b2FrictionJoint.cpp

Go to the documentation of this file.

/*
* 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/b2FrictionJoint.h>
#include <Box2D/Dynamics/b2Body.h>
#include <Box2D/Dynamics/b2TimeStep.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;

        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);

        // 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]

        float32 mA = m_invMassA, mB = m_invMassB;
        float32 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;
        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;

        float32 h = data.step.dt;

        // Solve angular friction
        {
                float32 Cdot = wB - wA;
                float32 impulse = -m_angularMass * Cdot;

                float32 oldImpulse = m_angularImpulse;
                float32 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;

                float32 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(float32 inv_dt) const
{
        return inv_dt * m_linearImpulse;
}

float32 b2FrictionJoint::GetReactionTorque(float32 inv_dt) const
{
        return inv_dt * m_angularImpulse;
}

void b2FrictionJoint::SetMaxForce(float32 force)
{
        b2Assert(b2IsValid(force) && force >= 0.0f);
        m_maxForce = force;
}

float32 b2FrictionJoint::GetMaxForce() const
{
        return m_maxForce;
}

void b2FrictionJoint::SetMaxTorque(float32 torque)
{
        b2Assert(b2IsValid(torque) && torque >= 0.0f);
        m_maxTorque = torque;
}

float32 b2FrictionJoint::GetMaxTorque() const
{
        return m_maxTorque;
}

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

        b2Log("  b2FrictionJointDef 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.maxForce = %.15lef;\n", m_maxForce);
        b2Log("  jd.maxTorque = %.15lef;\n", m_maxTorque);
        b2Log("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}