axmol/Box2D/Dynamics/Joints/b2DistanceJoint.h

/*
* Copyright (c) 2006-2007 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.
*/

#ifndef B2_DISTANCE_JOINT_H
#define B2_DISTANCE_JOINT_H

#include <Box2D/Dynamics/Joints/b2Joint.h>

/// Distance joint definition. This requires defining an
/// anchor point on both bodies and the non-zero length of the
/// distance joint. The definition uses local anchor points
/// so that the initial configuration can violate the constraint
/// slightly. This helps when saving and loading a game.
/// @warning Do not use a zero or short length.
struct b2DistanceJointDef : public b2JointDef
{
    b2DistanceJointDef()
    {
        type = e_distanceJoint;
        localAnchorA.Set(0.0f, 0.0f);
        localAnchorB.Set(0.0f, 0.0f);
        length = 1.0f;
        frequencyHz = 0.0f;
        dampingRatio = 0.0f;
    }

    /// Initialize the bodies, anchors, and length using the world
    /// anchors.
    void Initialize(b2Body* bodyA, b2Body* bodyB,
                    const b2Vec2& anchorA, const b2Vec2& anchorB);

    /// The local anchor point relative to bodyA's origin.
    b2Vec2 localAnchorA;

    /// The local anchor point relative to bodyB's origin.
    b2Vec2 localAnchorB;

    /// The natural length between the anchor points.
    float32 length;

    /// The mass-spring-damper frequency in Hertz. A value of 0
    /// disables softness.
    float32 frequencyHz;

    /// The damping ratio. 0 = no damping, 1 = critical damping.
    float32 dampingRatio;
};

/// A distance joint constrains two points on two bodies
/// to remain at a fixed distance from each other. You can view
/// this as a massless, rigid rod.
class b2DistanceJoint : public b2Joint
{
public:

    b2Vec2 GetAnchorA() const;
    b2Vec2 GetAnchorB() const;

    /// Get the reaction force given the inverse time step.
    /// Unit is N.
    b2Vec2 GetReactionForce(float32 inv_dt) const;

    /// Get the reaction torque given the inverse time step.
    /// Unit is N*m. This is always zero for a distance joint.
    float32 GetReactionTorque(float32 inv_dt) const;

    /// The local anchor point relative to bodyA's origin.
    const b2Vec2& GetLocalAnchorA() const { return m_localAnchorA; }

    /// The local anchor point relative to bodyB's origin.
    const b2Vec2& GetLocalAnchorB() const  { return m_localAnchorB; }

    /// Set/get the natural length.
    /// Manipulating the length can lead to non-physical behavior when the frequency is zero.
    void SetLength(float32 length);
    float32 GetLength() const;

    /// Set/get frequency in Hz.
    void SetFrequency(float32 hz);
    float32 GetFrequency() const;

    /// Set/get damping ratio.
    void SetDampingRatio(float32 ratio);
    float32 GetDampingRatio() const;

    /// Dump joint to dmLog
    void Dump();

protected:

    friend class b2Joint;
    b2DistanceJoint(const b2DistanceJointDef* data);

    void InitVelocityConstraints(const b2SolverData& data);
    void SolveVelocityConstraints(const b2SolverData& data);
    bool SolvePositionConstraints(const b2SolverData& data);

    float32 m_frequencyHz;
    float32 m_dampingRatio;
    float32 m_bias;

    // Solver shared
    b2Vec2 m_localAnchorA;
    b2Vec2 m_localAnchorB;
    float32 m_gamma;
    float32 m_impulse;
    float32 m_length;

    // Solver temp
    int32 m_indexA;
    int32 m_indexB;
    b2Vec2 m_u;
    b2Vec2 m_rA;
    b2Vec2 m_rB;
    b2Vec2 m_localCenterA;
    b2Vec2 m_localCenterB;
    float32 m_invMassA;
    float32 m_invMassB;
    float32 m_invIA;
    float32 m_invIB;
    float32 m_mass;
};

inline void b2DistanceJoint::SetLength(float32 length)
{
    m_length = length;
}

inline float32 b2DistanceJoint::GetLength() const
{
    return m_length;
}

inline void b2DistanceJoint::SetFrequency(float32 hz)
{
    m_frequencyHz = hz;
}

inline float32 b2DistanceJoint::GetFrequency() const
{
    return m_frequencyHz;
}

inline void b2DistanceJoint::SetDampingRatio(float32 ratio)
{
    m_dampingRatio = ratio;
}

inline float32 b2DistanceJoint::GetDampingRatio() const
{
    return m_dampingRatio;
}

#endif