<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>绘制圆形</title>
<style>
body{
width: 100%;
height: 100%;
background: black;
}
#canvas{
background:#ffffff;
}
</style>
</head>
<body>
<canvas id="canvas" width="400" height="400">
</canvas>
<script>
var oC=document.getElementById("canvas")
var oGC=oC.getContext("2d")
oGC.arc(100,100,50,0,180*Math.PI/180,false) //num*Math.PI/180转换成角度的公式，360° true是逆时针，false是顺时针
oGC.stroke() //填充
</script>
</body>
</html>