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<?php
/**
* A very simple algorithm for finding the dissimilarity between images,
* mainly useful for checking lossy compression.
*/
/**
* Gets the individual components of an RGB value.
*
* @param int $color
* @param int $red
* @param int $green
* @param int $blue
*
* @return void
*/
function get_rgb($color, &$red, &$green, &$blue)
{
// assumes $color is an RGB value
$red = ($color >> 16) & 0xFF;
$green = ($color >> 8) & 0xFF;
$blue = $color & 0xFF;
}
/**
* Calculates the euclidean distance of two RGB values.
*
* @param int $color1
* @param int $color2
*
* @return int
*/
function calc_pixel_distance($color1, $color2)
{
get_rgb($color1, $red1, $green1, $blue1);
get_rgb($color2, $red2, $green2, $blue2);
return sqrt(
pow($red1 - $red2, 2) + pow($green1 - $green2, 2) + pow($blue1 - $blue2, 2)
);
}
/**
* Calculates dissimilarity of two images.
*
* @param resource $image1
* @param resource $image2
*
* @return int The dissimilarity. 0 means the images are identical. The higher
* the value, the more dissimilar are the images.
*/
function calc_image_dissimilarity($image1, $image2)
{
// assumes image1 and image2 have same width and height
$dissimilarity = 0;
for ($i = 0, $n = imagesx($image1); $i < $n; $i++) {
for ($j = 0, $m = imagesy($image1); $j < $m; $j++) {
$color1 = imagecolorat($image1, $i, $j);
$color2 = imagecolorat($image2, $i, $j);
$dissimilarity += calc_pixel_distance($color1, $color2);
}
}
return $dissimilarity;
}