b2_pulley_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_body.h"
#include "box2d/b2_pulley_joint.h"
#include "box2d/b2_time_step.h"

// Pulley:
// length1 = norm(p1 - s1)
// length2 = norm(p2 - s2)
// C0 = (length1 + ratio * length2)_initial
// C = C0 - (length1 + ratio * length2)
// u1 = (p1 - s1) / norm(p1 - s1)
// u2 = (p2 - s2) / norm(p2 - s2)
// Cdot = -dot(u1, v1 + cross(w1, r1)) - ratio * dot(u2, v2 + cross(w2, r2))
// J = -[u1 cross(r1, u1) ratio * u2  ratio * cross(r2, u2)]
// K = J * invM * JT
//   = invMass1 + invI1 * cross(r1, u1)^2 + ratio^2 * (invMass2 + invI2 * cross(r2, u2)^2)

void b2PulleyJointDef::Initialize(b2Body* bA, b2Body* bB,
                                const b2Vec2& groundA, const b2Vec2& groundB,
                                const b2Vec2& anchorA, const b2Vec2& anchorB,
                                float r)
{
        bodyA = bA;
        bodyB = bB;
        groundAnchorA = groundA;
        groundAnchorB = groundB;
        localAnchorA = bodyA->GetLocalPoint(anchorA);
        localAnchorB = bodyB->GetLocalPoint(anchorB);
        b2Vec2 dA = anchorA - groundA;
        lengthA = dA.Length();
        b2Vec2 dB = anchorB - groundB;
        lengthB = dB.Length();
        ratio = r;
        b2Assert(ratio > b2_epsilon);
}

b2PulleyJoint::b2PulleyJoint(const b2PulleyJointDef* def)
: b2Joint(def)
{
        m_groundAnchorA = def->groundAnchorA;
        m_groundAnchorB = def->groundAnchorB;
        m_localAnchorA = def->localAnchorA;
        m_localAnchorB = def->localAnchorB;

        m_lengthA = def->lengthA;
        m_lengthB = def->lengthB;

        b2Assert(def->ratio != 0.0f);
        m_ratio = def->ratio;

        m_constant = def->lengthA + m_ratio * def->lengthB;

        m_impulse = 0.0f;
}

void b2PulleyJoint::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;
        float aA = data.positions[m_indexA].a;
        b2Vec2 vA = data.velocities[m_indexA].v;
        float wA = data.velocities[m_indexA].w;

        b2Vec2 cB = data.positions[m_indexB].c;
        float aB = data.positions[m_indexB].a;
        b2Vec2 vB = data.velocities[m_indexB].v;
        float 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);

        // Get the pulley axes.
        m_uA = cA + m_rA - m_groundAnchorA;
        m_uB = cB + m_rB - m_groundAnchorB;

        float lengthA = m_uA.Length();
        float lengthB = m_uB.Length();

        if (lengthA > 10.0f * b2_linearSlop)
        {
                m_uA *= 1.0f / lengthA;
        }
        else
        {
                m_uA.SetZero();
        }

        if (lengthB > 10.0f * b2_linearSlop)
        {
                m_uB *= 1.0f / lengthB;
        }
        else
        {
                m_uB.SetZero();
        }

        // Compute effective mass.
        float ruA = b2Cross(m_rA, m_uA);
        float ruB = b2Cross(m_rB, m_uB);

        float mA = m_invMassA + m_invIA * ruA * ruA;
        float mB = m_invMassB + m_invIB * ruB * ruB;

        m_mass = mA + m_ratio * m_ratio * mB;

        if (m_mass > 0.0f)
        {
                m_mass = 1.0f / m_mass;
        }

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

                // Warm starting.
                b2Vec2 PA = -(m_impulse) * m_uA;
                b2Vec2 PB = (-m_ratio * m_impulse) * m_uB;

                vA += m_invMassA * PA;
                wA += m_invIA * b2Cross(m_rA, PA);
                vB += m_invMassB * PB;
                wB += m_invIB * b2Cross(m_rB, PB);
        }
        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 b2PulleyJoint::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 vpA = vA + b2Cross(wA, m_rA);
        b2Vec2 vpB = vB + b2Cross(wB, m_rB);

        float Cdot = -b2Dot(m_uA, vpA) - m_ratio * b2Dot(m_uB, vpB);
        float impulse = -m_mass * Cdot;
        m_impulse += impulse;

        b2Vec2 PA = -impulse * m_uA;
        b2Vec2 PB = -m_ratio * impulse * m_uB;
        vA += m_invMassA * PA;
        wA += m_invIA * b2Cross(m_rA, PA);
        vB += m_invMassB * PB;
        wB += m_invIB * b2Cross(m_rB, PB);

