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<?php
// Limited Use License – March 1, 2025
// This source code is provided for public use under the following conditions :
// It may be downloaded, compiled, and executed, including in publicly accessible environments.
// Modification is strictly prohibited without the express written permission of the author.
// © Michel Leonard 2025
// The classic CIE ΔE2000 implementation, which operates on two L*a*b* colors, and returns their difference.
// "l" ranges from 0 to 100, while "a" and "b" are unbounded and commonly clamped to the range of -128 to 127.
function ciede_2000($l_1, $a_1, $b_1, $l_2, $a_2, $b_2) {
// Working in PHP with the CIEDE2000 color-difference formula.
// k_l, k_c, k_h are parametric factors to be adjusted according to
// different viewing parameters such as textures, backgrounds...
$k_l = $k_c = $k_h = 1.0;
$n = (sqrt($a_1 * $a_1 + $b_1 * $b_1) + sqrt($a_2 * $a_2 + $b_2 * $b_2)) * 0.5;
$n = $n * $n * $n * $n * $n * $n * $n;
// A factor involving chroma raised to the power of 7 designed to make
// the influence of chroma on the total color difference more accurate.
$n = 1.0 + 0.5 * (1.0 - sqrt($n / ($n + 6103515625.0)));
// Application of the chroma correction factor.
$c_1 = sqrt($a_1 * $a_1 * $n * $n + $b_1 * $b_1);
$c_2 = sqrt($a_2 * $a_2 * $n * $n + $b_2 * $b_2);
// atan2 is preferred over atan because it accurately computes the angle of
// a point (x, y) in all quadrants, handling the signs of both coordinates.
$h_1 = atan2($b_1, $a_1 * $n);
$h_2 = atan2($b_2, $a_2 * $n);
$h_1 += 2.0 * M_PI * ($h_1 < 0.0);
$h_2 += 2.0 * M_PI * ($h_2 < 0.0);
$n = abs($h_2 - $h_1);
// Cross-implementation consistent rounding.
if (M_PI - 1E-14 < $n && $n < M_PI + 1E-14)
$n = M_PI;
// When the hue angles lie in different quadrants, the straightforward
// average can produce a mean that incorrectly suggests a hue angle in
// the wrong quadrant, the next lines handle this issue.
$h_m = ($h_1 + $h_2) * 0.5;
$h_d = ($h_2 - $h_1) * 0.5;
if (M_PI < $n) {
$h_d += M_PI;
// 📜 Sharma’s formulation doesn’t use the next line, but the one after it,
// and these two variants differ by ±0.0003 on the final color differences.
$h_m += M_PI;
// $h_m += $h_m < M_PI ? M_PI : -M_PI;
}
$p = 36.0 * $h_m - 55.0 * M_PI;
$n = ($c_1 + $c_2) * 0.5;
$n = $n * $n * $n * $n * $n * $n * $n;
// The hue rotation correction term is designed to account for the
// non-linear behavior of hue differences in the blue region.
$r_t = -2.0 * sqrt($n / ($n + 6103515625.0))
* sin(M_PI / 3.0 * exp($p * $p / (-25.0 * M_PI * M_PI)));
$n = ($l_1 + $l_2) * 0.5;
$n = ($n - 50.0) * ($n - 50.0);
// Lightness.
$l = ($l_2 - $l_1) / ($k_l * (1.0 + 0.015 * $n / sqrt(20.0 + $n)));
// These coefficients adjust the impact of different harmonic
// components on the hue difference calculation.
$t = 1.0 + 0.24 * sin(2.0 * $h_m + M_PI * 0.5)
+ 0.32 * sin(3.0 * $h_m + 8.0 * M_PI / 15.0)
- 0.17 * sin($h_m + M_PI / 3.0)
- 0.20 * sin(4.0 * $h_m + 3.0 * M_PI / 20.0);
$n = $c_1 + $c_2;
// Hue.
$h = 2.0 * sqrt($c_1 * $c_2) * sin($h_d) / ($k_h * (1.0 + 0.0075 * $n * $t));
// Chroma.
$c = ($c_2 - $c_1) / ($k_c * (1.0 + 0.0225 * $n));
// Returning the square root ensures that dE00 accurately reflects the
// geometric distance in color space, which can range from 0 to around 185.
return sqrt($l * $l + $h * $h + $c * $c + $c * $h * $r_t);
}
// GitHub Project : https://github.com/michel-leonard/ciede2000-color-matching
// Online Tests : https://michel-leonard.github.io/ciede2000-color-matching
// L1 = 8.2 a1 = 45.7 b1 = 3.1
// L2 = 9.1 a2 = 50.8 b2 = -3.2
// CIE ΔE00 = 3.6399743999 (Bruce Lindbloom, Netflix’s VMAF, ...)
// CIE ΔE00 = 3.6399608019 (Gaurav Sharma, OpenJDK, ...)
// Deviation between implementations ≈ 1.4e-5
// See the source code comments for easy switching between these two widely used ΔE*00 implementation variants.
/////////////////////////////////////////////////
/////////////////////////////////////////////////
//////////// ////////////
//////////// CIEDE2000 Driver ////////////
//////////// ////////////
/////////////////////////////////////////////////
/////////////////////////////////////////////////
// Reads a CSV file specified as the first command-line argument. For each line, this program
// in PHP displays the original line with the computed Delta E 2000 color difference appended.
// The C driver can offer CSV files to process and programmatically check the calculations performed there.
// Example of a CSV input line : 38.5,-5,72.8,33,7,76
// Corresponding output line : 38.5,-5,72.8,33,7,76,8.470154878587844987814255112329
if (!isset($argv[1]))
die("Usage: php ciede-2000-driver.php <filename>");
$fp = fopen($argv[1], 'r');
while (($str = fgets($fp, 1024)) !== false)
echo rtrim($str), ',', ciede_2000(...array_map('floatval', explode(',', $str))), "\n";
fclose($fp);