Using raw image data from ajax request for data URI

I'm trying to use a combination of Ajax and data URIs to load a JPEG image and extract its EXIF data with a single HTTP request. I am modifying a library (https://github.com/kennydude/photosphere) to do this; currently this library uses two HTTP requests to set the source of the image and to get the EXIF data.

Getting the EXIF works, no problem. However I am having difficulty using the raw data from the ajax request as source for the image.

Source code for a small test of the technique:

<!DOCTYPE html>
<html>
<head>
<script type='text/javascript'>

function init()
{
    // own ajax library - using it to request a test jpg image
    new Ajax().sendRequest
    (
        "/images/photos/badger.jpg",
         { method : "GET",
            callback: function(xmlHTTP)
            {

                var encoded = btoa (unescape(encodeURIComponent(xmlHTTP.responseText)));
                var dataURL="data:image/jpeg;base64,"+encoded;

                document.getElementById("image").src = dataURL;
            }
        }
    );
}

</script>
<script type="text/javascript" src="http://www.free-map.org.uk/0.6/js/lib/Ajax.js"></script>
</head>
<body onload='init()'>
<img id="image" alt="data url loaded image" />
</body>
</html>

I get what looks like sensible jpeg data sent back, and the length (in bytes) of the raw data and the base64-encoded-then-unencoded-again raw data is the same. However the attempt to set the image src fails on both Firefox (25) and Chrome (31) (current versions) - chrome displays "broken image" icon suggesting the src is an invalid format.

I used this mozilla page for info on base64 encoding/decoding:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Base64_encoding_and_decoding

Any idea what might be wrong? Looking around I can create the base64 encoded image server side but can it be done client side like this? For one thing, base64 encoding server side obviously increases the data size and the whole purpose of this exercise is to cut down the amount of data being transferred from the server, as well as the number of requests.

Thanks, Nick

Answers:

Answer

Thanks for that. I've done a bit more digging on this and it turns out there is a solution at least on current versions of Firefox and Chrome (EDIT: IE10 works too). You can use XMLHttpRequest2 and use a typed array (Uint8Array). The following code works:

<!DOCTYPE html>
<html>
<head>
<script type='text/javascript'>

function init()
{
    var xmlHTTP = new XMLHttpRequest();
    xmlHTTP.open('GET','/images/photos/badger.jpg',true);

    // Must include this line - specifies the response type we want
    xmlHTTP.responseType = 'arraybuffer';

    xmlHTTP.onload = function(e)
    {

        var arr = new Uint8Array(this.response);


        // Convert the int array to a binary string
        // We have to use apply() as we are converting an *array*
        // and String.fromCharCode() takes one or more single values, not
        // an array.
        var raw = String.fromCharCode.apply(null,arr);

        // This works!!!
        var b64=btoa(raw);
        var dataURL="data:image/jpeg;base64,"+b64;
        document.getElementById("image").src = dataURL;
    };

    xmlHTTP.send();
}

</script>
</head>
<body onload='init()'>
<img id="image" alt="data url loaded image" />
</body>
</html>

Basically you ask for a binary response, then create an 8-bit unsigned int view of the data before converting it back into a (binary-friendly) string String.fromCharCode(). The apply is necessary as String.fromCharCode() does not accept an array argument. You then use btoa(), create your data url and it then works.

The following resources were useful for this:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Typed_arrays?redirectlocale=en-US&redirectslug=JavaScript%2FTyped_arrays

and

http://www.html5rocks.com/en/tutorials/file/xhr2/

Nick

Answer

I had trouble with the ArrayBuffer -> String -> Base64 method described above, but ran across another method using Blob that worked great. It's not a way to convert raw data to Base 64 (as in the title), but it is a way to display raw image data (as in the actual question):

var xhr = new XMLHttpRequest();
xhr.responseType = 'arraybuffer';
xhr.onload = function() {
    var blb = new Blob([xhr.response], {type: 'image/png'});
    var url = (window.URL || window.webkitURL).createObjectURL(blb);
    image.src = url;
}

xhr.open('GET', 'http://whatever.com/wherever');
xhr.send();

All credit goes to Jan Miksovsky, author of this fiddle. I just stumbled across it and thought it'd make a useful addition to this discussion.

Answer

You will have to do base64 encoding on the server side as the responseText is treated as a String, and the response data that the server is sending is binary.

Answer

I've been working for two days on this issue since I needed a solution to render the User's Outlook Profile Picture from the raw data received from Microsoft Graft. I have implemented all the solutions above, with no success. Then I found this git: get base64 raw data of image from responseBody using jquery ajax

In my case, I just replaced "data:image/png;base64," with "data:image/jpg;base64,"

It works like a charm.

Answer

Modern ES6 powered solution for image downloading: (without specifying image type)

async function downloadImageFromUrl(url) {    // returns dataURL
  const xmlHTTP = new XMLHttpRequest();
  xmlHTTP.open('GET', url, true);
  xmlHTTP.responseType = 'blob';
  const imageBlob = await new Promise((resolve, reject) => {
    xmlHTTP.onload = e => xmlHTTP.status >= 200 && xmlHTTP.status < 300 && xmlHTTP.response.type.startsWith('image/') ? resolve(xmlHTTP.response) : reject(Error(`wrong status or type: ${xmlHTTP.status}/${xmlHTTP.response.type}`));
    xmlHTTP.onerror = reject;
    xmlHTTP.send();
  });
  return blobToDataUrl(imageBlob);
}

function blobToDataUrl(blob) { return new Promise(resolve => {
  const reader = new FileReader();    // https://developer.mozilla.org/en-US/docs/Using_files_from_web_applications
  reader.onload = e => resolve(e.target.result);
  reader.readAsDataURL(blob);
})}

Usage:

downloadImageFromUrl('https://a.b/img.png').then(console.log, console.error)
Answer

Nick's answer works very well. But when I did this with a fairly large file, I got a stack overflow on

var raw = String.fromCharCode.apply(null,arr);

Generating the raw string in chunks worked well for me.

var raw = '';
var i,j,subArray,chunk = 5000;
for (i=0,j=arr.length; i<j; i+=chunk) {
   subArray = arr.subarray(i,i+chunk);
   raw += String.fromCharCode.apply(null, subArray);
}

Tags

Recent Questions

Top Questions

Home Tags Terms of Service Privacy Policy DMCA Contact Us

©2020 All rights reserved.