b2RevoluteJoint.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/b2RevoluteJoint.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)

// Motor constraint
// Cdot = w2 - w1
// J = [0 0 -1 0 0 1]
// K = invI1 + invI2

void b2RevoluteJointDef::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();
}

b2RevoluteJoint::b2RevoluteJoint(const b2RevoluteJointDef* def)
: b2Joint(def)
{
        m_localAnchorA = def->localAnchorA;
        m_localAnchorB = def->localAnchorB;
        m_referenceAngle = def->referenceAngle;

        m_impulse.SetZero();
        m_motorImpulse = 0.0f;

        m_lowerAngle = def->lowerAngle;
        m_upperAngle = def->upperAngle;
        m_maxMotorTorque = def->maxMotorTorque;
        m_motorSpeed = def->motorSpeed;
        m_enableLimit = def->enableLimit;
        m_enableMotor = def->enableMotor;
        m_limitState = e_inactiveLimit;
}

void b2RevoluteJoint::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;

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

        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);

        // 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]

        float32 mA = m_invMassA, mB = m_invMassB;
        float32 iA = m_invIA, iB = m_invIB;

        bool fixedRotation = (iA + iB == 0.0f);

        m_mass.ex.x = mA + mB + m_rA.y * m_rA.y * iA + m_rB.y * m_rB.y * iB;
        m_mass.ey.x = -m_rA.y * m_rA.x * iA - m_rB.y * m_rB.x * iB;
        m_mass.ez.x = -m_rA.y * iA - m_rB.y * iB;
        m_mass.ex.y = m_mass.ey.x;
        m_mass.ey.y = mA + mB + m_rA.x * m_rA.x * iA + m_rB.x * m_rB.x * iB;
        m_mass.ez.y = m_rA.x * iA + m_rB.x * iB;
        m_mass.ex.z = m_mass.ez.x;
        m_mass.ey.z = m_mass.ez.y;
        m_mass.ez.z = iA + iB;

        m_motorMass = iA + iB;
        if (m_motorMass > 0.0f)
        {
                m_motorMass = 1.0f / m_motorMass;
        }

        if (m_enableMotor == false || fixedRotation)
        {
                m_motorImpulse = 0.0f;
        }

        if (m_enableLimit && fixedRotation == false)
        {
                float32 jointAngle = aB - aA - m_referenceAngle;
                if (b2Abs(m_upperAngle - m_lowerAngle) < 2.0f * b2_angularSlop)
                {
                        m_limitState = e_equalLimits;
                }
                else if (jointAngle <= m_lowerAngle)
                {
                        if (m_limitState != e_atLowerLimit)
                        {
                                m_impulse.z = 0.0f;
                        }
                        m_limitState = e_atLowerLimit;
                }
                else if (jointAngle >= m_upperAngle)
                {
                        if (m_limitState != e_atUpperLimit)
                        {
                                m_impulse.z = 0.0f;
                        }
                        m_limitState = e_atUpperLimit;
                }
                else
                {
                        m_limitState = e_inactiveLimit;
                        m_impulse.z = 0.0f;
                }
        }
        else
        {
                m_limitState = e_inactiveLimit;
        }

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

                b2Vec2 P(m_impulse.x, m_impulse.y);

                vA -= mA * P;
                wA -= iA * (b2Cross(m_rA, P) + m_motorImpulse + m_impulse.z);

                vB += mB * P;
                wB += iB * (b2Cross(m_rB, P) + m_motorImpulse + m_impulse.z);
        }
        else
        {
                m_impulse.SetZero();
                m_motorImpulse = 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 b2RevoluteJoint::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;

        float32 mA = m_invMassA, mB = m_invMassB;
        float32 iA = m_invIA, iB = m_invIB;

        bool fixedRotation = (iA + iB == 0.0f);

        // Solve motor constraint.
        if (m_enableMotor && m_limitState != e_equalLimits && fixedRotation == false)
        {
                float32 Cdot = wB - wA - m_motorSpeed;
                float32 impulse = -m_motorMass * Cdot;
                float32 oldImpulse = m_motorImpulse;
                float32 maxImpulse = data.step.dt * m_maxMotorTorque;
                m_motorImpulse = b2Clamp(m_motorImpulse + impulse, -maxImpulse, maxImpulse);
                impulse = m_motorImpulse - oldImpulse;

