HTML5画布-绘制环形扇形

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
var strokewidth=100;
var cx=cw/2;
var cy=ch/2;
var PI=Math.PI;
var radius=Math.min(cw,ch)/2-strokewidth/2;
ctx.lineWidth=strokewidth;
ctx.lineCap='butt';
ctx.beginPath();
ctx.arc(cx,cy,radius,-PI*3/4,-PI/16);
ctx.strokeStyle='mediumVioletRed';
ctx.stroke();

body{ background-color:white; }
#canvas{border:1px solid red; }

<canvas id="canvas" width=378 height=378></canvas>