Overhaul GD test helpers and affected tests

ext/gd/tests/avif_decode_encode.phpt

@@ -58,8 +58,8 @@ gd
 
     // Note we could also forgive a certain number of pixel differences.
     // With the current test image, we just didn't need to.
-    echo 'How many pixels are different in the two images? ';
-    print_r(calc_image_dissimilarity($img, $img_from_avif));
+    echo 'What is the mean squared error of the two images? ';
+    print_r(mse($img, $img_from_avif));
 
     unlink($outfile);
 
@@ -79,4 +79,4 @@ Encoding AVIF with illegal quality: imageavif(): Argument #3 ($quality) must be
 Encoding AVIF with illegal speed: imageavif(): Argument #4 ($speed) must be between -1 and 10
 Encoding AVIF losslessly... ok
 Decoding the AVIF we just wrote...
-How many pixels are different in the two images? 0
+What is the mean squared error of the two images? 0

ext/gd/tests/func.inc

@@ -156,39 +156,22 @@ function trycatch_dump(...$tests) {
 }
 
 /**
- * A very simple algorithm for finding the dissimilarity between images,
- * mainly useful for checking lossy compression.
+ * Calculate the mean squared error (in range 0..255**2)
  */
-
-/**
- * Calculates the euclidean distance of two RGB values.
- */
-function calc_pixel_distance(int $color1, int $color2): float
-{
-    return sqrt(
-        ((($color1 >> 16) & 0xFF) - (($color2 >> 16) & 0xFF)) ** 2 +
-        ((($color1 >>  8) & 0xFF) - (($color2 >>  8) & 0xFF)) ** 2 +
-        ((($color1 >>  0) & 0xFF) - (($color2 >>  0) & 0xFF)) ** 2
-    );
-}
-
-/**
- * Calculates dissimilarity of two images.
- *
- * The dissimilarity. 0 means the images are identical. The higher
- * the value, the more dissimilar are the images.
- */
-function calc_image_dissimilarity(GdImage $image1, GdImage $image2): float
+function mse(GdImage $image1, GdImage $image2): float
 {
     assert(imageistruecolor($image1) && imageistruecolor($image2));
     assert(imagesx($image1) === imagesx($image2) && imagesy($image1) === imagesy($image2));
-    $dissimilarity = 0;
+    $e = $count = 0;
     for ($j = 0, $m = imagesy($image1); $j < $m; $j++) {
         for ($i = 0, $n = imagesx($image1); $i < $n; $i++) {
-            $color1 = imagecolorat($image1, $i, $j);
-            $color2 = imagecolorat($image2, $i, $j);
-            $dissimilarity += calc_pixel_distance($color1, $color2);
+            $c1 = imagecolorat($image1, $i, $j);
+            $c2 = imagecolorat($image2, $i, $j);
+            $e += ((($c1 >> 16) & 255) - (($c2 >> 16) & 255)) ** 2
+                + ((($c1 >>  8) & 255) - (($c2 >>  8) & 255)) ** 2
+                + ((($c1 >>  0) & 255) - (($c2 >>  0) & 255)) ** 2;
+            $count += 3;
         }
     }
-    return $dissimilarity;
+    return $e / $count;
 }

ext/gd/tests/webp_basic.phpt

@@ -30,12 +30,12 @@ imagewebp($im1, $filename);
 $im2 = imagecreatefromwebp($filename);
 imagewebp($im2, $filename);
 echo 'Is lossy conversion close enough? ';
-var_dump(calc_image_dissimilarity($im1, $im2) < 10e5);
+var_dump(mse($im1, $im2) < 500);
 
 imagewebp($im1, $filename, IMG_WEBP_LOSSLESS);
 $im_lossless = imagecreatefromwebp($filename);
 echo 'Does lossless conversion work? ';
-var_dump(calc_image_dissimilarity($im1, $im_lossless) == 0);
+var_dump(mse($im1, $im_lossless) == 0);
 
 try {
 	imagewebp($im1, $filename, -10);