canvas toDataUrl 跨域问题

使用canvas 的 toDataUrl方法会遇到跨域问题

chrome 会报下面的错误：

 Uncaught SecurityError: Failed to execute ‘toDataURL‘ on ‘HTMLCanvasElement‘: Tainted canvases may not be exported.

Firefox 会报下面的错误：

SecurityError: The operation is insecure.

解决方案分两步：

1、在服务端设置响应头部

Access-Control-Allow-Origin: *

2、js设置image crossOrigin 为 Anonymous

    image.crossOrigin = "Anonymous";
    image.onload = function(){
        context.drawImage(this, 0, 0, 400, 300);
        var base64Url = canvas.toDataURL("image/png");
        console.log(base64Url);
    };
    image.src = imageSrc;