Fabric.js 介绍
Fabric.js 是一个在服务器端运行的 Node.js
扩展模块，用于在Web上绘制各种图形的 JS 库。
示例代码：
var fabric = require('fabric').fabric,
canvas = fabric.createCanvasForNode(200, 200);
canvas.add(new fabric.Rect({
top: 100,
left: 100,
width: 100,
height: 50,
angle: 30,
fill: 'rgba(255,0,0,0.5)'
}));
var out = require('fs').createWriteStream(__dirname + '/rectangle.png'),
stream = canvas.createPNGStream();
stream.on('data', function(chunk) {
out.write(chunk);
});