Compare / calculate the difference between 2 images using the Root Mean Squared Error (RMSE) metric

compare_images_rmse.php

<?php

/**
 * Calculate the Root Mean Squared Error (RMSE) metric between 2 images.
 *
 * Quick and dirty pure PHP implementation
 * of the Root Mean Squared Error metric (RMSE)
 * used to compare two images and calculate their difference:
 * - Requires the php-gd extension to be available.
 * - Using the four (RGB+A) channels.
 * - 0-1 normalised using the max difference value.
 *
 * Note: The 2 images must be exactly the same size. Ensure it always.
 *
 * @copyright 2023 onwards Eloy Lafuente (stronk7) {@link https://stronk7.com}
 * @license   https://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */

$image1 = '1.png';
$image2 = '2.png';

$nrmse = nrmse($image1, $image2);
echo "PHP NRMSE: {$nrmse}" . PHP_EOL;

function nrmse($image1, $image2): float {
    $image1 = imagecreatefrompng($image1);
    $image2 = imagecreatefrompng($image2);

    $width = imagesx($image1);
    $height = imagesy($image1);

    $mse = 0;
    for ($x = 0; $x < $width; $x++) {
        for ($y = 0; $y < $height; $y++) {
            $rgb1 = imagecolorat($image1, $x, $y);
            $rgb2 = imagecolorat($image2, $x, $y);

            $red1 = ($rgb1 >> 16) & 0xFF;
            $green1 = ($rgb1 >> 8) & 0xFF;
            $blue1 = $rgb1 & 0xFF;
            $trans1 = ($rgb1 >> 24) & 0x7F;

            $red2 = ($rgb2 >> 16) & 0xFF;
            $green2 = ($rgb2 >> 8) & 0xFF;
            $blue2 = $rgb2 & 0xFF;
            $trans2 = ($rgb2 >> 24) & 0x7F;

            $mse += pow(($red1 - $red2) / 255, 2);
            $mse += pow(($green1 - $green2) / 255, 2);
            $mse += pow(($blue1 - $blue2) / 255, 2);
            $mse += pow(($trans1 - $trans2) / 127, 2);
        }
    }

    $mse /= $width * $height * 4;

    return sqrt($mse);
}