diff --git a/ext/gd/tests/avif_decode_encode.phpt b/ext/gd/tests/avif_decode_encode.phpt
index cb7f1ce95a67c..054eb52467637 100644
--- a/ext/gd/tests/avif_decode_encode.phpt
+++ b/ext/gd/tests/avif_decode_encode.phpt
@@ -14,7 +14,7 @@ gd
 --FILE--
 <?php
 
-    require_once __DIR__ . '/similarity.inc';
+    require_once __DIR__ . '/func.inc';
 
     $infile = __DIR__  . '/girl.avif';
     $outfile = __DIR__  . '/test.avif';
@@ -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, $img_from_avif));
+    echo 'What is the mean squared error of the two images? ',
+        mse($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
diff --git a/ext/gd/tests/bug43475.phpt b/ext/gd/tests/bug43475.phpt
index 454d2daad0f1a..4d3db399a30e2 100644
--- a/ext/gd/tests/bug43475.phpt
+++ b/ext/gd/tests/bug43475.phpt
@@ -13,7 +13,7 @@ gd
 ?>
 --FILE--
 <?php
-require_once __DIR__ . '/similarity.inc';
+require_once __DIR__ . '/func.inc';
 
 function setStyleAndThickness($im, $color, $thickness)
 {
@@ -59,8 +59,8 @@ imageline($im, 200, 100, 700, 100, IMG_COLOR_STYLED);
 imageline($im, 700, 100, 700, 600, IMG_COLOR_STYLED);
 imageline($im, 700, 600, 200, 100, IMG_COLOR_STYLED);
 
-$ex = imagecreatefrompng(__DIR__ . '/bug43475.png');
-var_dump(calc_image_dissimilarity($ex, $im) < 1e-5);
+imagepalettetotruecolor($im);
+test_image_equals_file(__DIR__ . '/bug43475.png', $im);
 ?>
 --EXPECT--
-bool(true)
+The images are equal.
diff --git a/ext/gd/tests/bug43475.png b/ext/gd/tests/bug43475.png
index 774270f63db86..d53a9a3322ae6 100644
Binary files a/ext/gd/tests/bug43475.png and b/ext/gd/tests/bug43475.png differ
diff --git a/ext/gd/tests/bug52070.phpt b/ext/gd/tests/bug52070.phpt
index 6bbe2363a650b..8bc653ae95ef8 100644
--- a/ext/gd/tests/bug52070.phpt
+++ b/ext/gd/tests/bug52070.phpt
@@ -20,6 +20,7 @@ imagedashedline($im, 800, 400, 500, 800, $color);
 imagedashedline($im, 800, 400, 600, 800, $color);
 imagedashedline($im, 800, 400, 700, 800, $color);
 imagedashedline($im, 800, 400, 800, 800, $color);
+imagepalettetotruecolor($im);
 include_once __DIR__ . '/func.inc';
 test_image_equals_file(__DIR__ . '/bug52070.png', $im);
 ?>
diff --git a/ext/gd/tests/bug52070.png b/ext/gd/tests/bug52070.png
index 5ce54b121bc2e..a6f9a3b875d56 100644
Binary files a/ext/gd/tests/bug52070.png and b/ext/gd/tests/bug52070.png differ
diff --git a/ext/gd/tests/bug64641.phpt b/ext/gd/tests/bug64641.phpt
index edfebee99c92a..d2e51817b9b48 100644
--- a/ext/gd/tests/bug64641.phpt
+++ b/ext/gd/tests/bug64641.phpt
@@ -13,7 +13,7 @@ if (!(imagetypes() & IMG_PNG)) {
 ?>
 --FILE--
 <?php
-require_once __DIR__ . '/similarity.inc';
+require_once __DIR__ . '/func.inc';
 
 $im = imagecreatetruecolor(640, 480);
 
@@ -31,15 +31,7 @@ $points = array(
 );
 imagefilledpolygon($im, $points, 0xFFFF00);
 
-$ex = imagecreatefrompng(__DIR__ . '/bug64641.png');
-if (($diss = calc_image_dissimilarity($ex, $im)) < 1e-5) {
-    echo "IDENTICAL";
-} else {
-    echo "DISSIMILARITY: $diss";
-}
-imagedestroy($ex);
-
-imagedestroy($im);
+test_image_equals_file(__DIR__ . '/bug64641.png', $im);
 ?>
 --EXPECT--
-IDENTICAL
+The images are equal.
diff --git a/ext/gd/tests/bug72913.phpt b/ext/gd/tests/bug72913.phpt
index 9eb9c24653530..eea77b6ed56b1 100644
--- a/ext/gd/tests/bug72913.phpt
+++ b/ext/gd/tests/bug72913.phpt
@@ -23,8 +23,16 @@ imagesavealpha($dst, true);
 
 imagecopy($dst, $src, 0,0, 0,0, 50,50);
 
