clip API实现遮罩

 1 (function () {
 2 
 3     var img;
 4     var canvas = document.getElementById("canvas");
 5     var con = canvas.getContext("2d");
 6     var circleX = 0, circleY = 100, speed = 1;
 7 
 8     function loadImage() {
 9         img = new Image();
10         img.src = "11.jpg";
11     }
12 
13     function render() {
14         circleX += speed;
15         if (circleX > 300) {
16             speed = -1;
17         }
18         if (circleX < 100) {
19             speed = 1;
20         }
21         con.clearRect(0,0,400,300);
22         con.save();
23         con.beginPath();
24         con.arc(circleX,circleY,100,0,2*Math.PI);
25         con.closePath();
26         con.clip();
27         con.drawImage(img,0,0);
28         con.restore();
29 
30         requestAnimationFrame(render);
31     }
32 
33     function init() {
34         loadImage();
35         render();
36     }
37 
38     init();
39 })();

 1 <!DOCTYPE html>
 2 <html lang="en">
 3 <head>
 4     <meta charset="UTF-8">
 5     <title>Title</title>
 6 </head>
 7 <body>
 8 <canvas id="canvas" width="400" height="300"></canvas>
 9 <script src="main.js"></script>
10 </body>
11 </html>