Stars Follow Cursor

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This is a neat little script that shows a chain of stars that act like gravity is affecting them.

Body Code:

// any of these URLs can be changed to any desired picture. You can also change the number of them by removing or adding more and changing the value of nDots below. Remember that the first picture is number 0 so the value of nDots would be 15, for 16 pictures.  //

<div id="dot0" style="position: absolute; visibility: hidden; height: 11; width: 11;"><img src="/data/ex_585/bullet.gif" height=11 width=11></div>
<div id="dot1" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-2.gif" height=30 width=30></div>
<div id="dot2" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-4.gif" height=30 width=30></div>
<div id="dot3" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-1.gif" height=30 width=30></div>
<div id="dot4" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-2.gif" height=25 width=25></div>
<div id="dot5" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-4.gif" height=25 width=25></div>
<div id="dot6" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-1.gif" height=25 width=25></div>
<div id="dot7" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-2.gif" height=20 width=20></div>
<div id="dot8" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-4.gif" height=20 width=20></div>
<div id="dot9" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-1.gif" height=20 width=20></div>
<div id="dot10" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-2.gif" height=15 width=15></div>
<div id="dot11" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-4.gif" height=15 width=15></div>
<div id="dot12" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-1.gif" height=15 width=15></div>
<div id="dot13" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-2.gif" height=10 width=10></div>
<div id="dot14" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-4.gif" height=10 width=10></div>
<div id="dot15" style="position: absolute; height: 11; width: 11;"><img src="/data/ex_585/star-1.gif" height=10 width=10></div>

<script LANGUAGE="JavaScript">
<!-- hide code

// Here is the total number of pictures to trail, minus one. You need a DIV tag above for each picture. //

var nDots = 16;
if (document.all&&window.print)
document.body.style.cssText="overflow-x:hidden;overflow-y:scroll"
var Xpos = 0;
var Ypos = 0;

  // fixed time step, no relation to real time
var DELTAT = .02;
  // 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;
var GRAVITY = 50;
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 isNetscape = navigator.appName=="Netscape";

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

var dots = new Array();
init();

function init()
{
    var i = 0;
    for (i = 0; i < nDots; i++) {
        dots[i] = new dot(i);
    }
    
    if (!isNetscape) {
        // I only know how to read the locations of the 
        // <LI> items in IE
        //skip this for now
        // setInitPositions(dots)
    }
    
    // set their positions
    for (i = 0; i < nDots; i++) {
        dots[i].obj.left = dots[i].X;
        dots[i].obj.top = dots[i].Y;
    }
    
    
    if (isNetscape) {
        // start right away since they are positioned
        // at 0, 0
        startanimate();
    } else {
        // let dots sit there for a few seconds
        // since they're hiding on the real bullets
        setTimeout("startanimate()", 2000);
    }
}



function dot(i) 
{
    this.X = Xpos;
    this.Y = Ypos;
    this.dx = 0;
    this.dy = 0;
    if (isNetscape) { 
        this.obj = eval("document.dot" + i);
    } else {
        this.obj = eval("dot" + i + ".style");
    }
}


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


// This is to line up the bullets with actual LI tags on the page
// Had to add -DOTSIZE to X and 2*DOTSIZE to Y for IE 5, not sure why
// Still doesn't work great
function setInitPositions(dots)
{
    // initialize dot positions to be on top 
    // of the bullets in the <ul>
    var startloc = document.all.tags("LI");
    var i = 0;
    for (i = 0; i < startloc.length && i < (nDots - 1); i++) {
        dots[i+1].X = startloc[i].offsetLeft
            startloc[i].offsetParent.offsetLeft - DOTSIZE;
        dots[i+1].Y = startloc[i].offsetTop +
            startloc[i].offsetParent.offsetTop + 2*DOTSIZE;
    }
    // put 0th dot above 1st (it is hidden)
    dots[0].X = dots[1].X;
    dots[0].Y = dots[1].Y - SEGLEN;
}

// just save mouse position for animate() to use
function MoveHandler(e)
{
    Xpos = e.pageX;
    Ypos = e.pageY;   
    return true;
}

// just save mouse position for animate() to use
function MoveHandlerIE() {
    Xpos = window.event.x + document.body.scrollLeft;
    Ypos = window.event.y + document.body.scrollTop;   
}

if (isNetscape) {
    document.captureEvents(Event.MOUSEMOVE);
    document.onMouseMove = MoveHandler;
} else {
    document.onmousemove = MoveHandlerIE;
}


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 = Xpos;
        dots[0].Y = Ypos; 
        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,
            (spring.Y + resist.Y)/ MASS + GRAVITY);
        
        // 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 &&
            Math.abs(dots[i].dy) < STOPVEL &&
            Math.abs(accel.X) < STOPACC &&
            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 (isNetscape) {
            height = window.innerHeight + document.scrollTop;
            width = window.innerWidth + document.scrollLeft;
        } else { 
            height = document.body.clientHeight + document.body.scrollTop;
            width = document.body.clientWidth + document.body.scrollLeft;
        }
        
        // bounce of 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; 
    }
}

// end code hiding -->
</script>

<HR>
These following image are needed to use to have them in your script - right click on each to download them:<BR>
<img src="/data/ex_585/bullet.gif" height=11 width=11>
<img src="/data/ex_585/star-2.gif" height=30 width=30>
<img src="/data/ex_585/star-4.gif" height=30 width=30>
<img src="/data/ex_585/star-1.gif" height=30 width=30>

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