<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
body{ background:black;}
#c1{ background:white;}
</style>
<script>
window.onload = function(){
var oC = document.getElementById('c1');
var oGC = oC.getContext('2d');
oGC.moveTo(200,200);
//弧度 = 角度*Math.PI/180
oGC.arc(200,200,150,0,90*Math.PI/180,true);//150为半径,200,200为圆心坐标,true顺时针方向,false逆时针画...Math.PI/180圆的一半,0-Math.PI看成是一个整圆
oGC.stroke();
};
</script>
</head>
<body>
<canvas id="c1" width="400" height="400"></canvas>
</body>
</html>