DataURL, Blob, File, Image之间的关系与转换

使用前景：

                移动端上传拍照图片太大，使用canvas压缩后base64=>blob二进制表单文件=>ajax-POST上传

// canvas转dataURL：canvas对象、转换格式、图像品质
function canvasToDataURL(canvas, format, quality) {
    return canvas.toDataURL(format||'image/jpeg', quality||1.0);
}


// DataURL转canvas
function dataURLToCanvas(dataurl, cb){
    var canvas = document.createElement('CANVAS');
    var ctx = canvas.getContext('2d');
    var img = new Image();
    img.onload = function(){
        canvas.width = img.width;
        canvas.height = img.height;
        ctx.drawImage(img, 0, 0);
        cb(canvas);
    };
    img.src = dataurl;
}

// image转canvas：图片地址
function imageToCanvas(src, cb){
    var canvas = document.createElement('CANVAS');
    var ctx = canvas.getContext('2d');
    var img = new Image();
    img.src = src;
    img.onload = function (){
        canvas.width = img.width;
        canvas.height = img.height;
        ctx.drawImage(img, 0, 0);
        cb(canvas);
    };
}

// canvas转image
function canvasToImage(canvas){
    var img = new Image();
    img.src = canvas.toDataURL('image/jpeg', 1.0);
    return img;
}

// File/Blob对象转DataURL
function fileOrBlobToDataURL(obj, cb){
    var a = new FileReader();
    a.readAsDataURL(obj);
    a.onload = function (e){
        cb(e.target.result);
    };
}
// DataURL转Blob对象
function dataURLToBlob(dataurl){
    var arr = dataurl.split(',');
    var mime = arr[0].match(/:(.*?);/)[1];
    var bstr = atob(arr[1]);
    var n = bstr.length;
    var u8arr = new Uint8Array(n);
    while(n--){
        u8arr[n] = bstr.charCodeAt(n);
    }
    return new Blob([u8arr], {type:mime});
}
// Blob转image
function blobToImage(blob, cb){
    fileOrBlobToDataURL(blob, function (dataurl){
        var img = new Image();
        img.src = dataurl;
        cb(img);
    });
}
// image转Blob
function imageToBlob(src, cb){
    imageToCanvas(src, function (canvas){
        cb(dataURLToBlob(canvasToDataURL(canvas)));
    });
}
// Blob转canvas
function BlobToCanvas(blob, cb){
    fileOrBlobToDataURL(blob, function (dataurl){
        dataURLToCanvas(dataurl, cb);
    });
}
// canvas转Blob
function canvasToBlob(canvas, cb){
    cb(dataURLToBlob(canvasToDataURL(canvas)));
}
// image转dataURL
function imageToDataURL(src, cb){
    imageToCanvas(src, function (canvas){
        cb(canvasToDataURL(canvas));
    });
}
// dataURL转image，这个不需要转，直接给了src就能用
function dataURLToImage(dataurl){
    var img = new Image();
    img.src = d;
    return img;
}