DistanceJoint.java :  » Java-2D » jbox2d » org » jbox2d » dynamics » joints » Java Open Source

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/*******************************************************************************
 * Copyright (c) 2011, Daniel Murphy
 * All rights reserved.
 * 
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 * modification, are permitted provided that the following conditions are met:
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 *       names of its contributors may be used to endorse or promote products
 *       derived from this software without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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 * DISCLAIMED. IN NO EVENT SHALL DANIEL MURPHY BE LIABLE FOR ANY
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 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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/*
 * JBox2D - A Java Port of Erin Catto's Box2D
 * 
 * JBox2D homepage: http://jbox2d.sourceforge.net/
 * Box2D homepage: 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.
 */

package org.jbox2d.dynamics.joints;

import org.jbox2d.common.Mat22;
import org.jbox2d.common.MathUtils;
import org.jbox2d.common.Settings;
import org.jbox2d.common.Vec2;
import org.jbox2d.dynamics.Body;
import org.jbox2d.dynamics.TimeStep;
import org.jbox2d.pooling.WorldPool;

//Updated to rev 56->130->142 of b2DistanceJoint.cpp/.h

//C = norm(p2 - p1) - L
//u = (p2 - p1) / norm(p2 - p1)
//Cdot = dot(u, v2 + cross(w2, r2) - v1 - cross(w1, r1))
//J = [-u -cross(r1, u) u cross(r2, u)]
//K = J * invM * JT
//= invMass1 + invI1 * cross(r1, u)^2 + invMass2 + invI2 * cross(r2, u)^2

/**
 * 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.
 */
public class DistanceJoint extends Joint {
  
  public final Vec2 m_localAnchor1;
  public final Vec2 m_localAnchor2;
  public final Vec2 m_u;
  public float m_impulse;
  public float m_mass; // effective mass for the constraint.
  public float m_length;
  public float m_frequencyHz;
  public float m_dampingRatio;
  public float m_gamma;
  public float m_bias;
  
  public DistanceJoint(WorldPool argWorld, final DistanceJointDef def) {
    super(argWorld, def);
    m_localAnchor1 = def.localAnchorA.clone();
    m_localAnchor2 = def.localAnchorB.clone();
    m_length = def.length;
    m_impulse = 0.0f;
    m_u = new Vec2();
    m_frequencyHz = def.frequencyHz;
    m_dampingRatio = def.dampingRatio;
    m_gamma = 0.0f;
    m_bias = 0.0f;
  }
  
  public void setFrequency(float hz) {
    m_frequencyHz = hz;
  }
  
  public float getFrequency() {
    return m_frequencyHz;
  }
  
  public float getLength() {
    return m_length;
  }
  
  public void setLength(float argLength) {
    m_length = argLength;
  }
  
  public void setDampingRatio(float damp) {
    m_dampingRatio = damp;
  }
  
  public float getDampingRatio() {
    return m_dampingRatio;
  }
  
  @Override
  public void getAnchorA(Vec2 argOut) {
    m_bodyA.getWorldPointToOut(m_localAnchor1, argOut);
  }
  
  @Override
  public void getAnchorB(Vec2 argOut) {
    m_bodyB.getWorldPointToOut(m_localAnchor2, argOut);
  }
  
  // djm pooled
  @Override
  public void getReactionForce(float inv_dt, Vec2 argOut) {
    argOut.x = m_impulse * m_u.x * inv_dt;
    argOut.y = m_impulse * m_u.y * inv_dt;
  }
  
  @Override
  public float getReactionTorque(float inv_dt) {
    return 0.0f;
  }
  
  @Override
  public void initVelocityConstraints(final TimeStep step) {
    
    // TODO: fully inline temp Vec2 ops
    final Body b1 = m_bodyA;
    final Body b2 = m_bodyB;
    
    Vec2 r1 = pool.popVec2();
    Vec2 r2 = pool.popVec2();
    
    // Compute the effective mass matrix.
    r1.set(m_localAnchor1).subLocal(b1.getLocalCenter());
    r2.set(m_localAnchor2).subLocal(b2.getLocalCenter());
    Mat22.mulToOut(b1.getTransform().R, r1, r1);
    Mat22.mulToOut(b2.getTransform().R, r2, r2);
    
    m_u.x = b2.m_sweep.c.x + r2.x - b1.m_sweep.c.x - r1.x;
    m_u.y = b2.m_sweep.c.y + r2.y - b1.m_sweep.c.y - r1.y;
    
    // Handle singularity.
    float length = m_u.length();
    if (length > Settings.linearSlop) {
      m_u.x *= 1.0f / length;
      m_u.y *= 1.0f / length;
    }
    else {
      m_u.set(0.0f, 0.0f);
    }
    
    float cr1u = Vec2.cross(r1, m_u);
    float cr2u = Vec2.cross(r2, m_u);
    
    float invMass = b1.m_invMass + b1.m_invI * cr1u * cr1u + b2.m_invMass + b2.m_invI * cr2u * cr2u;
    assert (invMass > Settings.EPSILON);
    m_mass = 1.0f / invMass;
    
    if (m_frequencyHz > 0.0f) {
      float C = length - m_length;
      
      // Frequency
      float omega = 2.0f * MathUtils.PI * m_frequencyHz;
      
