Trailing Image for a Cursor

<!--
// image src
var trailsrc = "images/Windows (Personalizada).png";

var nDots = 7;
var Xbpos = 0;
var Ybpos = 0;

// fixed time step, no relation to real time
var DELTAT = .01;
// size of one spring in pixels
var SEGLEN = 10;
// spring constant, stiffness of springs
var SPRINGK = 10;
// all the physics is bogus, just picked stuff to make it look okay
var MASS = 1;
// Positive XGRAVITY pulls right, negative pulls left
// Positive YGRAVITY pulls down, negative up
var XGRAVITY = 0;
var YGRAVITY = 50;
// RESISTANCE determines a slowing force proportional to velocity
var RESISTANCE = 10;
// stopping criterea to prevent endless jittering
// doesn't work when sitting on bottom since floor
// doesn't push back so acceleration always as big
// as gravity
var STOPVEL = 0.1;
var STOPACC = 0.1;
var DOTSIZE = 11;
// BOUNCE is percent of velocity retained when 
// bouncing off a wall
var BOUNCE = 0.75;

var ff=(document.getElementById&amp;&amp;!document.all);
var ns=(document.layers);
var ie=(document.all);

// always on for now, could be played with to
// let dots fall to botton, get thrown, etc.
var followmouse = true;
var dots = new Array();

function init()
{
    var i = 0;
    for (i = 0; i < nDots; i++) {
        dots[i] = new dot(i);
    }
    
    // set their positions
    for (i = 0; i < nDots; i++) {
        dots[i].obj.left = dots[i].X;
        dots[i].obj.top = dots[i].Y;
    }
    
    setTimeout("startanimate()", 10);
}

function dot(i) 
{
	this.X  = Xbpos;
	this.Y  = Ybpos;
	this.dx = 0;
	this.dy = 0;
	
	if (ns){
	  document.write("<layer id=\"mtrail"+ i +"\" ><img src='"+trailsrc+"' border=\"0\"><\/layer>");
	} else if (ie||ff) {
	if (i == 0) {
	  document.write("<div id=\"mtrail"+ i +"\" style=\"POSITION: absolute; VISIBILITY: hidden;\"><img src='"+trailsrc+"' border=\"0\"><\/div>");
	} else {
          document.write("<div id=\"mtrail"+ i +"\" style=\"POSITION: absolute; \"><img src='"+trailsrc+"' border=\"0\"><\/div>");
        }
        }	
	
	if (ie) 
	{
		this.obj = eval("mtrail" + i + ".style");
	} else if (ff) 
	{
		this.obj = document.getElementById("mtrail" + i).style;
	}
	else
	{
		this.obj = eval("document.mtrail" + i);
	}
}

function startanimate() {	
    setInterval("animate()", 20);
}

// just save mouse position for animate() to use
function MoveHandler(e)
{
    if (ie) {
    	Xbpos = window.event.x + document.body.scrollLeft;
    	Ybpos = window.event.y + document.body.scrollTop;
    }
    else {
    	Xbpos = e.pageX;
    	Ybpos = e.pageY;
    }
}

function vec(X, Y)
{
    this.X = X;
    this.Y = Y;
}

// adds force in X and Y to spring for dot[i] on dot[j]
function springForce(i, j, spring)
{
    var dx = (dots[i].X - dots[j].X);
    var dy = (dots[i].Y - dots[j].Y);
    var len = Math.sqrt(dx*dx + dy*dy);
    if (len > SEGLEN) {
        var springF = SPRINGK * (len - SEGLEN);
        spring.X += (dx / len) * springF;
        spring.Y += (dy / len) * springF;
    }
}


function animate() {	
    // dots[0] follows the mouse,
    // though no dot is drawn there
    var start = 0;
    if (followmouse) {
        dots[0].X = Xbpos;
        dots[0].Y = Ybpos;	
        start = 1;
    }
    
    for (i = start ; i < nDots; i++ ) {
        
        var spring = new vec(0, 0);
        if (i > 0) {
            springForce(i-1, i, spring);
        }
        if (i < (nDots - 1)) {
            springForce(i+1, i, spring);
        }
        
        // air resisitance/friction
        var resist = new vec(-dots[i].dx * RESISTANCE,
            -dots[i].dy * RESISTANCE);
        
        // compute new accel, including gravity
        var accel = new vec((spring.X + resist.X)/MASS + XGRAVITY,
            (spring.Y + resist.Y)/ MASS + YGRAVITY);
        
        // compute new velocity
        dots[i].dx += (DELTAT * accel.X);
        dots[i].dy += (DELTAT * accel.Y);
        
        // stop dead so it doesn't jitter when nearly still
        if (Math.abs(dots[i].dx) < STOPVEL &amp;&amp;
            Math.abs(dots[i].dy) < STOPVEL &amp;&amp;
            Math.abs(accel.X) < STOPACC &amp;&amp;
            Math.abs(accel.Y) < STOPACC) {
            dots[i].dx = 0;
            dots[i].dy = 0;
        }
        
        // move to new position
        dots[i].X += dots[i].dx;
        dots[i].Y += dots[i].dy;
        
        // get size of window
        var height, width;
        if (!ie) {
            height = window.innerHeight + window.pageYOffset;
            width = window.innerWidth + window.pageXOffset;
        } else {	
            height = document.body.clientHeight + document.body.scrollTop;
            width = document.body.clientWidth + document.body.scrollLeft;
        }
        
        // bounce off 3 walls (leave ceiling open)
        if (dots[i].Y >=  height - DOTSIZE - 1) {
            if (dots[i].dy > 0) {
                dots[i].dy = BOUNCE * -dots[i].dy;
            }
            dots[i].Y = height - DOTSIZE - 1;
        }
        if (dots[i].X >= width - DOTSIZE) {
            if (dots[i].dx > 0) {
                dots[i].dx = BOUNCE * -dots[i].dx;
            }
            dots[i].X = width - DOTSIZE - 1;
        }
        if (dots[i].X < 0) {
            if (dots[i].dx < 0) {
                dots[i].dx = BOUNCE * -dots[i].dx;
            }
            dots[i].X = 0;
        }
        
        // move img to new position
        dots[i].obj.left = dots[i].X;			
        dots[i].obj.top =  dots[i].Y;		
    }
}

init();
if(ns)window.captureEvents(Event.MOUSEMOVE);
document.onmousemove = MoveHandler;
-->