Function posterize

plugins/plugin-color/src/index.ts:413–437

* Apply a posterize effect * @param n the amount to adjust the contrast, minimum threshold is two * @example * ```ts * import { Jimp } from "jimp"; * * const image = await Jimp.read("test/image.png"); * * image.posterize(5); * ```

(image: I, n: number)

   * ```
   */
  posterize<I extends JimpClass>(image: I, n: number) {
    if (typeof n !== "number") {
      throw new Error("n must be numbers");
    }

    // minimum of 2 levels
    if (n < 2) {
      n = 2;
    }

    image.scan((_, __, idx) => {
      const r = image.bitmap.data[idx]!;
      const g = image.bitmap.data[idx + 1]!;
      const b = image.bitmap.data[idx + 2]!;

      image.bitmap.data[idx] =
        (Math.floor((r / 255) * (n - 1)) / (n - 1)) * 255;
      image.bitmap.data[idx + 1] =
        (Math.floor((g / 255) * (n - 1)) / (n - 1)) * 255;
      image.bitmap.data[idx + 2] =
        (Math.floor((b / 255) * (n - 1)) / (n - 1)) * 255;
    });

    return image;
  },

  /**
   * Removes colour from the image using ITU Rec 709 luminance values