物理动画
这里物理动画说的是，图形的动画效果和实际生活中的物理效果类似，比如说重力影响的效果。
其实动画的实现就是把一个动作拆分成间隔连续的去完成， 视觉上就感觉是在动的了。
比如把一个图从左边移到右边， 设总距离100的话， 每隔 1/40秒 移到 1 的话，效果就会有了。
这个时间间隔和距离间隔设置多少为最佳，美学上是有一些定义的，这里就不探讨了。
一个拽动和撕扯窗布的例子
三个文件
tearCloth.html ; tearCloth.css ; tearCloth.js
tearCloth.html
tearCloth.css
tearCloth.html ; tearCloth.css ; tearCloth.js
tearCloth.html
<!--Add by oscar999-->
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE> New Document </TITLE>
<META NAME="Author" CONTENT="oscar999">
<link rel="stylesheet" type="text/css" href="tearCloth.css" />
</HEAD>
<BODY>
<canvas id = "c" > </canvas>
<div id="info">
<div id="top">
<a id="close" href="">close</a>
</div>
<p>
<br>
- Tear the cloth with your mouse.<br><br>
- Right click and drag to cut the cloth<br><br>
- Reduce physics_accuracy if it's laggy.<br><br>
</p>
</div>
<script src="tearCloth.js"></script>
</BODY>
</HTML>
tearCloth.css
* {
margin: 0;
overflow:hidden;
-webkit-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
-o-user-select: none;
user-select: none;
}
body {
background:#333;
}
canvas {
background:#333;
100%;
height:376px;
margin:0 auto;
display:block;
}
#info {
position:absolute;
left:-1px;
top:-1px;
auto;
max-380px;
height:auto;
background:#f2f2f2;
border-bottom-right-radius:10px;
}
#top {
background:#fff;
100%;
height:auto;
position:relative;
border-bottom:1px solid #eee;
}
p {
font-family:Arial, sans-serif;
color:#666;
text-align:justify;
font-size: 16px;
margin:10px;
}
a {
font-family:sans-serif;
color:#444;
text-decoration:none;
font-size: 20px;
}
#site {
float:left;
margin: 10px;
color: #38a;
border-bottom:1px dashed #888;
}
#site:hover {
color: #7af;
}
#close {
float:right;
margin: 10px;
}
#p {
font-family: Verdana, sans-serif;
position:absolute;
right:10px;
bottom:10px;
color:#adf;
border: 1px dashed #555;
padding:4px 8px;
}
tearCloth.js
document.getElementById('close').onmousedown = function(e) {
e.preventDefault();
document.getElementById('info').style.display = 'none';
return false;
};
// settings
var physics_accuracy = 6,
mouse_influence = 20,
mouse_cut = 5,
gravity = 900,
cloth_height = 30,
cloth_width = 50,
start_y = 20,
spacing = 7,
tear_distance = 60;
window.requestAnimFrame =
window.requestAnimationFrame ||
window.webkitRequestAnimationFrame ||
window.mozRequestAnimationFrame ||
window.oRequestAnimationFrame ||
window.msRequestAnimationFrame ||
function(callback) {
window.setTimeout(callback, 1000 / 60);
};
var canvas,
ctx,
cloth,
boundsx,
boundsy,
mouse = {
down: false,
button: 1,
x: 0,
y: 0,
px: 0,
py: 0
};
window.onload = function() {
canvas = document.getElementById('c');
ctx = canvas.getContext('2d');
canvas.width = canvas.clientWidth;
canvas.height = 376;
canvas.onmousedown = function(e) {
mouse.button = e.which;
mouse.px = mouse.x;
mouse.py = mouse.y;
var rect = canvas.getBoundingClientRect();
mouse.x = e.clientX - rect.left,
mouse.y = e.clientY - rect.top,
mouse.down = true;
e.preventDefault();
};
canvas.onmouseup = function(e) {
mouse.down = false;
e.preventDefault();
};
canvas.onmousemove = function(e) {
mouse.px = mouse.x;
mouse.py = mouse.y;
var rect = canvas.getBoundingClientRect();
mouse.x = e.clientX - rect.left,
mouse.y = e.clientY - rect.top,
e.preventDefault();
};
canvas.oncontextmenu = function(e) {
e.preventDefault();
};
boundsx = canvas.width - 1;
boundsy = canvas.height - 1;
ctx.strokeStyle = 'rgba(222,222,222,0.6)';
cloth = new Cloth();
update();
};
var Point = function(x, y) {
this.x = x;
this.y = y;
this.px = x;
this.py = y;
this.vx = 0;
this.vy = 0;