                wA -= iA * impulse;
                wB += iB * impulse;
        }

        // Solve limit constraint.
        if (m_enableLimit && m_limitState != e_inactiveLimit && fixedRotation == false)
        {
                b2Vec2 Cdot1 = vB + b2Cross(wB, m_rB) - vA - b2Cross(wA, m_rA);
                float32 Cdot2 = wB - wA;
                b2Vec3 Cdot(Cdot1.x, Cdot1.y, Cdot2);

                b2Vec3 impulse = -m_mass.Solve33(Cdot);

                if (m_limitState == e_equalLimits)
                {
                        m_impulse += impulse;
                }
                else if (m_limitState == e_atLowerLimit)
                {
                        float32 newImpulse = m_impulse.z + impulse.z;
                        if (newImpulse < 0.0f)
                        {
                                b2Vec2 rhs = -Cdot1 + m_impulse.z * b2Vec2(m_mass.ez.x, m_mass.ez.y);
                                b2Vec2 reduced = m_mass.Solve22(rhs);
                                impulse.x = reduced.x;
                                impulse.y = reduced.y;
                                impulse.z = -m_impulse.z;
                                m_impulse.x += reduced.x;
                                m_impulse.y += reduced.y;
                                m_impulse.z = 0.0f;
                        }
                        else
                        {
                                m_impulse += impulse;
                        }
                }
                else if (m_limitState == e_atUpperLimit)
                {
                        float32 newImpulse = m_impulse.z + impulse.z;
                        if (newImpulse > 0.0f)
                        {
                                b2Vec2 rhs = -Cdot1 + m_impulse.z * b2Vec2(m_mass.ez.x, m_mass.ez.y);
                                b2Vec2 reduced = m_mass.Solve22(rhs);
                                impulse.x = reduced.x;
                                impulse.y = reduced.y;
                                impulse.z = -m_impulse.z;
                                m_impulse.x += reduced.x;
                                m_impulse.y += reduced.y;
                                m_impulse.z = 0.0f;
                        }
                        else
                        {
                                m_impulse += impulse;
                        }
                }

                b2Vec2 P(impulse.x, impulse.y);

                vA -= mA * P;
                wA -= iA * (b2Cross(m_rA, P) + impulse.z);

                vB += mB * P;
                wB += iB * (b2Cross(m_rB, P) + impulse.z);
        }
        else
        {
                // Solve point-to-point constraint
                b2Vec2 Cdot = vB + b2Cross(wB, m_rB) - vA - b2Cross(wA, m_rA);
                b2Vec2 impulse = m_mass.Solve22(-Cdot);

                m_impulse.x += impulse.x;
                m_impulse.y += impulse.y;

                vA -= mA * impulse;
                wA -= iA * b2Cross(m_rA, impulse);

                vB += mB * impulse;
                wB += iB * b2Cross(m_rB, impulse);
        }

        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 b2RevoluteJoint::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);

        float32 angularError = 0.0f;
        float32 positionError = 0.0f;

        bool fixedRotation = (m_invIA + m_invIB == 0.0f);

        // Solve angular limit constraint.
        if (m_enableLimit && m_limitState != e_inactiveLimit && fixedRotation == false)
        {
                float32 angle = aB - aA - m_referenceAngle;
                float32 limitImpulse = 0.0f;

                if (m_limitState == e_equalLimits)
                {
                        // Prevent large angular corrections
                        float32 C = b2Clamp(angle - m_lowerAngle, -b2_maxAngularCorrection, b2_maxAngularCorrection);
                        limitImpulse = -m_motorMass * C;
                        angularError = b2Abs(C);
                }
                else if (m_limitState == e_atLowerLimit)
                {
                        float32 C = angle - m_lowerAngle;
                        angularError = -C;

                        // Prevent large angular corrections and allow some slop.
                        C = b2Clamp(C + b2_angularSlop, -b2_maxAngularCorrection, 0.0f);
                        limitImpulse = -m_motorMass * C;
                }
                else if (m_limitState == e_atUpperLimit)
                {
                        float32 C = angle - m_upperAngle;
                        angularError = C;

                        // Prevent large angular corrections and allow some slop.
                        C = b2Clamp(C - b2_angularSlop, 0.0f, b2_maxAngularCorrection);
                        limitImpulse = -m_motorMass * C;
                }

                aA -= m_invIA * limitImpulse;
                aB += m_invIB * limitImpulse;
        }