-include_once __DIR__ . '/func.inc';
-test_image_equals_file(__DIR__ . '/bug72913.png', $dst);
+var_dump(imagecolorsforindex($dst, imagecolorat($dst, 0, 0)));
 ?>
 --EXPECT--
-The images are equal.
+array(4) {
+  ["red"]=>
+  int(255)
+  ["green"]=>
+  int(255)
+  ["blue"]=>
+  int(255)
+  ["alpha"]=>
+  int(127)
+}
diff --git a/ext/gd/tests/bug72913.png b/ext/gd/tests/bug72913.png
deleted file mode 100644
index 83eeb81fb16ac..0000000000000
Binary files a/ext/gd/tests/bug72913.png and /dev/null differ
diff --git a/ext/gd/tests/func.inc b/ext/gd/tests/func.inc
index 0f10aa7d83dee..0668742d2b8a7 100644
--- a/ext/gd/tests/func.inc
+++ b/ext/gd/tests/func.inc
@@ -78,9 +78,7 @@ function test_image_equals_file(string $filename, GdImage $actual): void
         save_actual_image($actual);
         return;
     }
-    $actual = test_to_truecolor($actual);
     $expected = imagecreatefrompng($filename);
-    $expected = test_to_truecolor($expected);
     test_image_equals_image($expected, $actual, true);
 }
 
@@ -95,6 +93,7 @@ function test_image_equals_file(string $filename, GdImage $actual): void
  */
 function test_image_equals_image(GdImage $expected, GdImage $actual, bool $save_actual = false): void
 {
+    assert(imageistruecolor($expected) && imageistruecolor($actual));
     $exp_x = imagesx($expected);
     $exp_y = imagesy($expected);
     $act_x = imagesx($actual);
@@ -126,35 +125,13 @@ function test_image_equals_image(GdImage $expected, GdImage $actual, bool $save_
     }
 }
 
-/**
- * Returns the truecolor version of an image.
- *
- * @param resource $image
- * @return resource
- */
-function test_to_truecolor($image)
-{
-    if (imageistruecolor($image)) {
-        return $image;
-    } else {
-        $width = imagesx($image);
-        $height = imagesy($image);
-        $result = imagecreatetruecolor($width, $height);
-        imagecopy($result, $image, 0,0, 0,0, $width, $height);
-        return $result;
-    }
-}
-
 /**
  * Saves an actual image to disk.
  *
  * The image is saved right beside the temporary .php test file with the
  * extension .out.png.
- *
- * @param resource $image
- * @return void
  */
-function save_actual_image($image)
+function save_actual_image(GdImage $image): void
 {
     $pathinfo = pathinfo($_SERVER['SCRIPT_FILENAME']);
     $filename = "{$pathinfo['dirname']}/{$pathinfo['filename']}.out.png";
@@ -178,3 +155,23 @@ function trycatch_dump(...$tests) {
     }
 }
 