        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 b2PulleyJoint::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;

        b2Rot qA(aA), qB(aB);

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

        // Get the pulley axes.
        b2Vec2 uA = cA + rA - m_groundAnchorA;
        b2Vec2 uB = cB + rB - m_groundAnchorB;

        float lengthA = uA.Length();
        float lengthB = uB.Length();

        if (lengthA > 10.0f * b2_linearSlop)
        {
                uA *= 1.0f / lengthA;
        }
        else
        {
                uA.SetZero();
        }

        if (lengthB > 10.0f * b2_linearSlop)
        {
                uB *= 1.0f / lengthB;
        }
        else
        {
                uB.SetZero();
        }

        // Compute effective mass.
        float ruA = b2Cross(rA, uA);
        float ruB = b2Cross(rB, uB);

        float mA = m_invMassA + m_invIA * ruA * ruA;
        float mB = m_invMassB + m_invIB * ruB * ruB;

        float mass = mA + m_ratio * m_ratio * mB;

        if (mass > 0.0f)
        {
                mass = 1.0f / mass;
        }

        float C = m_constant - lengthA - m_ratio * lengthB;
        float linearError = b2Abs(C);

        float impulse = -mass * C;

        b2Vec2 PA = -impulse * uA;
        b2Vec2 PB = -m_ratio * impulse * uB;

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

        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 linearError < b2_linearSlop;
}

b2Vec2 b2PulleyJoint::GetAnchorA() const
{
        return m_bodyA->GetWorldPoint(m_localAnchorA);
}

b2Vec2 b2PulleyJoint::GetAnchorB() const
{
        return m_bodyB->GetWorldPoint(m_localAnchorB);
}

b2Vec2 b2PulleyJoint::GetReactionForce(float inv_dt) const
{
        b2Vec2 P = m_impulse * m_uB;
        return inv_dt * P;
}

float b2PulleyJoint::GetReactionTorque(float inv_dt) const
{
        B2_NOT_USED(inv_dt);
        return 0.0f;
}

b2Vec2 b2PulleyJoint::GetGroundAnchorA() const
{
        return m_groundAnchorA;
}

b2Vec2 b2PulleyJoint::GetGroundAnchorB() const
{
        return m_groundAnchorB;
}

float b2PulleyJoint::GetLengthA() const
{
        return m_lengthA;
}

float b2PulleyJoint::GetLengthB() const
{
        return m_lengthB;
}

float b2PulleyJoint::GetRatio() const
{
        return m_ratio;
}

float b2PulleyJoint::GetCurrentLengthA() const
{
        b2Vec2 p = m_bodyA->GetWorldPoint(m_localAnchorA);
        b2Vec2 s = m_groundAnchorA;
        b2Vec2 d = p - s;
        return d.Length();
}

float b2PulleyJoint::GetCurrentLengthB() const
{
        b2Vec2 p = m_bodyB->GetWorldPoint(m_localAnchorB);
        b2Vec2 s = m_groundAnchorB;
        b2Vec2 d = p - s;
        return d.Length();
}

void b2PulleyJoint::Dump()
{
        int32 indexA = m_bodyA->m_islandIndex;
        int32 indexB = m_bodyB->m_islandIndex;

        b2Dump("  b2PulleyJointDef 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.groundAnchorA.Set(%.9g, %.9g);\n", m_groundAnchorA.x, m_groundAnchorA.y);
        b2Dump("  jd.groundAnchorB.Set(%.9g, %.9g);\n", m_groundAnchorB.x, m_groundAnchorB.y);
        b2Dump("  jd.localAnchorA.Set(%.9g, %.9g);\n", m_localAnchorA.x, m_localAnchorA.y);
        b2Dump("  jd.localAnchorB.Set(%.9g, %.9g);\n", m_localAnchorB.x, m_localAnchorB.y);
        b2Dump("  jd.lengthA = %.9g;\n", m_lengthA);
        b2Dump("  jd.lengthB = %.9g;\n", m_lengthB);
        b2Dump("  jd.ratio = %.9g;\n", m_ratio);
        b2Dump("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}

void b2PulleyJoint::ShiftOrigin(const b2Vec2& newOrigin)
{
        m_groundAnchorA -= newOrigin;
        m_groundAnchorB -= newOrigin;
}