this.pin_x = null;
this.pin_y = null;
this.constraints = [];
};
Point.prototype.update = function(delta) {
if (mouse.down) {
var diff_x = this.x - mouse.x,
diff_y = this.y - mouse.y,
dist = Math.sqrt(diff_x * diff_x + diff_y * diff_y);
if (mouse.button == 1) {
if(dist < mouse_influence) {
this.px = this.x - (mouse.x - mouse.px) * 1.8;
this.py = this.y - (mouse.y - mouse.py) * 1.8;
}
} else if (dist < mouse_cut) this.constraints = [];
}
this.add_force(0, gravity);
delta *= delta;
nx = this.x + ((this.x - this.px) * .99) + ((this.vx / 2) * delta);
ny = this.y + ((this.y - this.py) * .99) + ((this.vy / 2) * delta);
this.px = this.x;
this.py = this.y;
this.x = nx;
this.y = ny;
this.vy = this.vx = 0
};
Point.prototype.draw = function() {
if (this.constraints.length <= 0) return;
var i = this.constraints.length;
while(i--) this.constraints[i].draw();
};
Point.prototype.resolve_constraints = function() {
if (this.pin_x != null && this.pin_y != null) {
this.x = this.pin_x;
this.y = this.pin_y;
return;
}
var i = this.constraints.length;
while(i--) this.constraints[i].resolve();
this.x > boundsx ? this.x = 2 * boundsx - this.x : 1 > this.x && (this.x = 2 - this.x);
this.y < 1 ? this.y = 2 - this.y : this.y > boundsy && (this.y = 2 * boundsy - this.y);
};
Point.prototype.attach = function(point) {
this.constraints.push(
new Constraint(this, point)
);
};
Point.prototype.remove_constraint = function(lnk) {
var i = this.constraints.length;
while(i--) if(this.constraints[i] == lnk) this.constraints.splice(i, 1);
};
Point.prototype.add_force = function(x, y ) {
this.vx += x;
this.vy += y;
};
Point.prototype.pin = function(pinx, piny) {
this.pin_x = pinx;
this.pin_y = piny;
};
var Constraint = function(p1, p2) {
this.p1 = p1;
this.p2 = p2;
this.length = spacing;
};
Constraint.prototype.resolve = function() {
var diff_x = this.p1.x - this.p2.x,
diff_y = this.p1.y - this.p2.y,
dist = Math.sqrt(diff_x * diff_x + diff_y * diff_y),
diff = (this.length - dist) / dist;
if (dist > tear_distance) this.p1.remove_constraint(this);
var px = diff_x * diff * 0.5;
var py = diff_y * diff * 0.5;
this.p1.x += px;
this.p1.y += py;
this.p2.x -= px;
this.p2.y -= py;
};
Constraint.prototype.draw = function() {
ctx.moveTo(this.p1.x, this.p1.y);
ctx.lineTo(this.p2.x, this.p2.y);
};
var Cloth = function() {
this.points = [];
var start_x = canvas.width / 2 - cloth_width * spacing / 2;
for(var y = 0; y <= cloth_height; y++) {
for(var x = 0; x <= cloth_width; x++) {
var p = new Point(start_x + x * spacing, start_y + y * spacing);
x != 0 && p.attach(this.points[this.points.length - 1]);
y == 0 && p.pin(p.x, p.y);
y != 0 && p.attach(this.points[x + (y - 1) * (cloth_width + 1)])
this.points.push(p);
}
}
};
Cloth.prototype.update = function() {
var i = physics_accuracy;
while(i--) {
var p = this.points.length;
while(p--) this.points[p].resolve_constraints();
}
i = this.points.length;
while(i--) this.points[i].update(.016);
};
Cloth.prototype.draw = function() {
ctx.beginPath();
var i = cloth.points.length;
while(i--) cloth.points[i].draw();
ctx.stroke();
};
function update() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
cloth.update();
cloth.draw();
requestAnimFrame(update);
}
打开html 文件就可以看到效果了，最好是在Chrome和Firefox 下查看效果。
也可以通过以下link 在线看效果：
http://lonely-pixel.com/lab/cloth/
其他的例子
除了以上撕扯窗布的效果，还可以看一些更多的类似效果
框架
在这个JS框架横行的年代，你有可能会问，有没有一个JS框架,让让我很容易的实现这些效果。
答案是肯定的： 有，
verlet-js 就是比较好一个。
verlet-js的定位就是一个简单的使用javascript实现的物理引擎。
官方的网址是：
http://subprotocol.com/verlet-js/
可以看的例子有。
Examples
那个蜘蛛在网上爬行的例子很炫。。