html5 canvas arcTo()

<!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>html5 canvas从圆开始</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(){
		//颜色粉色，线宽10,
	
		
		 
		 context.beginPath();
	         context.strokeStyle="red";
                 context.lineWidth=4;
		 context.lineTo(100,200);
		 context.arcTo(350,350,100,100,20);
		 context.stroke();
		 context.closePath();
		 
		 //以下为了给个直观的感觉，画的参考线
		 context.beginPath();
		 context.strokeStyle="#ddd";
		 context.lineWidth=2;
		 context.moveTo(0,350);
		 context.lineTo(350,350);
		 context.stroke();
		 context.closePath();
		  
		 context.beginPath();
		 context.moveTo(350,0);
		 context.lineTo(350,350);
		 context.stroke();
		 context.closePath();
		 
		 context.beginPath();
		 context.moveTo(100,0);
		 context.lineTo(100,100);
		 context.stroke();
		 context.closePath();
		 
		 
		 context.beginPath();
		 context.moveTo(0,100);
		 context.lineTo(100,100);
		 context.stroke();
		 context.closePath();
		 
		 
		 context.beginPath();
		 context.moveTo(350,350);
		 context.lineTo(100,200);
		 context.stroke();
		 context.closePath();
		 
		 
		 context.beginPath();
		 context.moveTo(100,100);
		 context.lineTo(350,350);
		 context.stroke();
		 context.closePath();
   }
}


</script>
<canvas id="canvas" width="500" height="500">
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</canvas>
</body>
</html>

context.arcTo(350,350,100,100,20);

x1,y1,x2,y2,radius

首先需要有个路径context.lineTo(100,200);

然后从创建一个小弧线

然后这个过程比较复杂加蛋疼！！！

大概的意思是（x1,y1）到（x2,y2）的路径相切

看着这个图大概能理解了