Canvas Check before submission

Question

I have a page where a user can draw on the canvas and save the image to a file on the server. The canvas has a default black background. Is there a way to check if the user has drawn anything on the canvas before submitting the data URL representation of the image of a canvas with the toDataURL() function? So if the user doesn't draw anything on the canvas(it will be a blank canvas with a black background), the image wont be created on the server. Should I loop through each and every pixel of the canvas to determine this? Here is what I'm doing currently:

var currentPixels  = context.getImageData(0, 0, 600, 400);
    for (var y = 0; y < currentPixels.height; y += 1) {

         for (var x = 0; x < currentPixels.width; x += 1) {
                for (var c = 0; c < 3; c += 1) {

                var i = (y*currentPixels.width + x)*4 + c;

                if(currentPixels.data[i]!=0)
                break;

                }

         }

  }

Answer 1

Assuming that something is drawn on the canvas when you click on it, you could bind a click even to the canvas that set a flag indicating that the canvas has been drawn on.

For example using jQuery:

var canvasDrawnOn = false;

$("#canvas").click(function(){
    canvasDrawnOn = true;
});