W3C Relative Luminance

luminance.ts

// Get the luminance of an RGB color [0x000000, 0xFFFFFF] using the w3c procedure for relative luminance
// https://www.w3.org/TR/WCAG20-TECHS/G17.html#G17-tests
// 
// This code is untested

const getChannelLuminance = (color: number, shift: number) => {
  let result = color;
  result &= 0xff << shift;
  result >>= shift;
  result /= 255;

  if (result <= 0.03928) {
    result /= 12.92;
  } else {
    result += 0.055;
    result /= 1.055;
    result **= 2.4;
  }

  return result;
};

export const luminance = (color: number) => {
  const r = getChannelLuminance(color, 16);
  const g = getChannelLuminance(color, 8);
  const b = getChannelLuminance(color, 0);

  const l = 0.2126 * r + 0.7152 * g + 0.0722 * b;

  return l;
};