html5-canvas Animation Don't draw animations in your event handlers (a simple sketch app)


Example

During mousemove you get flooded with 30 mouse events per second. You might not be able to redraw your drawings at 30 times per second. Even if you can, you're probably wasting computing power by drawing when the browser is not ready to draw (wasted == across display refresh cycles).

Therefore it makes sense to separate your users input events (like mousemove) from the drawing of your animations.

  • In event handlers, save all the event variables that control where drawings are positioned on the Canvas. But don't actually draw anything.

  • In a requestAnimationFrame loop, render all the drawings to the Canvas using the saved information.

By not drawing in the event handlers, you are not forcing Canvas to try to refresh complex drawings at mouse event speeds.

By doing all drawing in requestAnimationFrame you gain all the benefits described in here Use 'requestanimationFrame' not 'setInterval' for animation loops.

Annotated Code:

<!doctype html>
<html>
<head>
<style>
    body{ background-color: ivory; }
    #canvas{border:1px solid red; }
</style>
<script>
window.onload=(function(){

    function log(){console.log.apply(console,arguments);}

    // canvas variables
    var canvas=document.getElementById("canvas");
    var ctx=canvas.getContext("2d");
    var cw=canvas.width;
    var ch=canvas.height;
    // set canvas styling
    ctx.strokeStyle='skyblue';
    ctx.lineJoint='round';
    ctx.lineCap='round';
    ctx.lineWidth=6;

    // handle windows scrolling & resizing
    function reOffset(){
        var BB=canvas.getBoundingClientRect();
        offsetX=BB.left;
        offsetY=BB.top;        
    }
    var offsetX,offsetY;
    reOffset();
    window.onscroll=function(e){ reOffset(); }
    window.onresize=function(e){ reOffset(); }

    // vars to save points created during mousemove handling
    var points=[];
    var lastLength=0;

    // start the  animation loop
    requestAnimationFrame(draw);

    canvas.onmousemove=function(e){handleMouseMove(e);}


    function handleMouseMove(e){
        // tell the browser we're handling this event
        e.preventDefault();
        e.stopPropagation();

        // get the mouse position
        mouseX=parseInt(e.clientX-offsetX);
        mouseY=parseInt(e.clientY-offsetY);

        // save the mouse position in the points[] array
        // but don't draw anything
        points.push({x:mouseX,y:mouseY});
    }

    function draw(){
        // No additional points? Request another frame an return
        var length=points.length;
        if(length==lastLength){requestAnimationFrame(draw);return;}
        
        // draw the additional points
        var point=points[lastLength];
        ctx.beginPath();
        ctx.moveTo(point.x,point.y)
        for(var i=lastLength;i<length;i++){
            point=points[i];
            ctx.lineTo(point.x,point.y);
        }
        ctx.stroke();
        
        // request another animation loop
        requestAnimationFrame(draw);
    }

}); // end window.onload
</script>
</head>
<body>
    <h4>Move mouse over Canvas to sketch</h4>
    <canvas id="canvas" width=512 height=512></canvas>
</body>
</html>