把其他域的图片在canvas中转换为base64时，会遇到跨域安全限制。

目前，唯一可行的方案是，把图片文件以arraybuffer的形式ajax下载下来，然后直接转base4。

但是，这样有个毛病，就是可能会浪费带宽，多下载一次。

/*    var getBase64ByUrl = function(src, callback, outputFormat) {        var canvas = document.createElement('canvas'),            ctx = canvas.getContext('2d'),            img = new Image;        img.crossOrigin = 'Anonymous';        img.onload = function() {            canvas.height = img.height;            canvas.width = img.width;            ctx.drawImage(img, 0, 0); //默认图片文件的原始size 缩小base串需加第4、5个参数            alert(5)            try{var dataURL = canvas.toDataURL(outputFormat || 'image/png');            } catch(e) { alert(e.name + ": " + e.message);                alert(JSON.stringify(e));            }            dataURL =dataURL.replace("data:image/png;base64,", "");            //dataURL =encodeURIComponent(dataURL);            alert(dataURL);            callback(dataURL);            canvas = null;        };        img.src = src;alert(6.5)    }*/var getBase64ByUrl = function(src, callback, outputFormat) {      var xhr = new XMLHttpRequest();      xhr.open('GET', src, true);      xhr.responseType = 'arraybuffer';      xhr.onload = function(e) {        if (xhr.status == 200) {          var uInt8Array = new Uint8Array(xhr.response);          var i = uInt8Array.length;          var binaryString = new Array(i);          while (i--) {            binaryString[i] = String.fromCharCode(uInt8Array[i]);          }          var data = binaryString.join('');          var base64 = window.btoa(data);          var dataUrl = "data:" + (outputFormat || "image/png") + ";base64," + base64;          alert(dataUrl)          callback.call(this, dataUrl);        }      };      xhr.send();    }

 

参考

http://stackoverflow.com/questions/22783368/android-browser-only-canvas-todataurl-throws-uncaught-error-securityerror-dom