m_ratio * (m_mD + m_mB) + m_iD * m_JwD * m_JwD + m_iB * m_JwB * m_JwB;
    }

    // Compute effective mass.
    m_mass = m_mass > 0.0f ? 1.0f / m_mass : 0.0f;

    if (data.step.warmStarting)
    {
        vA += (m_mA * m_impulse) * m_JvAC;
        wA += m_iA * m_impulse * m_JwA;
        vB += (m_mB * m_impulse) * m_JvBD;
        wB += m_iB * m_impulse * m_JwB;
        vC -= (m_mC * m_impulse) * m_JvAC;
        wC -= m_iC * m_impulse * m_JwC;
        vD -= (m_mD * m_impulse) * m_JvBD;
        wD -= m_iD * m_impulse * m_JwD;
    }
    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;
    data.velocities[m_indexC].v = vC;
    data.velocities[m_indexC].w = wC;
    data.velocities[m_indexD].v = vD;
    data.velocities[m_indexD].w = wD;
}

void b2GearJoint::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;
    b2Vec2 vC = data.velocities[m_indexC].v;
    float32 wC = data.velocities[m_indexC].w;
    b2Vec2 vD = data.velocities[m_indexD].v;
    float32 wD = data.velocities[m_indexD].w;

    float32 Cdot = b2Dot(m_JvAC, vA - vC) + b2Dot(m_JvBD, vB - vD);
    Cdot += (m_JwA * wA - m_JwC * wC) + (m_JwB * wB - m_JwD * wD);

    float32 impulse = -m_mass * Cdot;
    m_impulse += impulse;

    vA += (m_mA * impulse) * m_JvAC;
    wA += m_iA * impulse * m_JwA;
    vB += (m_mB * impulse) * m_JvBD;
    wB += m_iB * impulse * m_JwB;
    vC -= (m_mC * impulse) * m_JvAC;
    wC -= m_iC * impulse * m_JwC;
    vD -= (m_mD * impulse) * m_JvBD;
    wD -= m_iD * impulse * m_JwD;

    data.velocities[m_indexA].v = vA;
    data.velocities[m_indexA].w = wA;
    data.velocities[m_indexB].v = vB;
    data.velocities[m_indexB].w = wB;
    data.velocities[m_indexC].v = vC;
    data.velocities[m_indexC].w = wC;
    data.velocities[m_indexD].v = vD;
    data.velocities[m_indexD].w = wD;
}

bool b2GearJoint::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;
    b2Vec2 cC = data.positions[m_indexC].c;
    float32 aC = data.positions[m_indexC].a;
    b2Vec2 cD = data.positions[m_indexD].c;
    float32 aD = data.positions[m_indexD].a;

    b2Rot qA(aA), qB(aB), qC(aC), qD(aD);

    float32 linearError = 0.0f;

    float32 coordinateA, coordinateB;

    b2Vec2 JvAC, JvBD;
    float32 JwA, JwB, JwC, JwD;
    float32 mass = 0.0f;

    if (m_typeA == e_revoluteJoint)
    {
        JvAC.SetZero();
        JwA = 1.0f;
        JwC = 1.0f;
        mass += m_iA + m_iC;

        coordinateA = aA - aC - m_referenceAngleA;
    }
    else
    {
        b2Vec2 u = b2Mul(qC, m_localAxisC);
        b2Vec2 rC = b2Mul(qC, m_localAnchorC - m_lcC);
        b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_lcA);
        JvAC = u;
        JwC = b2Cross(rC, u);
        JwA = b2Cross(rA, u);
        mass += m_mC + m_mA + m_iC * JwC * JwC + m_iA * JwA * JwA;

        b2Vec2 pC = m_localAnchorC - m_lcC;
        b2Vec2 pA = b2MulT(qC, rA + (cA - cC));
        coordinateA = b2Dot(pA - pC, m_localAxisC);
    }

    if (m_typeB == e_revoluteJoint)
    {
        JvBD.SetZero();
        JwB = m_ratio;
        JwD = m_ratio;
        mass += m_ratio * m_ratio * (m_iB + m_iD);

        coordinateB = aB - aD - m_referenceAngleB;
    }
    else
    {
        b2Vec2 u = b2Mul(qD, m_localAxisD);
        b2Vec2 rD = b2Mul(qD, m_localAnchorD - m_lcD);
        b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_lcB);
        JvBD = m_ratio * u;
        JwD = m_ratio * b2Cross(rD, u);
        JwB = m_ratio * b2Cross(rB, u);
        mass += m_ratio * m_ratio * (m_mD + m_mB) + m_iD * JwD * JwD + m_iB * JwB * JwB;

        b2Vec2 pD = m_localAnchorD - m_lcD;
        b2Vec2 pB = b2MulT(qD, rB + (cB - cD));
        coordinateB = b2Dot(pB - pD, m_localAxisD);
    }

    float32 C = (coordinateA + m_ratio * coordinateB) - m_constant;

    float32 impulse = 0.0f;
    if (mass > 0.0f)
    {
        impulse = -C / mass;
    }

    cA += m_mA * impulse * JvAC;
    aA += m_iA * impulse * JwA;
    cB += m_mB * impulse * JvBD;
    aB += m_iB * impulse * JwB;
    cC -= m_mC * impulse * JvAC;
    aC -= m_iC * impulse * JwC;
    cD -= m_mD * impulse * JvBD;
    aD -= m_iD * impulse * JwD;

    data.positions[m_indexA].c = cA;
    data.positions[m_indexA].a = aA;
    data.positions[m_indexB].c = cB;
    data.positions[m_indexB].a = aB;
    data.positions[m_indexC].c = cC;
    data.positions[m_indexC].a = aC;
    data.positions[m_indexD].c = cD;
    data.positions[m_indexD].a = aD;

    // TODO_ERIN not implemented
    return linearError < b2_linearSlop;
}

b2Vec2 b2GearJoint::GetAnchorA() const
{
    return m_bodyA->GetWorldPoint(m_localAnchorA);
}

b2Vec2 b2GearJoint::GetAnchorB() const
{
    return m_bodyB->GetWorldPoint(m_localAnchorB);
}

b2Vec2 b2GearJoint::GetReactionForce(float32 inv_dt) const
{
    b2Vec2 P = m_impulse * m_JvAC;
    return inv_dt * P;
}

float32 b2GearJoint::GetReactionTorque(float32 inv_dt) const
{
    float32 L = m_impulse * m_JwA;
    return inv_dt * L;
}

void b2GearJoint::SetRatio(float32 ratio)
{
    b2Assert(b2IsValid(ratio));
    m_ratio = ratio;
}

float32 b2GearJoint::GetRatio() const
{
    return m_ratio;
}

void b2GearJoint::Dump()
{
    int32 indexA = m_bodyA->m_islandIndex;
    int32 indexB = m_bodyB->m_islandIndex;

    int32 index1 = m_joint1->m_index;
    int32 index2 = m_joint2->m_index;

    b2Log("  b2GearJointDef 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.joint1 = joints[%d];\n", index1);
    b2Log("  jd.joint2 = joints[%d];\n", index2);
    b2Log("  jd.ratio = %.15lef;\n", m_ratio);
    b2Log("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}