arcTo(x1,y1,x2,y2,r);
当前点x0,y0;圆弧与(x0,y0-x1,y1)相切，与(x1,y1-x2,y2)相切；
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>arcTo</title> </head> <body> <canvas id=‘myCanvas‘ width=‘200‘ height=‘100‘> your browser does not support canvas </canvas> <script type="text/javascript"> var c = document.getElementById(‘myCanvas‘); var cxt = c.getContext(‘2d‘); cxt.beginPath(); cxt.moveTo(20, 20); cxt.lineTo(70, 20); cxt.arcTo(120, 20, 120, 70, 50); cxt.lineTo(120, 120); cxt.stroke(); cxt.moveTo(70, 20); cxt.lineTo(120, 20); cxt.stroke(); </script> </body> </html>
原文：http://www.cnblogs.com/stono/p/4666236.html