arcTo（路径命令）

Created: November-22, 2018

context.arcTo(pointX1, pointY1, pointX2, pointY2, radius);

绘制具有给定半径的圆弧。在当前笔位置形成的楔形内部顺时针绘制弧，并给出两点：Point1 和 Point2。

在弧之前自动绘制连接当前笔位置和弧开始的线。

StackOverflow 文档

<!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 it's context
    var canvas=document.getElementById("canvas");
    var ctx=canvas.getContext("2d");

    // arguments
    var pointX0=25;
    var pointY0=80;
    var pointX1=75;
    var pointY1=0;
    var pointX2=125;
    var pointY2=80;
    var radius=25;

    // A circular arc drawn using the "arcTo" command. The line is automatically drawn.
    ctx.beginPath();
    ctx.moveTo(pointX0,pointY0);
    ctx.arcTo(pointX1, pointY1, pointX2, pointY2, radius);
    ctx.stroke();

}); // end window.onload
</script>
</head>
<body>
    <canvas id="canvas" width=200 height=150></canvas>
</body>
</html>