gilbarbara/colorizr

import hex2rgb from './hex2rgb';
import { invariant, isString, messages, round } from './modules/utils';
import parseCSS from './parse-css';

/**
 * Get the luminance of a color.
 */
export default function luminance(input: string): number {
  invariant(isString(input), messages.inputString);

  const { r, g, b } = hex2rgb(parseCSS(input));

  const rgb = [r / 255, g / 255, b / 255];

  for (let index = 0; index < rgb.length; index++) {
    if (rgb[index] <= 0.03928) {
      rgb[index] /= 12.92;
    } else {
      rgb[index] = ((rgb[index] + 0.055) / 1.055) ** 2.4;
    }
  }

  return round(0.2126 * rgb[0] + 0.7152 * rgb[1] + 0.0722 * rgb[2], 4);
}