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将html+css页面生成canvas：html2canvas.js

html2canvas可以通过纯JS对浏览器端进行"截屏"，但"截图"的精确度还有待提高，部分css不可识别，如css3的filter等，所以在canvas中不能完全呈现原画面样式。

支持的浏览器

Firefox 3.5+
Google Chrome
Opera 12+
IE11+
Safari 6+

基本语法 
//两个参数：所需要截图的元素id，截图后要执行的函数， canvas为截图后返回的最后一个canvas
html2canvas(document.getElementById('id')).then(function(canvas) {document.body.appendChild(canvas);});
可用参数

参数名称 类型 默认值 描述
allowTaint boolean false Whether to allow cross-origin images to taint the canvas---允许跨域
background string #fff Canvas background color, if none is specified in DOM. Set undefined for transparent---canvas的背景颜色，如果没有设定默认透明
height number null Define the heigt of the canvas in pixels. If null, renders with full height of the window.---canvas高度设定
letterRendering boolean false Whether to render each letter seperately. Necessary if letter-spacing is used.---在设置了字间距的时候有用
logging boolean false Whether to log events in the console.---在console.log()中输出信息
proxy string undefined Url to the proxy which is to be used for loading cross-origin images. If left empty, cross-origin images won't be loaded.---代理地址
taintTest boolean true Whether to test each image if it taints the canvas before drawing them---是否在渲染前测试图片
timeout number 0 Timeout for loading images, in milliseconds. Setting it to 0 will result in no timeout.---图片加载延迟，默认延迟为0，单位毫秒
width number null Define the width of the canvas in pixels. If null, renders with full width of the window.---canvas宽度
useCORS boolean false Whether to attempt to load cross-origin images as CORS served, before reverting back to proxy--这个我也不知道是干嘛的

例子 
<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <title>html2canvas example</title>
    <script type="text/javascript" src="html2canvas.js"></script>
</head>
<script type="text/javascript">
function takeScreenshot() {
  console.log('test');
    html2canvas(document.getElementById('view'), {
        onrendered: function(canvas) {
            document.body.appendChild(canvas);
        },
      // width: 300,
    // height: 300
    });
}
</script>
<body>
    <div id="view" style="background:url(test.jpg) 50%; width: 700px; height: 500px;">
        <input type="button" value="截图" onclick="takeScreenshot()">
    </div>
</body>
</html>
下面是我爱模板网利用 html2canvas 生成的图片：



遇到的问题
    1、由于 html2canvas.js 不支持css3的高斯模糊滤镜，所以，我是先用js将图片生成高斯模糊的canvas定位到了底层，再使用html2canvas.js生成整个页面，参见：canvas实现高斯模糊
    2、html2canvas.js使用了Promise，IE11-浏览器不支持，要做下兼容处理： 
if (!window.Promise) {document.write('<script src="//5.jimth.com/download/js/promise/es6-promise.min.js"><\/script><script>ES6Promise.polyfill()<\/script>');')

TAGS: html2canvas.js