Question

I’d like to build a simple HTML page that includes JavaScript to perform a form POST with image data that is embedded in the HTML vs a file off disk.

I’ve looked at this post which would work with regular form data but I’m stumped on the image data.

JavaScript post request like a form submit

Answer 1

** UPDATE ** Feb. 2014 **

New and improved version available as a jQuery plugin:
https://github.com/CoeJoder/jquery.image.blob

Usage:

$('img').imageBlob().ajax('/upload', {
    complete: function(jqXHR, textStatus) { console.log(textStatus); } 
});

---

Requirements

• the canvas element (HTML 5)
• FormData
• XMLHttpRequest.send(:FormData)
• Blob constructor
• Uint8Array
• atob(), escape()

Thus the browser requirements are:

• Chrome: 20+
• Firefox: 13+
• Internet Explorer: 10+
• Opera: 12.5+
• Safari: 6+

Note: The images must be of the same-origin as your JavaScript, or else the browser security policy will prevent calls to canvas.toDataURL() (for more details, see this SO question: Why does canvas.toDataURL() throw a security exception?). A proxy server can be used to circumvent this limitation via response header injection, as described in the answers to that post.

Here is a jsfiddle of the below code. It should throw an error message, because it’s not submitting to a real URL (‘/some/url’). Use firebug or a similar tool to inspect the request data and verify that the image is serialized as form data (click “Run” after the page loads):

Example Markup

<img id="someImage" src="../img/logo.png"/>

The JavaScript

(function() {
    // access the raw image data
    var img = document.getElementById('someImage');
    var canvas = document.createElement('canvas');
    var ctx = canvas.getContext('2d');
    canvas.width = img.width;
    canvas.height = img.height;
    ctx.drawImage(img, 0, 0);
    var dataUrl = canvas.toDataURL('image/png');
    var blob = dataUriToBlob(dataUrl);

    // submit as a multipart form, along with any other data
    var form = new FormData();
    var xhr = new XMLHttpRequest();
    xhr.open('POST', '/some/url', true);    // plug-in desired URL
    xhr.onreadystatechange = function() {
        if (xhr.readyState == 4) {
            if (xhr.status == 200) {
                alert('Success: ' + xhr.responseText);
            } else {
                alert('Error submitting image: ' + xhr.status);
            }
        }
    };
    form.append('param1', 'value1');
    form.append('param2', 'value2');
    form.append('theFile', blob);
    xhr.send(form);

    function dataUriToBlob(dataURI) {
        // serialize the base64/URLEncoded data
        var byteString;
        if (dataURI.split(',')[0].indexOf('base64') >= 0) {
            byteString = atob(dataURI.split(',')[1]);
        }
        else {
            byteString = unescape(dataURI.split(',')[1]);
        }

        // parse the mime type
        var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0]

        // construct a Blob of the image data
        var array = [];
        for(var i = 0; i < byteString.length; i++) {
            array.push(byteString.charCodeAt(i));
        }
        return new Blob(
            [new Uint8Array(array)],
            {type: mimeString}
        );
    }
})();

References

SO: ‘Convert DataURI to File and append to FormData