JavaScript 圖片與Base64數據互相轉換腳本

JavaScript 圖片與Base64數據互相轉換腳本

注: 轉換過程當中注意跨域問題、測試頁是否支持相關標籤建立、dom結構.

方法一：非Html 5使用FileReader

使用XMLHttpRequest將圖像加載爲blob，接着使用FileReader API將其轉換爲dataURL。

function toDataURL(url, callback) {
  var xhr = new XMLHttpRequest();
  xhr.onload = function() {
    var reader = new FileReader();
    reader.onloadend = function() {
      callback(reader.result);
    }
    reader.readAsDataURL(xhr.response);
  };
  xhr.open('GET', url);
  xhr.responseType = 'blob';
  xhr.send();
}



toDataURL('https://www.gravatar.com/avatar/00000000000000000000000000000000?d=identicon&f=y', function(dataUrl) {
  console.log('結果:', dataUrl)
})

方法二：Html 5使用Canvas

將圖像加載到Image對象中，將其繪製到Canvas上，最後轉換爲dataURL

function toDataURL(src, callback, outputFormat) {
  var img = new Image();
  img.crossOrigin = 'Anonymous';
  img.onload = function() {
    var canvas = document.createElement('CANVAS');
    var ctx = canvas.getContext('2d');
    var dataURL;
    canvas.height = this.naturalHeight;
    canvas.width = this.naturalWidth;
    ctx.drawImage(this, 0, 0);
    dataURL = canvas.toDataURL(outputFormat);
    callback(dataURL);
  };
  img.src = src;
  if (img.complete || img.complete === undefined) {
    img.src = "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==";
    img.src = src;
  }
}



toDataURL(
  'https://www.gravatar.com/avatar/00000000000000000000000000000000?d=identicon&f=y',
  function(dataUrl) {
    console.log('結果:', dataUrl)
  }
)

Base64數據轉換爲圖片
這裏是把Base64的字符串轉換爲Blob ，這樣就能夠把數據傳到一些圖片服務器。

function base64toBlob(base64Data, contentType) {
    contentType = contentType || '';
    var sliceSize = 1024;
    var byteCharacters = atob(base64Data);
    var bytesLength = byteCharacters.length;
    var slicesCount = Math.ceil(bytesLength / sliceSize);
    var byteArrays = new Array(slicesCount);



    for (var sliceIndex = 0; sliceIndex < slicesCount; ++sliceIndex) {
        var begin = sliceIndex * sliceSize;
        var end = Math.min(begin + sliceSize, bytesLength);



        var bytes = new Array(end - begin);
        for (var offset = begin, i = 0; offset < end; ++i, ++offset) {
            bytes[i] = byteCharacters[offset].charCodeAt(0);
        }
        byteArrays[sliceIndex] = new Uint8Array(bytes);
    }
    return new Blob(byteArrays, { type: contentType });
}

附: blob 對象 轉 blob url

window.URL = window.URL || window.webkitURL;
var url = window.URL.createObjectURL(blob); // base64toBlob 方法轉出的 blob 對象, 轉 url
// $0 是一個 img 標籤
$0.src = url

/*-----------------------------------------------------------------------*/
// 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，這個不須要轉，直接給了src就能用
function dataURLToImage(dataurl){
    var img = new Image();
    img.src = d;
    return img;
}
/*-----------------------------------------------------------------------*/

// 使用示例

imageToDataURL('https://www.baidu.com/favicon.ico', res=> console.log(res))

參考:

• http://www.javashuo.com/article/p-ohvauvhu-ka.html
• https://majing.io/posts/10000008231204

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