原理：
1.使用FileReader 读取图片
2.使用canvas 的 getImageData 获取图片像素信息
3.将像素信息转换为CSS3 box-shadow
代码：
<!DOCTYPE HTML PUBLIC>
<html>
<head>
<meta charset="utf-8">
<title>获取图片像素颜色,转换为css3 box-shadow</title>
<style type="text/css">
body{margin: 0px; background:#f2f2f0;}
p{margin:0px;}
.title{color:#FFFF00; background:#000000; text-align:center; font-size:24px; line-height:50px; font-weight:bold;}
.file{position:absolute; 100%; font-size:90px;}
.filebtn{display:block; position:relative; height:110px; color:#FFFFFF; background:#06980e; font-size:48px; line-height:110px; text-align:center; cursor:pointer; border: 3px solid #cccccc;}
.filebtn:hover{background:#04bc0d;}
.showimg{margin:10px auto 10px auto; -webkit-transition: all .3s; transition: all .3s;}
.showimg span{1px; height:1px; display:block; margin: -1px 0px 0px -1px;}
.css_code{margin:10px; padding:10px; display:none; border:1px solid #FFCC00; font-size:12px; background:#F1F1F1; white-space:pre-wrap; word-wrap:break-word; height:300px; overflow:auto;}
.css_code:hover{
background:#DEFEDE;
}
</style>
<script type="text/javascript">
window.onload = function(){
var canvas = document.createElement('canvas');
var context = canvas.getContext('2d');
var showimg = document.getElementById('showimg');
var shadow = showimg.getElementsByTagName('span')[0];
var css_code = document.getElementById('css_code');
document.getElementById('img').onchange = function(){
var img = event.target.files[0];
// 检查能否读取图片
if(!img){
return ;
}
// 检查图片类型
if(!(img.type.indexOf('image')==0 || !img.type && /.(?:jpg|png|gif)$/.test(img.name)) ){
alert('图片只能是jpg,gif,png');
return ;
}
// 检查图片尺寸
if(img.size > 120*1024){
alert('图片不能大于120K');
return ;
}
// file reader
var reader = new FileReader();
reader.readAsDataURL(img);
reader.onload = function(e){ // reader onload start
var image = new Image();
image.src = e.target.result;
image.onload = function(){ // image onload start
var img_width = this.width;
var img_height = this.height;
// 设置画布尺寸
canvas.width = img_width;
canvas.height = img_height;
// 将图片按像素写入画布
context.drawImage(this, 0, 0, img_width, img_height);
// 读取图片像素信息
var imageData = context.getImageData(0, 0, img_width, img_height);
var arrbox = [],
length = imageData.data.length;
// 生成box-shadow
for(var i=0; i<length; i++){
if(i%4 === 0){ // 每四个元素为一个像素数据 r,g,b,alpha
var x = i/4%img_width + 1; // 横坐标
var y = Math.floor(i/4/img_width) + 1; // 纵坐标
var alpha = Math.round(imageData.data[i+3]/255*100)/100; // alpha 值
if(imageData.data[i+3]==255){ // 没有alpha 值
var hex = gethex(imageData.data[i], imageData.data[i+1], imageData.data[i+2]);
arrbox.push(x + 'px ' + y + 'px #' + hex);
}else if(alpha>0){ // 有alpha 值
var rgba = imageData.data[i] + ',' + imageData.data[i+1] + ',' + imageData.data[i+2] + ',' + alpha;
arrbox.push(x + 'px ' + y + 'px rgba(' + rgba + ')');
}
}
}
// 将数据写入dom
showimg.style.width = img_width + 'px';
showimg.style.height = img_height + 'px';
shadow.style.boxShadow = arrbox.join(',');
// 输出CSS3 box-shadow
css_code.style.display = 'block';
css_code.innerHTML = 'box-shadow: ' + arrbox.join(',');
// 获取16进制颜色
function gethex(r,g,b){
r = r.toString(16);
g = g.toString(16);
b = b.toString(16);
// 补0
r.length==1? r = '0' + r : '';
g.length==1? g = '0' + g : '';
b.length==1? b = '0' + b : '';
var hex = r + g + b;
// 简化处理,如 FFEEDD 可以写为 FED
if(r.slice(0,1)==r.slice(1,1) && g.slice(0,1)==g.slice(1,1) && b.slice(0,1)==b.slice(1,1)){
hex = r.slice(0,1) + g.slice(0,1) + b.slice(0,1);
}
return hex;
}
} // image onload end
} // reader onload end
}
}
</script>
</head>
<body>
<p class="title">获取图片像素颜色,转换为CSS3 box-shadow</p>
<p><input type="file" class="file" id="img"><label class="filebtn" for="img" title="JPG,GIF,PNG">请选择图片(小于120K)</label></p>
<p class="showimg" id="showimg"><span></span></p>
<p class="css_code" id="css_code"></p>
</body>
</html>