画像を2値化する JavaScript + canvas 【画像処理】

const canvas = document.getElementById("canvas")
let image = new Image()
image.src = "./path/to/image.png"
const THRESHOLD = 128
let ctx = canvas.getContext("2d");
ctx.drawImage(image, 0, 0, image.width, image.height)
let src = ctx.getImageData(0, 0, image.width, image.height)
let dst = ctx.createImageData(image.width, image.height)
for (let i = 0; i < src.data.length; i += 4) {
  let y = 0.2126 * src.data[i] + 0.7152 * src.data[i + 1] + 0.0722 * src.data[i + 2]
  y = parseInt(y, 10)
  if (y > THRESHOLD) {
    y = 255
  } else {
    y = 0
  }
  dst.data[i] = y
  dst.data[i + 1] = y
  dst.data[i + 2] = y
  dst.data[i + 3] = src.data[i + 3]
}
ctx.putImageData(dst, 0, 0)