I used this in the end:
function draw(imgData, frameCount) {
var r = new FileReader();
r.readAsBinaryString(imgData);
r.onload = function(){
var img=new Image();
img.onload = function() {
cxt.drawImage(img, 0, 0, canvas.width, canvas.height);
}
img.src = "data:image/jpeg;base64,"+window.btoa(r.result);
};
}
I needed to read the bytes into a string before using btoa().