示例
在这个示例中，我们使用 XHTML、SVG、JavaScript 和 DOM 来动画化一群“微粒”。这些微粒受两个简单的原则控制。首先，每个微粒都试图向鼠标光标移动，其次每个微粒都试图远离平均微粒位置。结合起来，我们得到了这种非常自然的视觉效果。
查看示例。链接的示例使用 2006 年的最佳实践编写。下面的示例已更新为现代 JavaScript 最佳实践。两者都有效。
xml
<?xml version='1.0'?>
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:svg="http://www.w3.org/2000/svg">
<head>
<title>A swarm of motes</title>
<style>
<![CDATA[
label, input
{
width: 150px;
display: block;
float: left;
margin-bottom: 10px;
}
label
{
text-align: right;
width: 75px;
padding-right: 20px;
}
br
{
clear: left;
}
]]>
</style>
</head>
<body onload='update()'>
<svg:svg id='display' width='400' height='300'>
<svg:circle id='cursor' cx='200'
cy='150' r='7' fill='#0000ff' fill-opacity='0.5'/>
</svg:svg>
<p>A swarm of motes, governed by two simple principles.
First, each mote tries to move towards the cursor, and
second each mote tries to move away from the average
mote position. Combined, we get this very natural
looking behavior.
</p>
<div>
(C) 2006 <a id='emailme' href='#'>Nick Johnson</a>
<script type='text/javascript'>
<![CDATA[
// foil spam bots
let email = '@riovia.net';
email ='nick' + email;
document.getElementById('emailme').href = 'mailto:'+email;
]]>
</script>
This software is free for you to use in any way whatsoever,
and comes with no warranty at all.
</div>
<form action="" onsubmit="return false;">
<p>
<label>Number of motes:</label>
<input id='num_motes' value='5'/>
<br/>
<label>Max. Velocity:</label>
<input id='max_velocity' value='15'/>
<br/>
<label>Attraction to cursor:</label>
<input id='attract_cursor' value='6'/>
<br/>
<label>Repulsion from peers:</label>
<input id='repel_peer' value='5'/>
<br/>
</p>
</form>
<script type='text/javascript'>
<![CDATA[
// Array of motes
let motes;
// Get the display element.
function Display() {
return document.getElementById('display');
}
// Determine dimensions of the display element.
// Return this as a 2-tuple (x,y) in an array
function Dimensions() {
// Our Rendering Element
const display = Display();
const width = parseInt(display.getAttributeNS(null, 'width'));
const height = parseInt(display.getAttributeNS(null, 'height'));
return [width, height];
}
// This is called by mouse move events
const mouse_x = 200;
const mouse_y = 150;
function OnMouseMove(evt) {
mouse_x = evt.clientX;
mouse_y = evt.clientY;
const widget = document.getElementById('cursor');
widget.setAttributeNS(null,'cx',mouse_x);
widget.setAttributeNS(null,'cy',mouse_y);
}
document.onmousemove = OnMouseMove;
// Determine (x,y) of the cursor
function Cursor() {
return [mouse_x, mouse_y];
}
// Determine average (x,y) of the swarm
function AverageMotePosition() {
if (!motes || motes.length === 0) {
return [0, 0];
}
const sum_x = 0;
const sum_y = 0;
for (const mote of motes) {
sum_x += mote.x;
sum_y += mote.y;
}
return [sum_x / motes.length, sum_y / motes.length];
}
// A nicer, integer random
function Rand(modulo)
{
return Math.round(Math.random() * (modulo - 1));
}
// Class Mote
function Mote() {
// Dimensions of drawing area.
const dims = Dimensions();
const width = dims[0];
const height = dims[1];
// Choose a random coordinate to start at.
this.x = Rand(width);
this.y = Rand(height);
// Nil initial velocity.
this.vx = this.vy = 0;
// A visual element, initially none
this.elt = null;
}
// Turn this into a class.
new Mote();
// Mote::applyForce() — Adjust velocity
// towards the given position.
// Warning: Pseudo-physics — not really
// governed by any /real/ physical principles.
Mote.prototype.applyForce = function (pos, mag) {
if (pos[0] > this.x) {
this.vx += mag;
} else if (pos[0] < this.x) {
this.vx -= mag;
}
if (pos[1] > this.y) {
this.vy += mag;
} else if (pos[1] < this.y) {
this.vy -= mag;
}
}
// Mote::capVelocity() — Apply an upper limit
// on mote velocity.
Mote.prototype.capVelocity = function () {
const max = parseInt(document.getElementById('max_velocity').value);
if (max < this.vx) {
this.vx = max;
} else if (-max > this.vx) {
this.vx = -max;
}
if (max < this.vy) {
this.vy = max;
} else if (-max > this.vy) {
this.vy = -max;
}
}
// Mote::capPosition() — Apply an upper/lower limit
// on mote position.
Mote.prototype.capPosition = function () {
const dims = Dimensions();
if (this.x < 0) {
this.x = 0;
} else if (this.x >= dims[0]) {
this.x = dims[0] - 1;
}
if (this.y < 0) {
this.y = 0;
} else if (this.y >= dims[1]) {
this.y = dims[1] - 1;
}
}
// Mote::move() — move a mote, update the screen.
Mote.prototype.move = function () {
// Apply attraction to cursor.
const attract = parseInt(document.getElementById('attract_cursor').value);
const cursor = Cursor();
this.applyForce(cursor, attract);
// Apply repulsion from average mote position.
const repel = parseInt(document.getElementById('repel_peer').value);
const average = AverageMotePosition();
this.applyForce(average, -repel);
// Add some randomness to the velocity.
this.vx += Rand(3) - 1;
this.vy += Rand(3) - 1;
// Put an upper limit on velocity.
this.capVelocity();
// Apply velocity.
const old_x = this.x;
const old_y = this.y;
this.x += this.vx;
this.y += this.vy;
this.capPosition();
// Draw it.
if (this.elt === null) {
const svg = 'http://www.w3.org/2000/svg';
this.elt = document.createElementNS(svg, 'line');
this.elt.setAttributeNS(null, 'stroke', 'green');
this.elt.setAttributeNS(null, 'stroke-width', '3');
this.elt.setAttributeNS(null, 'stroke-opacity', '0.5');
Display().appendChild(this.elt);
}
this.elt.setAttributeNS(null, 'x1', old_x);
this.elt.setAttributeNS(null, 'y1', old_y);
this.elt.setAttributeNS(null, 'x2', this.x);
this.elt.setAttributeNS(null, 'y2', this.y);
}
function update() {
// First call?
if (!motes) {
motes = [];
}
// How many motes should there be?
let num = parseInt( document.getElementById('num_motes').value );
if ( num < 0 ) {
num = 0;
}
// Make sure we have exactly that many...
// Too few?
while (motes.length < num) {
motes.push(new Mote());
}
// Or too many?
if (num === 0) {
motes = [];
} else if (motes.length > num) {
motes = motes.slice(0, num - 1);
}
// Move a random mote
if (motes.length > 0) {
motes[Rand(motes.length)].move();
}
// And do this again in 1/100 sec
setTimeout('update()', 10);
}
]]>
</script>
</body>
</html>