HTML如何利用css3和js实现截取图片功能

img {
  -webkit-clip-path: circle(50%);
  clip-path: circle(50%);
}

//canvas
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var img = new Image();
img.src = "pic.jpg";
img.onload = function() {
  ctx.drawImage(img, 0, 0, 400, 400, 0, 0, 200, 200);
};