convert base64 to raw binary data held in a string

dataURItoBlob

/** 
via http://stackoverflow.com/questions/4998908/convert-data-uri-to-file-then-append-to-formdata/5100158

via http://www.smartjava.org/content/face-detection-using-html5-javascript-webrtc-websockets-jetty-and-javacvopencv
**/

function dataURItoBlob(dataURI) {
    var binary = atob(dataURI.split(',')[1]);
    var array = [];
    for(var i = 0; i < binary.length; i++) {
        array.push(binary.charCodeAt(i));
    }
    return new Blob([new Uint8Array(array)], {type: 'image/jpeg'});
}


///

From there, appending the data to a form such that it will be uploaded as a file is easy:

var dataURL = canvas.toDataURL('image/jpeg', 0.5);
var blob = dataURItoBlob(dataURL);
var fd = new FormData(document.forms[0]);
fd.append("canvasImage", blob);