threshold effect

threshold.js

function(context, threshold, width, height) {
    let image, data, r, g, b, color;

    image = context.getImageData(0, 0, width, height);

    data = image.data;

    for (let i = 0; i< data.length; i = i+4) {
      r = data[i];
      g = data[i+1];
      b = data[i+2];

      if ((r + b + g) / 3 < threshold) {
        color = 0; // black
      } else {
        color = 255; // white
      }

      data[i] = data[i+1] = data[i+2] = color;
    }

    image.data = data;

    context.putImageData(image, 0, 0);
  }