Pulley Script Correctness

Unity Engine
1

I’ve been scripting a pulley system, and was hoping to get some critique of its implementation so far. One of my main problems is that with a low drag, the system has this whole perpetual bouncing thing going on, but with high drag, it doesn’t respond very well to things pushing on it.

using UnityEngine;
using System.Collections;

public class PulleyBehaviour : MonoBehaviour {
	public Rigidbody bodyA;
	public Rigidbody bodyB;
	
	public GameObject pivotA;
	public GameObject pivotB;
	
	private Vector3 lastVelA;
	private Vector3 lastVelB;

	private float totalLength;
	
	// Use this for initialization
	void Start () {
		Vector3 posA = bodyA.position;
		Vector3 posB = bodyB.position;
		
		float lengthA = (pivotA.transform.position - posA).magnitude;
		float lengthB = (pivotB.transform.position - posB).magnitude;
		
		totalLength = lengthA + lengthB;
		
		lastVelA = bodyA.velocity;
		lastVelB = bodyB.velocity;
	}
	
	// Update is called once per frame
	void FixedUpdate () {
		float lengthA = (pivotA.transform.position - bodyA.position).magnitude;
		float lengthB = (pivotB.transform.position - bodyB.position).magnitude;
		
		if( lengthA + lengthB > totalLength ) {
			Vector3 d1 = bodyA.position - pivotA.transform.position;
			Vector3 d2 = bodyB.position - pivotB.transform.position;
			
			d1.Normalize();
			d2.Normalize();
			
			Vector3 forceA = (bodyA.velocity - lastVelA) * -bodyA.mass;
			Vector3 forceB = (bodyB.velocity - lastVelB) * -bodyB.mass;
			
			bodyB.AddForce( forceA );
			bodyA.AddForce( forceB );
			

			float diff = -(totalLength - lengthA - lengthB);
			float aMoves = diff * (bodyB.mass / (bodyA.mass + bodyB.mass));
			float bMoves = diff - aMoves;			

			bodyA.position = bodyA.position - aMoves * d1;
			bodyB.position = bodyB.position - bMoves * d2;
		}
	}
}
2

Hi BeShifty,
Thanks for the code. Will you please explain the code from if( lengthA + lengthB > totalLength ) ?