html5-canvas Animation Animate at a specified time (an animated clock)
Example
This example animates a clock showing the seconds as a filled wedge
Annotated Code:
<!doctype html>
<html>
<head>
<style>
body{ background-color:white; }
#canvas{border:1px solid red; }
</style>
<script>
window.onload=(function(){
// canvas related variables
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
// canvas styling for the clock
ctx.strokeStyle='lightgray';
ctx.fillStyle='skyblue';
ctx.lineWidth=5;
// cache often used values
var PI=Math.PI;
var fullCircle=PI*2;
var sa=-PI/2; // == the 12 o'clock angle in context.arc
// start the animation
requestAnimationFrame(animate);
function animate(currentTime){
// get the current seconds value from the system clock
var date=new Date();
var seconds=date.getSeconds();
// clear the canvas
ctx.clearRect(0,0,cw,ch);
// draw a full circle (== the clock face);
ctx.beginPath();
ctx.moveTo(100,100);
ctx.arc(100,100,75,0,fullCircle);
ctx.stroke();
// draw a wedge representing the current seconds value
ctx.beginPath();
ctx.moveTo(100,100);
ctx.arc(100,100,75,sa,sa+fullCircle*seconds/60);
ctx.fill();
// request another loop of animation
requestAnimationFrame(animate);
}
}); // end $(function(){});
</script>
</head>
<body>
<canvas id="canvas" width=512 height=512></canvas>
</body>
</html>
