DataURL, Blob, File, Image之間的關係與轉換

使用前景：

移動端上傳拍照圖片太大，使用canvas壓縮後base64=>blob二進位表單文件=>ajax-POST上傳

// canvas轉dataURL：canvas對象、轉換格式、圖像品質
function canvasToDataURL(canvas, format, quality) {
return canvas.toDataURL(format||"image/jpeg", quality||1.0);
}
// DataURL轉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轉canvas：圖片地址
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轉image
function canvasToImage(canvas){
var img = new Image();
img.src = canvas.toDataURL("image/jpeg", 1.0);
return img;
}
// File/Blob對象轉DataURL
function fileOrBlobToDataURL(obj, cb){
var a = new FileReader();
a.readAsDataURL(obj);
a.onload = function (e){
cb(e.target.result);
};
}
// DataURL轉Blob對象
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轉image
function blobToImage(blob, cb){
fileOrBlobToDataURL(blob, function (dataurl){
var img = new Image();
img.src = dataurl;
cb(img);
});
}
// image轉Blob
function imageToBlob(src, cb){
imageToCanvas(src, function (canvas){
cb(dataURLToBlob(canvasToDataURL(canvas)));
});
}
// Blob轉canvas
function BlobToCanvas(blob, cb){
fileOrBlobToDataURL(blob, function (dataurl){
dataURLToCanvas(dataurl, cb);
});
}
// canvas轉Blob
function canvasToBlob(canvas, cb){
cb(dataURLToBlob(canvasToDataURL(canvas)));
}
// image轉dataURL
function imageToDataURL(src, cb){
imageToCanvas(src, function (canvas){
cb(canvasToDataURL(canvas));
});
}
// dataURL轉image，這個不需要轉，直接給了src就能用
function dataURLToImage(dataurl){
var img = new Image();
img.src = d;
return img;
}

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