getLuminance.ts

import colorString from 'color-string'

/**
 * Calculate a color's relative luminance based on the following algorithm:
 * https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
 *
 * @param {string} color A string representing a color (supported by {@link colorString})
 * @returns {number} The color's relative luminance 0-1
 */
export default function getLuminance(color: Parameters<typeof colorString.get.rgb>[0]): number {
  const [rInt, gInt, bInt] = colorString.get.rgb(color)!

  const rFloat = rInt / 255
  const gFloat = gInt / 255
  const bFloat = bInt / 255

  const rLuminance = rFloat < 0.03928 ? rFloat / 12.92 : luminanceList[rInt]
  const gLuminance = gFloat < 0.03928 ? gFloat / 12.92 : luminanceList[gInt]
  const bLuminance = bFloat < 0.03928 ? bFloat / 12.92 : luminanceList[bInt]

  return rLuminance * 0.2126 + gLuminance * 0.7152 + bLuminance * 0.0722
}

// This list represents ((RsRGB+0.055)/1.055) ^ 2.4 for all values from 0 to 255
// All numbers have been rounded to 4 decimals
const luminanceList = [
  0.0008, 0.001, 0.0011, 0.0013, 0.0015, 0.0017, 0.002, 0.0022, 0.0025, 0.0027, 0.003, 0.0033,
  0.0037, 0.004, 0.0044, 0.0048, 0.0052, 0.0056, 0.006, 0.0065, 0.007, 0.0075, 0.008, 0.0086,
  0.0091, 0.0097, 0.0103, 0.011, 0.0116, 0.0123, 0.013, 0.0137, 0.0144, 0.0152, 0.016, 0.0168,
  0.0176, 0.0185, 0.0194, 0.0203, 0.0212, 0.0222, 0.0232, 0.0242, 0.0252, 0.0262, 0.0273, 0.0284,
  0.0296, 0.0307, 0.0319, 0.0331, 0.0343, 0.0356, 0.0369, 0.0382, 0.0395, 0.0409, 0.0423, 0.0437,
  0.0452, 0.0467, 0.0482, 0.0497, 0.0513, 0.0529, 0.0545, 0.0561, 0.0578, 0.0595, 0.0612, 0.063,
  0.0648, 0.0666, 0.0685, 0.0704, 0.0723, 0.0742, 0.0762, 0.0782, 0.0802, 0.0823, 0.0844, 0.0865,
  0.0887, 0.0908, 0.0931, 0.0953, 0.0976, 0.0999, 0.1022, 0.1046, 0.107, 0.1095, 0.1119, 0.1144,
  0.117, 0.1195, 0.1221, 0.1248, 0.1274, 0.1301, 0.1329, 0.1356, 0.1384, 0.1413, 0.1441, 0.147,
  0.15, 0.1529, 0.1559, 0.159, 0.162, 0.1651, 0.1683, 0.1714, 0.1746, 0.1779, 0.1812, 0.1845,
  0.1878, 0.1912, 0.1946, 0.1981, 0.2016, 0.2051, 0.2086, 0.2122, 0.2159, 0.2195, 0.2232, 0.227,
  0.2307, 0.2346, 0.2384, 0.2423, 0.2462, 0.2502, 0.2542, 0.2582, 0.2623, 0.2664, 0.2705, 0.2747,
  0.2789, 0.2831, 0.2874, 0.2918, 0.2961, 0.3005, 0.305, 0.3095, 0.314, 0.3185, 0.3231, 0.3278,
  0.3325, 0.3372, 0.3419, 0.3467, 0.3515, 0.3564, 0.3613, 0.3663, 0.3712, 0.3763, 0.3813, 0.3864,
  0.3916, 0.3968, 0.402, 0.4072, 0.4125, 0.4179, 0.4233, 0.4287, 0.4342, 0.4397, 0.4452, 0.4508,
  0.4564, 0.4621, 0.4678, 0.4735, 0.4793, 0.4851, 0.491, 0.4969, 0.5029, 0.5089, 0.5149, 0.521,
  0.5271, 0.5333, 0.5395, 0.5457, 0.552, 0.5583, 0.5647, 0.5711, 0.5776, 0.5841, 0.5906, 0.5972,
  0.6038, 0.6105, 0.6172, 0.624, 0.6308, 0.6376, 0.6445, 0.6514, 0.6584, 0.6654, 0.6724, 0.6795,
  0.6867, 0.6939, 0.7011, 0.7084, 0.7157, 0.7231, 0.7305, 0.7379, 0.7454, 0.7529, 0.7605, 0.7682,
  0.7758, 0.7835, 0.7913, 0.7991, 0.807, 0.8148, 0.8228, 0.8308, 0.8388, 0.8469, 0.855, 0.8632,
  0.8714, 0.8796, 0.8879, 0.8963, 0.9047, 0.9131, 0.9216, 0.9301, 0.9387, 0.9473, 0.956, 0.9647,
  0.9734, 0.9823, 0.9911, 1,
]