var c=document.getElementById("php100");
var p100=c.getContext("2d");
p100.clearRect(x,y,x,y) // 擦除画布一个区域x-y、x-y
---
---
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<title></title>
</head>
<body>
<canvas id="php100" width="500" height="200"></canvas>
<div id="ds"></div>
<script type="text/javascript">
var canvas = document.getElementById("php100");
var p100 = canvas.getContext("2d");
var dir = 100;
var height = 500;
var width = 300;
var exp = 1;//像素移动的位置
function fff()
{
p100.clearRect(0,0,height,width);//清除上一次的痕迹
p100.fillStyle = "red";
p100.beginPath();
p100.arc(190, dir, 20, 0, Math.PI * 2, 1);
p100.closePath();
p100.fill();
dir = dir + exp;
if(dir==0||dir==height)
{
exp = exp * -1;
}
}
</script>
<button onclick="tt=setInterval(fff,70);">开始</button>
<button onclick="clearInterval(tt);" >停止</button>
</body>
</html>
原文来源：http://www.qwangxiao.com/
