b2_gear_joint.cpp

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// MIT License

// Copyright (c) 2019 Erin Catto

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#include "box2d/b2_gear_joint.h"
#include "box2d/b2_revolute_joint.h"
#include "box2d/b2_prismatic_joint.h"
#include "box2d/b2_body.h"
#include "box2d/b2_time_step.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);

        float 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();

        // Body B on joint1 must be dynamic
        b2Assert(m_bodyA->m_type == b2_dynamicBody);

        // Get geometry of joint1
        b2Transform xfA = m_bodyA->m_xf;
        float aA = m_bodyA->m_sweep.a;
        b2Transform xfC = m_bodyC->m_xf;
        float 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();

        // Body B on joint2 must be dynamic
        b2Assert(m_bodyB->m_type == b2_dynamicBody);

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

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

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

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

        float aD = data.positions[m_indexD].a;
        b2Vec2 vD = data.velocities[m_indexD].v;
        float 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;
        float wA = data.velocities[m_indexA].w;
        b2Vec2 vB = data.velocities[m_indexB].v;
        float wB = data.velocities[m_indexB].w;
        b2Vec2 vC = data.velocities[m_indexC].v;
        float wC = data.velocities[m_indexC].w;
        b2Vec2 vD = data.velocities[m_indexD].v;
        float wD = data.velocities[m_indexD].w;

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

        float 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;
        float aA = data.positions[m_indexA].a;
        b2Vec2 cB = data.positions[m_indexB].c;
        float aB = data.positions[m_indexB].a;
        b2Vec2 cC = data.positions[m_indexC].c;
        float aC = data.positions[m_indexC].a;
        b2Vec2 cD = data.positions[m_indexD].c;
        float aD = data.positions[m_indexD].a;

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

        float linearError = 0.0f;

        float coordinateA, coordinateB;

        b2Vec2 JvAC, JvBD;
        float JwA, JwB, JwC, JwD;
        float 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);
        }

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

        float 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(float inv_dt) const
{
        b2Vec2 P = m_impulse * m_JvAC;
        return inv_dt * P;
}

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

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

float 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;

        b2Dump("  b2GearJointDef jd;\n");
        b2Dump("  jd.bodyA = bodies[%d];\n", indexA);
        b2Dump("  jd.bodyB = bodies[%d];\n", indexB);
        b2Dump("  jd.collideConnected = bool(%d);\n", m_collideConnected);
        b2Dump("  jd.joint1 = joints[%d];\n", index1);
        b2Dump("  jd.joint2 = joints[%d];\n", index2);
        b2Dump("  jd.ratio = %.9g;\n", m_ratio);
        b2Dump("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}