b2_friction_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_friction_joint.h"
#include "box2d/b2_body.h"
#include "box2d/b2_time_step.h"

// Point-to-point constraint
// 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)

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

void b2FrictionJointDef::Initialize(b2Body* bA, b2Body* bB, const b2Vec2& anchor)
{
        bodyA = bA;
        bodyB = bB;
        localAnchorA = bodyA->GetLocalPoint(anchor);
        localAnchorB = bodyB->GetLocalPoint(anchor);
}

b2FrictionJoint::b2FrictionJoint(const b2FrictionJointDef* def)
: b2Joint(def)
{
        m_localAnchorA = def->localAnchorA;
        m_localAnchorB = def->localAnchorB;

        m_linearImpulse.SetZero();
        m_angularImpulse = 0.0f;

        m_maxForce = def->maxForce;
        m_maxTorque = def->maxTorque;
}

void b2FrictionJoint::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;

        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;

        b2Rot qA(aA), qB(aB);

        // Compute the effective mass matrix.
        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]

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

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

        m_linearMass = K.GetInverse();

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

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

                b2Vec2 P(m_linearImpulse.x, m_linearImpulse.y);
                vA -= mA * P;
                wA -= iA * (b2Cross(m_rA, P) + m_angularImpulse);
                vB += mB * P;
                wB += iB * (b2Cross(m_rB, P) + m_angularImpulse);
        }
        else
        {
                m_linearImpulse.SetZero();
                m_angularImpulse = 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 b2FrictionJoint::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;

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

        float h = data.step.dt;

        // Solve angular friction
        {
                float Cdot = wB - wA;
                float impulse = -m_angularMass * Cdot;

                float oldImpulse = m_angularImpulse;
                float maxImpulse = h * m_maxTorque;
                m_angularImpulse = b2Clamp(m_angularImpulse + impulse, -maxImpulse, maxImpulse);
                impulse = m_angularImpulse - oldImpulse;

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

        // Solve linear friction
        {
                b2Vec2 Cdot = vB + b2Cross(wB, m_rB) - vA - b2Cross(wA, m_rA);

                b2Vec2 impulse = -b2Mul(m_linearMass, Cdot);
                b2Vec2 oldImpulse = m_linearImpulse;
                m_linearImpulse += impulse;

                float maxImpulse = h * m_maxForce;

                if (m_linearImpulse.LengthSquared() > maxImpulse * maxImpulse)
                {
                        m_linearImpulse.Normalize();
                        m_linearImpulse *= maxImpulse;
                }

                impulse = m_linearImpulse - oldImpulse;

                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 b2FrictionJoint::SolvePositionConstraints(const b2SolverData& data)
{
        B2_NOT_USED(data);

        return true;
}

b2Vec2 b2FrictionJoint::GetAnchorA() const
{
        return m_bodyA->GetWorldPoint(m_localAnchorA);
}

b2Vec2 b2FrictionJoint::GetAnchorB() const
{
        return m_bodyB->GetWorldPoint(m_localAnchorB);
}

b2Vec2 b2FrictionJoint::GetReactionForce(float inv_dt) const
{
        return inv_dt * m_linearImpulse;
}

float b2FrictionJoint::GetReactionTorque(float inv_dt) const
{
        return inv_dt * m_angularImpulse;
}

void b2FrictionJoint::SetMaxForce(float force)
{
        b2Assert(b2IsValid(force) && force >= 0.0f);
        m_maxForce = force;
}

float b2FrictionJoint::GetMaxForce() const
{
        return m_maxForce;
}

void b2FrictionJoint::SetMaxTorque(float torque)
{
        b2Assert(b2IsValid(torque) && torque >= 0.0f);
        m_maxTorque = torque;
}

float b2FrictionJoint::GetMaxTorque() const
{
        return m_maxTorque;
}

void b2FrictionJoint::Dump()
{
        int32 indexA = m_bodyA->m_islandIndex;
        int32 indexB = m_bodyB->m_islandIndex;

        b2Dump("  b2FrictionJointDef 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.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.maxForce = %.9g;\n", m_maxForce);
        b2Dump("  jd.maxTorque = %.9g;\n", m_maxTorque);
        b2Dump("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}