Beispiel
In diesem Beispiel verwenden wir XHTML, SVG, JavaScript und das DOM, um einen Schwarm von "Motes" zu animieren. Diese Motes folgen zwei grundlegenden Prinzipien. Erstens versucht jedes Mote, sich auf den Mauszeiger zuzubewegen, und zweitens versucht jedes Mote, sich von der durchschnittlichen Position der Motes zu entfernen. In Kombination ergibt sich dieses sehr natürlich wirkende Verhalten.
Sehen Sie sich das Beispiel an. Das verlinkte Beispiel wurde mit Best Practices aus dem Jahr 2006 geschrieben. Das untenstehende Beispiel wurde auf die aktuellen Best Practices für JavaScript aktualisiert. Beide funktionieren.
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 basic 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='email-me' href='#'>Nick Johnson</a>
<script type='text/javascript'>
<![CDATA[
// foil spam bots
let email = '@riovia.net';
email ='nick' + email;
document.getElementById('email-me').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>