generate.mjs

// node@v20.11 or higher needed

import { ColorType, colorTypeToChannels, encode } from "png-tools";
import fs from "fs/promises";
import { deflate } from "pako";
import {
  lab_to_lch,
  lch_to_lab,
  rgb_to_oklab,
  angle_delta,
  oklab_to_rgb,
} from "./util/color.mjs";
import * as path from "path";
import { fileURLToPath } from "url";

const colorType = ColorType.RGB;
const channels = colorTypeToChannels(colorType);
const size = 512;
const data = new Uint8ClampedArray(size * size * channels);

const SelectiveColor = (hueTarget, hueRange) => {
  return (rgb) => {
    let [R, G, B] = rgb;
    let [L, C, H] = lab_to_lch(rgb_to_oklab([R, G, B]));

    const hueDelta = Math.abs(angle_delta(H, hueTarget));
    const hueFact = 1 - Math.max(0, Math.min(1, hueDelta / hueRange));

    // desaturate
    C *= hueFact;

    return oklab_to_rgb(lch_to_lab([L, C, H]));
  };
};

// Set this to null if you don't want to filter the LUT
// and would like to just generate the base PNG
// const filter = null;

// Here we target a specific hue (34º) and smoothly desaturate as we
// move away from that by 25º in either dir
const filter = SelectiveColor(34, 25);

for (let by = 0; by < 8; by++) {
  for (let bx = 0; bx < 8; bx++) {
    for (let g = 0; g < 64; g++) {
      for (let r = 0; r < 64; r++) {
        const x = r + bx * 64;
        const y = g + by * 64;
        const idx = x + y * size;

        let R = Math.floor((r * 255.0) / 63.0 + 0.5);
        let G = Math.floor((g * 255.0) / 63.0 + 0.5);
        let B = Math.floor(((bx + by * 8.0) * 255.0) / 63.0 + 0.5);

        if (filter) {
          [R, G, B] = filter([R, G, B]);
        }

        data[idx * channels + 0] = R;
        data[idx * channels + 1] = G;
        data[idx * channels + 2] = B;
      }
    }
  }
}

const png = encode(
  {
    colorType,
    width: size,
    height: size,
    data,
  },
  deflate
);

const __dirname = path.dirname(fileURLToPath(import.meta.url));
const file = path.resolve(__dirname, "lookup_selective_color.png");
await fs.writeFile(file, png);