html5 canvas裁剪区域

<!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>canvas裁剪区域</title>
<script src="js/modernizr.js"></script>
</head>
 
<body>
<script type="text/javascript">
window.addEventListener('load',eventWindowLoaded,false);
function eventWindowLoaded(){
    canvasApp();
}
function canvasSupport(){
    return Modernizr.canvas;
}
function canvasApp(){
    if(!canvasSupport()){
        return;
    }else{
        var theCanvas = document.getElementById('canvas')
        var context = theCanvas.getContext("2d")
 
    }
    drawScreen();
    function drawScreen(){
        context.fillStyle = "green";
        context.fillRect(10,10,200,200);
        context.save();
        context.beginPath();
         
        context.rect(0,0,50,50);
        context.clip();
        //红色圆
        context.beginPath();
        context.strokeStyle="red";
        context.lineWidth=5;
        context.arc(100,100,100,(Math.PI/180)*0,(Math.PI/180)*360,false);
        //整理
        context.stroke();
        context.closePath();
        context.restore();
        //再次裁剪整个画布
        context.beginPath();
        context.rect(0,0,200,200);
        context.clip();
        //绘制一个没有裁剪的蓝线
        context.beginPath();
        context.strokeStyle="blue";
        context.lineWidth=5;
        context.arc(100,100,50,(Math.PI/180)*0,(Math.PI/180)*360,false);
        //整圆
        context.stroke();
        context.closePath();
         
         
         
    }
         
}
 
 
</script>
<canvas id="canvas" width="500" height="500">
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</canvas>
</body>
</html>