+/**
+ * Calculate the mean squared error (in range 0..255**2)
+ */
+function mse(GdImage $image1, GdImage $image2): float
+{
+    assert(imageistruecolor($image1) && imageistruecolor($image2));
+    assert(imagesx($image1) === imagesx($image2) && imagesy($image1) === imagesy($image2));
+    $e = $count = 0;
+    for ($j = 0, $m = imagesy($image1); $j < $m; $j++) {
+        for ($i = 0, $n = imagesx($image1); $i < $n; $i++) {
+            $c1 = imagecolorat($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 $e / $count;
+}
diff --git a/ext/gd/tests/gd276.phpt b/ext/gd/tests/gd276.phpt
index a12bce3983049..ad7b6343f520d 100644
--- a/ext/gd/tests/gd276.phpt
+++ b/ext/gd/tests/gd276.phpt
@@ -15,6 +15,8 @@ $filename = __DIR__ . "/gd276.bmp";
 imagebmp($orig, $filename, true);
 $saved = imagecreatefrombmp($filename);
 var_dump($saved !== false);
+imagepalettetotruecolor($orig);
+imagepalettetotruecolor($saved);
 test_image_equals_image($orig, $saved);
 ?>
 --EXPECT--
diff --git a/ext/gd/tests/imagebmp_basic.phpt b/ext/gd/tests/imagebmp_basic.phpt
index b3d1212d014cd..31a62e674a845 100644
--- a/ext/gd/tests/imagebmp_basic.phpt
+++ b/ext/gd/tests/imagebmp_basic.phpt
@@ -13,7 +13,7 @@ $im = imagecreate(100, 100);
 imagecolorallocate($im, 0, 0, 0);
 $white = imagecolorallocate($im, 255, 255, 255);
 imageline($im, 10,10, 89,89, $white);
-
+imagepalettetotruecolor($im);
 require __DIR__ . "/func.inc";
 test_image_equals_file(__DIR__ . "/imagebmp_basic.png", $im);
 ?>
diff --git a/ext/gd/tests/imagebmp_basic.png b/ext/gd/tests/imagebmp_basic.png
index 26fc7d0afcf9f..d52dd04fb0987 100644
Binary files a/ext/gd/tests/imagebmp_basic.png and b/ext/gd/tests/imagebmp_basic.png differ
diff --git a/ext/gd/tests/imagecolorset_basic.phpt b/ext/gd/tests/imagecolorset_basic.phpt
index 5ed9a1ab8600e..7677b85948aa4 100644
--- a/ext/gd/tests/imagecolorset_basic.phpt
+++ b/ext/gd/tests/imagecolorset_basic.phpt
@@ -25,6 +25,7 @@ $bg = imagecolorat($im, 0, 0);
 // Set the background to be blue
 imagecolorset($im, $bg, 0, 0, 255);
 
+imagepalettetotruecolor($im);
 include_once __DIR__ . '/func.inc';
 test_image_equals_file(__DIR__ . '/imagecolorset_basic.png', $im);
 imagedestroy($im);
diff --git a/ext/gd/tests/imagecolorset_basic.png b/ext/gd/tests/imagecolorset_basic.png
index b9e77d116932d..6a37d1d87176f 100644
Binary files a/ext/gd/tests/imagecolorset_basic.png and b/ext/gd/tests/imagecolorset_basic.png differ
diff --git a/ext/gd/tests/imagecreatefromstring_bmp.phpt b/ext/gd/tests/imagecreatefromstring_bmp.phpt
index 34db04297e3aa..746e566fdbfea 100644
--- a/ext/gd/tests/imagecreatefromstring_bmp.phpt
+++ b/ext/gd/tests/imagecreatefromstring_bmp.phpt
@@ -22,7 +22,7 @@ $bmp = "\x42\x4D\x3E\x00\x00\x00\x00\x00\x00\x00\x3E\x00\x00\x00\x28\x00"
      . "\x01\x00\x05\x00\x00\x00\x02\x00\x01\x01\x01\x00\x06\x00\x00\x00"
      . "\x0A\x00\x00\x00\x0A\x00\x00\x00\x00\x01";
 $im = imagecreatefromstring($bmp);
-
+imagepalettetotruecolor($im);
 include_once __DIR__ . '/func.inc';
 test_image_equals_file(__DIR__ . '/imagecreatefromstring_bmp.png', $im);
 ?>
diff --git a/ext/gd/tests/imagecreatefromstring_bmp.png b/ext/gd/tests/imagecreatefromstring_bmp.png
index 04ffc83c468e3..a9ae65fdc9827 100644
Binary files a/ext/gd/tests/imagecreatefromstring_bmp.png and b/ext/gd/tests/imagecreatefromstring_bmp.png differ
diff --git a/ext/gd/tests/imagegammacorrect_variation1.phpt b/ext/gd/tests/imagegammacorrect_variation1.phpt
index 55370aa49cf87..67a8e403377ca 100644
--- a/ext/gd/tests/imagegammacorrect_variation1.phpt
+++ b/ext/gd/tests/imagegammacorrect_variation1.phpt
@@ -26,7 +26,7 @@ $half2 =  imagefilledarc ( $image, 75, 75, 70, 70, 0, -180, $gray, IMG_ARC_PIE )
 
 $gamma = imagegammacorrect($image, 1, 5);
 var_dump((bool) $gamma);
-
+imagepalettetotruecolor($image);
 include_once __DIR__ . '/func.inc';
 test_image_equals_file(__DIR__ . '/imagegammacorrect_variation1.png', $image);
 ?>
diff --git a/ext/gd/tests/imagegammacorrect_variation1.png b/ext/gd/tests/imagegammacorrect_variation1.png
index f9718890968a2..8b0e80c5ea034 100644
Binary files a/ext/gd/tests/imagegammacorrect_variation1.png and b/ext/gd/tests/imagegammacorrect_variation1.png differ
diff --git a/ext/gd/tests/imagegammacorrect_variation2.phpt b/ext/gd/tests/imagegammacorrect_variation2.phpt
index 33c89d205a1a7..7621ff98c8945 100644
--- a/ext/gd/tests/imagegammacorrect_variation2.phpt
+++ b/ext/gd/tests/imagegammacorrect_variation2.phpt
@@ -33,6 +33,9 @@ function test_gamma($in, $out, $constructor)
 
     imagegammacorrect($im, $in, $out);
 