        // Solve point-to-point constraint.
        {
                qA.Set(aA);
                qB.Set(aB);
                b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
                b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_localCenterB);

                b2Vec2 C = cB + rB - cA - rA;
                positionError = C.Length();

                float32 mA = m_invMassA, mB = m_invMassB;
                float32 iA = m_invIA, iB = m_invIB;

                b2Mat22 K;
                K.ex.x = mA + mB + iA * rA.y * rA.y + iB * rB.y * rB.y;
                K.ex.y = -iA * rA.x * rA.y - iB * rB.x * rB.y;
                K.ey.x = K.ex.y;
                K.ey.y = mA + mB + iA * rA.x * rA.x + iB * rB.x * rB.x;

                b2Vec2 impulse = -K.Solve(C);

                cA -= mA * impulse;
                aA -= iA * b2Cross(rA, impulse);

                cB += mB * impulse;
                aB += iB * b2Cross(rB, impulse);
        }

        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 positionError <= b2_linearSlop && angularError <= b2_angularSlop;
}

b2Vec2 b2RevoluteJoint::GetAnchorA() const
{
        return m_bodyA->GetWorldPoint(m_localAnchorA);
}

b2Vec2 b2RevoluteJoint::GetAnchorB() const
{
        return m_bodyB->GetWorldPoint(m_localAnchorB);
}

b2Vec2 b2RevoluteJoint::GetReactionForce(float32 inv_dt) const
{
        b2Vec2 P(m_impulse.x, m_impulse.y);
        return inv_dt * P;
}

float32 b2RevoluteJoint::GetReactionTorque(float32 inv_dt) const
{
        return inv_dt * m_impulse.z;
}

float32 b2RevoluteJoint::GetJointAngle() const
{
        b2Body* bA = m_bodyA;
        b2Body* bB = m_bodyB;
        return bB->m_sweep.a - bA->m_sweep.a - m_referenceAngle;
}

float32 b2RevoluteJoint::GetJointSpeed() const
{
        b2Body* bA = m_bodyA;
        b2Body* bB = m_bodyB;
        return bB->m_angularVelocity - bA->m_angularVelocity;
}

bool b2RevoluteJoint::IsMotorEnabled() const
{
        return m_enableMotor;
}

void b2RevoluteJoint::EnableMotor(bool flag)
{
        m_bodyA->SetAwake(true);
        m_bodyB->SetAwake(true);
        m_enableMotor = flag;
}

float32 b2RevoluteJoint::GetMotorTorque(float32 inv_dt) const
{
        return inv_dt * m_motorImpulse;
}

void b2RevoluteJoint::SetMotorSpeed(float32 speed)
{
        m_bodyA->SetAwake(true);
        m_bodyB->SetAwake(true);
        m_motorSpeed = speed;
}

void b2RevoluteJoint::SetMaxMotorTorque(float32 torque)
{
        m_bodyA->SetAwake(true);
        m_bodyB->SetAwake(true);
        m_maxMotorTorque = torque;
}

bool b2RevoluteJoint::IsLimitEnabled() const
{
        return m_enableLimit;
}

void b2RevoluteJoint::EnableLimit(bool flag)
{
        if (flag != m_enableLimit)
        {
                m_bodyA->SetAwake(true);
                m_bodyB->SetAwake(true);
                m_enableLimit = flag;
                m_impulse.z = 0.0f;
        }
}

float32 b2RevoluteJoint::GetLowerLimit() const
{
        return m_lowerAngle;
}

float32 b2RevoluteJoint::GetUpperLimit() const
{
        return m_upperAngle;
}

void b2RevoluteJoint::SetLimits(float32 lower, float32 upper)
{
        b2Assert(lower <= upper);
        
        if (lower != m_lowerAngle || upper != m_upperAngle)
        {
                m_bodyA->SetAwake(true);
                m_bodyB->SetAwake(true);
                m_impulse.z = 0.0f;
                m_lowerAngle = lower;
                m_upperAngle = upper;
        }
}

void b2RevoluteJoint::Dump()
{
        int32 indexA = m_bodyA->m_islandIndex;
        int32 indexB = m_bodyB->m_islandIndex;

        b2Log("  b2RevoluteJointDef 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.referenceAngle = %.15lef;\n", m_referenceAngle);
        b2Log("  jd.enableLimit = bool(%d);\n", m_enableLimit);
        b2Log("  jd.lowerAngle = %.15lef;\n", m_lowerAngle);
        b2Log("  jd.upperAngle = %.15lef;\n", m_upperAngle);
        b2Log("  jd.enableMotor = bool(%d);\n", m_enableMotor);
        b2Log("  jd.motorSpeed = %.15lef;\n", m_motorSpeed);
        b2Log("  jd.maxMotorTorque = %.15lef;\n", m_maxMotorTorque);
        b2Log("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}