code generating a sketch of mark twain. This should really be in codepen

sketch.html

<canvas id="image" width=374 height=480>
</canvas>
<canvas id="sketch" width=374 height=480>
</canvas>

sketch.js

const img = new Image();
img.crossOrigin = "anonymous";
img.src = "https://upload.wikimedia.org/wikipedia/commons/thumb/8/84/Twain1909.jpg/374px-Twain1909.jpg";
const canvas = document.getElementById("image");
const ctx = canvas.getContext("2d");
const sk = document.getElementById("sketch");
const sketch = sk.getContext("2d");
function gray9(c, x, y) {
  let d = c.getImageData(x, y, 3, 3).data;
  let gray = [];
  for (let i = 0; i < 9; i++) {
    gray[i] = 0.299*d[4*i] + 0.587*d[4*i + 1] + 0.114*d[4*i + 2];
  }
  return gray;
}
function gx(a) {
  return -a[0]+a[2]-2*a[3]+2*a[5]-a[6]+a[8];
}
function gy(a) {
  return -a[0]-2*a[1]-a[2]+a[6]+2*a[7]+a[8];
}
function styleFor(g) {
  if (g > 128 && Math.random() < (240-g)/80) {
     return 'white'
  } else if (g < 128 && Math.random() > (g-40)/80) {
     return 'black'     
  }
  return 'gray'
}
img.addEventListener("load", () => {
  ctx.drawImage(img, 0, 0);
  img.style.display = "none";
  sketch.fillStyle = "gray";
  sketch.fillRect(0, 0, 374, 480);
  let theta,x1,y1;
  for(var i = 0; i < 50000; i++) {
      console.log(theta)
    if (Math.random() < .1 || theta == null) {
      x1=Math.floor(Math.random() * 374);
      y1=Math.floor(Math.random() * 480);
      theta=null
    } else {
      x1 = Math.min(374, Math.max(0, Math.floor(x1 + (Math.random() * 17) - 9)));
      y1 = Math.min(480, Math.max(0, Math.floor(y1 + (Math.random() * 17) - 9)));
    }
    let a = gray9(ctx, x1, y1);
    let dx = gx(a);
    let dy = gy(a);
    if (Math.abs(dx)+Math.abs(dy) > 60) {
      theta = Math.atan2(dy, dx) + Math.PI/2;
    } else if (theta==null) {
      continue;
    }
    let r=Math.random()*8+2;
    dx=r*Math.cos(theta);
    dy=r*Math.sin(theta);
    sketch.beginPath();
    sketch.strokeStyle = styleFor(a[4])
    sketch.moveTo(x1-dx, y1-dy);
    sketch.lineTo(x1+dx, y1+dy);
    sketch.stroke();
  }
});