HTML5 Canvas - How to Draw a line over an image background?

Question

I am trying to draw a line on top of an image background - in an HTML5 Canvas . However always the line gets drawn behind the image . Actually the line gets drawn first and

Answer 1

To be more efficient, assuming you are going to be doing multiple redraws of this line or lines, would be to set the CSS background-image of the canvas to be your image.

<canvas style="background-image:url('images/main_timerand3papers.png');"></canvas>

Answer 2

Totally untested code, but did you tried something like this?

function drawbackground(canvas, context, onload){

    var imagePaper = new Image();


        imagePaper.onload = function(){


            context.drawImage(imagePaper,100, 20, 500,500);
            onload(canvas, context);
        };

      imagePaper.src = "images/main_timerand3papers.png";
}

and then call the method like this...

 drawbackground(canvas, context, drawlines);

Answer 3

Or you can try this:

drawbackground(canvas, context);
context.globalCompositeOperation = 'destination-atop';
drawlines(canvas, context);

Answer 4

Change your image onload to something like this:

imagePaper.onload = function () {
    context.drawImage( imagePaper, 100, 20, 500, 500 );
    drawLines( canvas, context );
};

Then make sure you remove the earlier call to drawLines.

The important take away to this solution, is that the onload function will be executed sometime in the future, whereas the drawLines function is executed immediately. You must always be careful of how you structure your callbacks, especially when nesting them.