arc（路径命令）
context.arc(centerX, centerY, radius, startingRadianAngle, endingRadianAngle)
给定中心点，半径和起始和结束角度的圆弧。角度表示为弧度。要将度数转换为弧度，你可以使用以下公式：radians = degrees * Math.PI / 180;。
角度 0 直接面向弧的中心。
默认情况下，弧顺时针绘制，可选[true | false]参数指示弧逆时针绘制：context.arc(10,10,20,0,Math.PI*2,true)
<!doctype html>
<html>
<head>
<style>
body{ background-color:white; }
#canvas{border:1px solid red; }
</style>
<script>
window.onload=(function(){
// get a reference to the canvas element and its context
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
// arguments
var centerX=50;
var centerY=50;
var radius=30;
var startingRadianAngle=Math.PI*2*; // start at 90 degrees == centerY+radius
var endingRadianAngle=Math.PI*2*.75; // end at 270 degrees (==PI*2*.75 in radians)
// A partial circle (i.e. arc) drawn using the "arc" command
ctx.beginPath();
ctx.arc(centerX, centerY, radius, startingRadianAngle, endingRadianAngle);
ctx.stroke();
}); // end window.onload
</script>
</head>
<body>
<canvas id="canvas" width=200 height=150></canvas>
</body>
</html>
要绘制一个完整的圆，你可以使 endsAngle = startingAngle + 360 度（360 度== Math.PI2）。
<!doctype html>
<html>
<head>
<style>
body{ background-color:white; }
#canvas{border:1px solid red; }
</style>
<script>
window.onload=(function(){
// get a reference to the canvas element and its context
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
// arguments
var centerX=50;
var centerY=50;
var radius=30;
var startingRadianAngle=0; // start at 0 degrees
var endingRadianAngle=Math.PI*2; // end at 360 degrees (==PI*2 in radians)
// A complete circle drawn using the "arc" command
ctx.beginPath();
ctx.arc(centerX, centerY, radius, startingRadianAngle, endingRadianAngle);
ctx.stroke();
}); // end window.onload
</script>
</head>
<body>
<canvas id="canvas" width=200 height=150></canvas>
</body>
</html>