##图片URL Blob Base64相互转换 ###urlToBase64 实现：

var b64datas = {};
function urlToBase64(url, callback) {
    if (b64datas[url]) {
        callback && callback(b64datas[url]);
        return;
    }
    var image = new Image();
    image.onload = function() {
        var canvas = document.createElement('CANVAS');
        var ctx = canvas.getContext('2d');

        canvas.width = this.width;
        canvas.height = this.height;
        ctx.drawImage(this, 0, 0);

        var b64data = b64datas[url] = canvas.toDataURL('base64');
        callback && callback(b64data);
    };
    image.src = url;
};

示例：

urlToBase64('http://b6.hucdn.com/upload/im/1607/27/85273548789370_100x100.png!imthumb.jpg', function(b64data){
	//剩余逻辑
});

###base64ToBlob 实现：

function base64ToBlob(b64data, contentType, sliceSize) {
	sliceSize || (sliceSize = 512);

    var byteCharacters = atob(b64data);
    var byteArrays = [];

    for (var offset = 0; offset < byteCharacters.length; offset += sliceSize) {
        var slice = byteCharacters.slice(offset, offset + sliceSize);

        var byteNumbers = [];
        for (var i = 0; i < slice.length; i++) {
            byteNumbers.push(slice.charCodeAt(i));
        }

        byteArrays.push(new Uint8Array(byteNumbers));
    }
    return new Blob(byteArrays, {
        type: contentType
    });
};

示例：

var url = 'http://b6.hucdn.com/upload/im/1607/27/85273548789370_100x100.png!imthumb.jpg';
urlToBase64(url, function(b64data){		
	var blob = base64ToBlob(b64data.split(',')[1], 'image/png');
	//剩余逻辑
});

blobToBase64

实现：

function blobToBase64(blob, callback) {
    var fileReader = new FileReader();
    fileReader.onload = function(e) {
        callback && callback(e.target.result);
    };
    fileReader.readAsDataURL(blob);
};

示例：

blobToBase64(blob, function(b64data){
	//剩余逻辑
});