axmol/Box2D/Dynamics/Joints/b2PulleyJoint.h

/*
* 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.
*/

#ifndef B2_PULLEY_JOINT_H
#define B2_PULLEY_JOINT_H

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

const float32 b2_minPulleyLength = 2.0f;

/// Pulley joint definition. This requires two ground anchors,
/// two dynamic body anchor points, and a pulley ratio.
struct b2PulleyJointDef : public b2JointDef
{
    b2PulleyJointDef()
    {
        type = e_pulleyJoint;
        groundAnchorA.Set(-1.0f, 1.0f);
        groundAnchorB.Set(1.0f, 1.0f);
        localAnchorA.Set(-1.0f, 0.0f);
        localAnchorB.Set(1.0f, 0.0f);
        lengthA = 0.0f;
        lengthB = 0.0f;
        ratio = 1.0f;
        collideConnected = true;
    }

    /// Initialize the bodies, anchors, lengths, max lengths, and ratio using the world anchors.
    void Initialize(b2Body* bodyA, b2Body* bodyB,
                    const b2Vec2& groundAnchorA, const b2Vec2& groundAnchorB,
                    const b2Vec2& anchorA, const b2Vec2& anchorB,
                    float32 ratio);

    /// The first ground anchor in world coordinates. This point never moves.
    b2Vec2 groundAnchorA;

    /// The second ground anchor in world coordinates. This point never moves.
    b2Vec2 groundAnchorB;

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

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

    /// The a reference length for the segment attached to bodyA.
    float32 lengthA;

    /// The a reference length for the segment attached to bodyB.
    float32 lengthB;

    /// The pulley ratio, used to simulate a block-and-tackle.
    float32 ratio;
};

/// The pulley joint is connected to two bodies and two fixed ground points.
/// The pulley supports a ratio such that:
/// length1 + ratio * length2 <= constant
/// Yes, the force transmitted is scaled by the ratio.
/// Warning: the pulley joint can get a bit squirrelly by itself. They often
/// work better when combined with prismatic joints. You should also cover the
/// the anchor points with static shapes to prevent one side from going to
/// zero length.
class b2PulleyJoint : public b2Joint
{
public:
    b2Vec2 GetAnchorA() const;
    b2Vec2 GetAnchorB() const;

    b2Vec2 GetReactionForce(float32 inv_dt) const;
    float32 GetReactionTorque(float32 inv_dt) const;

    /// Get the first ground anchor.
    b2Vec2 GetGroundAnchorA() const;

    /// Get the second ground anchor.
    b2Vec2 GetGroundAnchorB() const;

    /// Get the current length of the segment attached to bodyA.
    float32 GetLengthA() const;

    /// Get the current length of the segment attached to bodyB.
    float32 GetLengthB() const;

    /// Get the pulley ratio.
    float32 GetRatio() const;

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

protected:

    friend class b2Joint;
    b2PulleyJoint(const b2PulleyJointDef* data);

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

    b2Vec2 m_groundAnchorA;
    b2Vec2 m_groundAnchorB;
    float32 m_lengthA;
    float32 m_lengthB;
    
    // Solver shared
    b2Vec2 m_localAnchorA;
    b2Vec2 m_localAnchorB;
    float32 m_constant;
    float32 m_ratio;
    float32 m_impulse;

    // Solver temp
    int32 m_indexA;
    int32 m_indexB;
    b2Vec2 m_uA;
    b2Vec2 m_uB;
    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;
};

#endif