--- /dev/null
+<?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 $dissimilarity;
+}
--- /dev/null
+--TEST--\r
+imagewebp() and imagecreatefromwebp() - basic test\r
+--SKIPIF--\r
+<?php\r
+if (!extension_loaded('gd')) die('skip gd extension not available');\r
+if (!function_exists('imagewebp') || !function_exists('imagecreatefromwebp'))\r
+ die('skip WebP support not available');\r
+?>\r
+--FILE--\r
+<?php\r
+require_once __DIR__ . '/similarity.inc';\r
+\r
+$filename = __DIR__ . '/webp_basic.webp';\r
+\r
+$im1 = imagecreatetruecolor(75, 75);\r
+$white = imagecolorallocate($im1, 255, 255, 255);\r
+$red = imagecolorallocate($im1, 255, 0, 0);\r
+$green = imagecolorallocate($im1, 0, 255, 0);\r
+$blue = imagecolorallocate($im1, 0, 0, 255);\r
+imagefilledrectangle($im1, 0, 0, 74, 74, $white);\r
+imageline($im1, 3, 3, 71, 71, $red);\r
+imageellipse($im1, 18, 54, 36, 36, $green);\r
+imagerectangle($im1, 41, 3, 71, 33, $blue);\r
+imagewebp($im1, $filename);\r
+\r
+$im2 = imagecreatefromwebp($filename);\r
+imagewebp($im2, $filename);\r
+var_dump(calc_image_dissimilarity($im1, $im2) < 10e5);\r
+?>\r
+--CLEAN--\r
+<?php\r
+@unlink(__DIR__ . '/webp_basic.webp');\r
+?>\r
+--EXPECT--\r
+bool(true)\r