PHP getcolorluminance

getcolorluminance.php

function getcolorluminance($r, $g, $b) {

  // type checking
  if (!is_numeric($r) || !is_numeric($g) || !is_numeric($b)) return false;

  // range checking
  if (!($r >= 0 && $r <= 255)) return false;
  if (!($g >= 0 && $g <= 255)) return false;
  if (!($b >= 0 && $b <= 255)) return false;

  // https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef

  $r_srgb = $r/255;
  $g_srgb = $g/255;
  $b_srgb = $b/255;

  $r_srgb = $r_srgb <= 0.03928 ? ($r_srgb / 12.92) : pow((($r_srgb + 0.055)/1.055),2.4);
  $g_srgb = $g_srgb <= 0.03928 ? ($g_srgb / 12.92) : pow((($g_srgb + 0.055)/1.055),2.4);
  $b_srgb = $b_srgb <= 0.03928 ? ($b_srgb / 12.92) : pow((($b_srgb + 0.055)/1.055),2.4);

  return  0.2126 * $r_srgb + 0.7152 * $g_srgb + 0.0722 * $b_srgb;
}