+    if ($constructor === "imagecreate") {
+        imagepalettetotruecolor($im);
+    }
     $filename = __DIR__ . DIRECTORY_SEPARATOR
         . "imagegammacorrect_variation2_{$in}_{$out}.png";
     $kind = $constructor === 'imagecreate' ? 'palette' : 'truecolor';
diff --git a/ext/gd/tests/imagetruecolortopalette_basic.phpt b/ext/gd/tests/imagetruecolortopalette_basic.phpt
index 2f1c2961a603c..4abe6cf0b37ad 100644
--- a/ext/gd/tests/imagetruecolortopalette_basic.phpt
+++ b/ext/gd/tests/imagetruecolortopalette_basic.phpt
@@ -25,7 +25,7 @@ $half =  imagefilledarc ( $image, 75, 75, 70, 70, 0, 180, $a, IMG_ARC_PIE );
 $half2 =  imagefilledarc ( $image, 75, 55, 80, 70, 0, -180, $b, IMG_ARC_PIE );
 
 var_dump(imagetruecolortopalette($image, true, 2));
-
+imagepalettetotruecolor($image);
 include_once __DIR__ . '/func.inc';
 test_image_equals_file(__DIR__ . '/imagetruecolortopalette_basic.png', $image);
 ?>
diff --git a/ext/gd/tests/imagetruecolortopalette_basic.png b/ext/gd/tests/imagetruecolortopalette_basic.png
index 7d042f2ac3da2..79e14eef35908 100644
Binary files a/ext/gd/tests/imagetruecolortopalette_basic.png and b/ext/gd/tests/imagetruecolortopalette_basic.png differ
diff --git a/ext/gd/tests/similarity.inc b/ext/gd/tests/similarity.inc
deleted file mode 100644
index cb0dba77f2064..0000000000000
--- a/ext/gd/tests/similarity.inc
+++ /dev/null
@@ -1,64 +0,0 @@
-<?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;
-}
diff --git a/ext/gd/tests/test_image_equals_file_palette.phpt b/ext/gd/tests/test_image_equals_file_palette.phpt
index 2e294c04c4066..d8f6a0556b20c 100644
--- a/ext/gd/tests/test_image_equals_file_palette.phpt
+++ b/ext/gd/tests/test_image_equals_file_palette.phpt
@@ -18,6 +18,7 @@ $red = imagecolorallocate($im, 255, 0, 0);
 imagefilledrectangle($im, 3,3, 7,7, $red);
 
 $filename = __DIR__ . DIRECTORY_SEPARATOR . 'test_image_equals_file_palette.png';
+imagepalettetotruecolor($im);
 imagepng($im, $filename);
 
 $im = imagecreate(10, 10);
@@ -25,6 +26,7 @@ imagecolorallocate($im, 255, 255, 255);
 $blue = imagecolorallocate($im, 0, 0, 255);
 imagefilledrectangle($im, 3,3, 7,7, $blue);
 
+imagepalettetotruecolor($im);
 test_image_equals_file($filename, $im);
 
 $im = imagecreate(10, 10);
@@ -33,6 +35,7 @@ imagecolorallocate($im, 0, 0, 0);
 $red = imagecolorallocate($im, 255, 0, 0);
 imagefilledrectangle($im, 3,3, 7,7, $red);
 
+imagepalettetotruecolor($im);
 test_image_equals_file($filename, $im);
 ?>
 --EXPECT--
diff --git a/ext/gd/tests/webp_basic.phpt b/ext/gd/tests/webp_basic.phpt
index 97b0451c09b8d..ef2fae8652560 100644
--- a/ext/gd/tests/webp_basic.phpt
+++ b/ext/gd/tests/webp_basic.phpt
@@ -12,7 +12,7 @@ if (!function_exists('imagewebp') || !function_exists('imagecreatefromwebp'))
 ?>
 --FILE--
 <?php
-require_once __DIR__ . '/similarity.inc';
+require_once __DIR__ . '/func.inc';
 
 $filename = __DIR__ . '/webp_basic.webp';
 
@@ -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);