Dynamic image loading in Processing.js

Question

I'm building something that uses processing.js to operate on JPEG images dragged from a users machine into a canvas element. Currently, it works like so:

canvas.addEventListener("dragenter", dragEnter, false);
canvas.addEventListener("dragexit", dragExit, false);
canvas.addEventListener("dragover", dragOver, false);
canvas.addEventListener("drop", drop, false);
...
function drop(evt) {
...
    var files = evt.dataTransfer.files;
    handleFiles(files);
}
function handleFiles(files) {
    var file = files[0];
    reader = new FileReader();
    reader.onloadend = handleReaderLoadEnd;
    reader.readAsDataURL(file);
}
function handleReaderLoadEnd(evt) {
    var p=Processing.getInstanceById('canvas');
    p.setImage( evt.target.result );
}

...in JS, then in the .pjs script attached to the canvas (<canvas datasrc="/assets/draw.pjs" id="canvas" width="978" height="652">):

PImage imported_image;
void setImage(s) {
    imported_image = requestImage(s , "" , image_loaded_callback());
}

All working fine so far. Now the problem: I would have thought the callback on requestImage (or loadImage for that matter) would happen when the image is ready to render. However if I do this:

void image_loaded_callback() {
    image(imported_image, 0, 0);
}

nothing renders. I can get round the problem by waiting 10 frames and then rendering, but that seems like a very ugly (and probably unreliable) solution. Are there any better ways to do this? Any help much appreciated!

Answer 1

if the image loading failed, the callback will never call. but imported_image.sourceImg referent to the real img element, may be this will help. you can use it to detect the image load state. ex: imported_image.sourceImg.complete ;imported_image.sourceImg.onerror;

Answer 2

Actually, the best way I've found to do this is checking the loaded property of the image in the draw loop. So:

void draw() {
    if(imported_image != null ) {
        if(imported_image.loaded) {
           image(imported_image,0,0,500,500);
        }
    }
}

Not a callback, but same end result. Had more luck with that property than @DouO's sourceImg.complete.