我正在通过画布将一张图片裁剪成一个形状。现在我想在它的外面添加一个投影。我该怎么做呢？

var canvas1 = document.createElement("canvas");
var ctx = canvas1.getContext("2d");
var canvas1_img = new Image();
canvas1_img.src = "https://placeimg.com/300/300/nature";
canvas1_img.addEventListener("load", draw);

function draw() {
  var canvas_wd = canvas1.width;
  var canvas_ht = canvas1.height;
  var y1 = 20;
  var y2 = 130;

  ctx.beginPath();
  ctx.moveTo(0, y1);
  ctx.lineTo(0, y2);
  ctx.lineTo(canvas_wd, canvas_ht);
  ctx.lineTo(canvas_wd, 0);
  ctx.closePath();
  ctx.clip();

  ctx.drawImage(canvas1_img, 0, 0);
  document.body.appendChild(canvas1);
}