1<?php 2 3/** 4 * A very simple algorithm for finding the dissimilarity between images, 5 * mainly useful for checking lossy compression. 6 */ 7 8/** 9 * Gets the individual components of an RGB value. 10 * 11 * @param int $color 12 * @param int $red 13 * @param int $green 14 * @param int $blue 15 * 16 * @return void 17 */ 18function get_rgb($color, &$red, &$green, &$blue) 19{ 20 // assumes $color is an RGB value 21 $red = ($color >> 16) & 0xFF; 22 $green = ($color >> 8) & 0xFF; 23 $blue = $color & 0xFF; 24} 25 26/** 27 * Calculates the euclidean distance of two RGB values. 28 * 29 * @param int $color1 30 * @param int $color2 31 * 32 * @return int 33 */ 34function calc_pixel_distance($color1, $color2) 35{ 36 get_rgb($color1, $red1, $green1, $blue1); 37 get_rgb($color2, $red2, $green2, $blue2); 38 return sqrt( 39 pow($red1 - $red2, 2) + pow($green1 - $green2, 2) + pow($blue1 - $blue2, 2) 40 ); 41} 42 43/** 44 * Calculates dissimilarity of two images. 45 * 46 * @param resource $image1 47 * @param resource $image2 48 * 49 * @return int The dissimilarity. 0 means the images are identical. The higher 50 * the value, the more dissimilar are the images. 51 */ 52function calc_image_dissimilarity($image1, $image2) 53{ 54 // assumes image1 and image2 have same width and height 55 $dissimilarity = 0; 56 for ($i = 0, $n = imagesx($image1); $i < $n; $i++) { 57 for ($j = 0, $m = imagesy($image1); $j < $m; $j++) { 58 $color1 = imagecolorat($image1, $i, $j); 59 $color2 = imagecolorat($image2, $i, $j); 60 $dissimilarity += calc_pixel_distance($color1, $color2); 61 } 62 } 63 return $dissimilarity; 64} 65