      // Damping coefficient
      float d = 2.0f * m_mass * m_dampingRatio * omega;
      
      // Spring stiffness
      float k = m_mass * omega * omega;
      
      // magic formulas
      m_gamma = step.dt * (d + step.dt * k);
      m_gamma = m_gamma != 0.0f ? 1.0f / m_gamma : 0.0f;
      m_bias = C * step.dt * k * m_gamma;
      
      m_mass = invMass + m_gamma;
      m_mass = m_mass != 0.0f ? 1.0f / m_mass : 0.0f;
    }
    
    if (step.warmStarting) {
      
      // Scale the impulse to support a variable time step.
      m_impulse *= step.dtRatio;
      
      Vec2 P = pool.popVec2();
      P.set(m_u).mulLocal(m_impulse);
      
      b1.m_linearVelocity.x -= b1.m_invMass * P.x;
      b1.m_linearVelocity.y -= b1.m_invMass * P.y;
      b1.m_angularVelocity -= b1.m_invI * Vec2.cross(r1, P);
      
      b2.m_linearVelocity.x += b2.m_invMass * P.x;
      b2.m_linearVelocity.y += b2.m_invMass * P.y;
      b2.m_angularVelocity += b2.m_invI * Vec2.cross(r2, P);
      
      pool.pushVec2(1);
    }
    else {
      m_impulse = 0.0f;
    }
    pool.pushVec2(2);
  }
  
  @Override
  public void solveVelocityConstraints(final TimeStep step) {
    final Body b1 = m_bodyA;
    final Body b2 = m_bodyB;
    
    final Vec2 r1 = pool.popVec2();
    final Vec2 r2 = pool.popVec2();
    
    r1.set(m_localAnchor1).subLocal(b1.getLocalCenter());
    r2.set(m_localAnchor2).subLocal(b2.getLocalCenter());
    Mat22.mulToOut(b1.getTransform().R, r1, r1);
    Mat22.mulToOut(b2.getTransform().R, r2, r2);
    
    final Vec2 v1 = pool.popVec2();
    final Vec2 v2 = pool.popVec2();
    
    // Cdot = dot(u, v + cross(w, r))
    Vec2.crossToOut(b1.m_angularVelocity, r1, v1);
    Vec2.crossToOut(b2.m_angularVelocity, r2, v2);
    v1.set(b1.m_linearVelocity).addLocal(b1.m_linearVelocity);
    v2.set(b2.m_linearVelocity).addLocal(b2.m_linearVelocity);
    
    float Cdot = Vec2.dot(m_u, v2.subLocal(v1));
    
    float impulse = -m_mass * (Cdot + m_bias + m_gamma * m_impulse);
    m_impulse += impulse;
    
    float Px = impulse * m_u.x;
    float Py = impulse * m_u.y;
    b1.m_linearVelocity.x -= b1.m_invMass * Px;
    b1.m_linearVelocity.y -= b1.m_invMass * Py;
    b1.m_angularVelocity -= b1.m_invI * (r1.x * Py - r1.y * Px);// b2Cross(r1, P);
    b2.m_linearVelocity.x += b2.m_invMass * Px;
    b2.m_linearVelocity.y += b2.m_invMass * Py;
    b2.m_angularVelocity += b2.m_invI * (r2.x * Py - r2.y * Px);// b2Cross(r2, P);
    
    pool.pushVec2(4);
  }
  
  @Override
  public boolean solvePositionConstraints(float baumgarte) {
    if (m_frequencyHz > 0.0f) {
      return true;
    }
    
    final Body b1 = m_bodyA;
    final Body b2 = m_bodyB;
    
    final Vec2 r1 = pool.popVec2();
    final Vec2 r2 = pool.popVec2();
    final Vec2 d = pool.popVec2();
    
    r1.set(m_localAnchor1).subLocal(b1.getLocalCenter());
    r2.set(m_localAnchor2).subLocal(b2.getLocalCenter());
    Mat22.mulToOut(b1.getTransform().R, r1, r1);
    Mat22.mulToOut(b2.getTransform().R, r2, r2);
    
    d.x = b2.m_sweep.c.x + r2.x - b1.m_sweep.c.x - r1.x;
    d.y = b2.m_sweep.c.y + r2.y - b1.m_sweep.c.y - r1.y;
    
    float length = d.normalize();
    float C = length - m_length;
    C = MathUtils.clamp(C, -Settings.maxLinearCorrection, Settings.maxLinearCorrection);
    
    float impulse = -m_mass * C;
    m_u.set(d);
    float Px = impulse * m_u.x;
    float Py = impulse * m_u.y;
    
    b1.m_sweep.c.x -= b1.m_invMass * Px;
    b1.m_sweep.c.y -= b1.m_invMass * Py;
    b1.m_sweep.a -= b1.m_invI * (r1.x * Py - r1.y * Px);// b2Cross(r1, P);
    
    b2.m_sweep.c.x += b2.m_invMass * Px;
    b2.m_sweep.c.y += b2.m_invMass * Py;
    b2.m_sweep.a += b2.m_invI * (r2.x * Py - r2.y * Px);// b2Cross(r2, P);
    
    b1.synchronizeTransform();
    b2.synchronizeTransform();
    
    pool.pushVec2(3);
    
    return MathUtils.abs(C) < Settings.linearSlop;
  }
}
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