PulleyJointDef.java

/*
 * 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.Settings;
import org.jbox2d.common.Vec2;
import org.jbox2d.dynamics.Body;

// Updated to rev 130 of b2PulleyJoint.cpp/.h

/**
 * 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.
 * The pulley also enforces a maximum length limit on both sides. This is
 * useful to prevent one side of the pulley hitting the top.
 */
public class PulleyJointDef extends JointDef {

	public PulleyJointDef() {
		type = JointType.PULLEY_JOINT;
		groundAnchor1 = new Vec2(-1.0f, 1.0f);
		groundAnchor2 = new Vec2(1.0f, 1.0f);
		localAnchor1 = new Vec2(-1.0f, 0.0f);
		localAnchor2 = new Vec2(1.0f, 0.0f);
		length1 = 0.0f;
		maxLength1 = 0.0f;
		length2 = 0.0f;
		maxLength2 = 0.0f;
		ratio = 1.0f;
		collideConnected = true;
	}

	/// Initialize the bodies, anchors, lengths, max lengths, and ratio using the world anchors.
	public void initialize(final Body b1, final Body b2,
	                       final Vec2 ga1, final Vec2 ga2,
	                       final Vec2 anchor1, final Vec2 anchor2,
	                       final float r){
		body1 = b1;
		body2 = b2;
		groundAnchor1 = ga1;
		groundAnchor2 = ga2;
		localAnchor1 = body1.getLocalPoint(anchor1);
		localAnchor2 = body2.getLocalPoint(anchor2);
		final Vec2 d1 = anchor1.sub(ga1);
		length1 = d1.length();
		final Vec2 d2 = anchor2.sub(ga2);
		length2 = d2.length();
		ratio = r;
		assert(ratio > Settings.EPSILON);
		final float C = length1 + ratio * length2;
		maxLength1 = C - ratio * PulleyJoint.MIN_PULLEY_LENGTH;
		maxLength2 = (C - PulleyJoint.MIN_PULLEY_LENGTH) / ratio;
	}

	/// The first ground anchor in world coordinates. This point never moves.
	public Vec2 groundAnchor1;

	/// The second ground anchor in world coordinates. This point never moves.
	public Vec2 groundAnchor2;

	/// The local anchor point relative to body1's origin.
	public Vec2 localAnchor1;

	/// The local anchor point relative to body2's origin.
	public Vec2 localAnchor2;

	/// The a reference length for the segment attached to body1.
	public float length1;

	/// The maximum length of the segment attached to body1.
	public float maxLength1;

	/// The a reference length for the segment attached to body2.
	public float length2;

	/// The maximum length of the segment attached to body2.
	public float maxLength2;

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