diff --git a/core/tests/Drupal/KernelTests/Core/Image/ToolkitGdTest.php b/core/tests/Drupal/KernelTests/Core/Image/ToolkitGdTest.php
index 3854f3ef8f1d49ec9a95a9a13480d56b2266996c..8db24885962a23e43d62d3dd497139b40d314d69 100644
--- a/core/tests/Drupal/KernelTests/Core/Image/ToolkitGdTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Image/ToolkitGdTest.php
@@ -4,54 +4,46 @@
 
 use Drupal\Core\File\FileSystemInterface;
 use Drupal\Core\Image\ImageInterface;
-use Drupal\Component\Render\FormattableMarkup;
-use Drupal\Core\Site\Settings;
 use Drupal\KernelTests\KernelTestBase;
 
 /**
- * Tests that core image manipulations work properly: scale, resize, rotate,
- * crop, scale and crop, and desaturate.
+ * Tests for the GD image toolkit.
  *
+ * @coversDefaultClass \Drupal\system\Plugin\ImageToolkit\GDToolkit
  * @group Image
  * @requires extension gd
  */
 class ToolkitGdTest extends KernelTestBase {
 
   /**
-   * The image factory service.
-   *
-   * @var \Drupal\Core\Image\ImageFactory
+   * Colors that are used in testing.
    */
-  protected $imageFactory;
+  protected const BLACK              = [0, 0, 0, 0];
+  protected const RED                = [255, 0, 0, 0];
+  protected const GREEN              = [0, 255, 0, 0];
+  protected const BLUE               = [0, 0, 255, 0];
+  protected const YELLOW             = [255, 255, 0, 0];
+  protected const WHITE              = [255, 255, 255, 0];
+  protected const TRANSPARENT        = [0, 0, 0, 127];
+  protected const FUCHSIA            = [255, 0, 255, 0];
+  protected const ROTATE_TRANSPARENT = [255, 255, 255, 127];
 
   /**
-   * Colors that are used in testing.
+   * The image factory service.
    *
-   * @var array
+   * @var \Drupal\Core\Image\ImageFactory
    */
-  protected $black       = [0, 0, 0, 0];
-  protected $red         = [255, 0, 0, 0];
-  protected $green       = [0, 255, 0, 0];
-  protected $blue        = [0, 0, 255, 0];
-  protected $yellow      = [255, 255, 0, 0];
-  protected $white       = [255, 255, 255, 0];
-  protected $transparent = [0, 0, 0, 127];
+  protected $imageFactory;
 
   /**
-   * Used as rotate background colors.
+   * A directory where test image files can be saved to.
    *
-   * @var array
+   * @var string
    */
-  protected $fuchsia           = [255, 0, 255, 0];
-  protected $rotateTransparent = [255, 255, 255, 127];
-
-  protected $width = 40;
-  protected $height = 20;
+  protected $directory;
 
   /**
-   * Modules to enable.
-   *
-   * @var array
+   * {@inheritdoc}
    */
   protected static $modules = ['system'];
 
@@ -62,32 +54,41 @@ protected function setUp(): void {
     parent::setUp();
 
     $this->installConfig(['system']);
+
     // Set the image factory service.
     $this->imageFactory = $this->container->get('image.factory');
+    $this->assertEquals('gd', $this->imageFactory->getToolkitId(), 'The image factory is set to use the \'gd\' image toolkit.');
+
+    // Prepare a directory for test file results.
+    $this->directory = 'public://imagetest';
+    \Drupal::service('file_system')->prepareDirectory($this->directory, FileSystemInterface::CREATE_DIRECTORY);
   }
 
   /**
-   * Function to compare two colors by RGBa.
+   * Assert two colors are equal by RGBA, net of full transparency.
+   *
+   * @param int[] $expected
+   *   The expected RGBA array.
+   * @param int[] $actual
+   *   The actual RGBA array.
+   * @param int $tolerance
+   *   The acceptable difference between the colors.
+   * @param string $message
+   *   The assertion message.
    */
-  public function colorsAreEqual($color_a, $color_b) {
-    // Fully transparent pixels are equal, regardless of RGB.
-    if ($color_a[3] == 127 && $color_b[3] == 127) {
-      return TRUE;
+  protected function assertColorsAreEqual(array $expected, array $actual, int $tolerance, string $message = ''): void {
+    // Fully transparent colors are equal, regardless of RGB.
+    if ($actual[3] == 127 && $expected[3] == 127) {
+      return;
     }
-
-    foreach ($color_a as $key => $value) {
-      if ($color_b[$key] != $value) {
-        return FALSE;
-      }
-    }
-
-    return TRUE;
+    $distance = pow(($actual[0] - $expected[0]), 2) + pow(($actual[1] - $expected[1]), 2) + pow(($actual[2] - $expected[2]), 2) + pow(($actual[3] - $expected[3]), 2);
+    $this->assertLessThanOrEqual($tolerance, $distance, $message . " - Actual: {" . implode(',', $actual) . "}, Expected: {" . implode(',', $expected) . "}, Distance: " . $distance . ", Tolerance: " . $tolerance);
   }
 
   /**
    * Function for finding a pixel's RGBa values.
    */
-  public function getPixelColor(ImageInterface $image, $x, $y) {
+  public function getPixelColor(ImageInterface $image, int $x, int $y): array {
     $toolkit = $image->getToolkit();
     $color_index = imagecolorat($toolkit->getResource(), $x, $y);
 
@@ -100,123 +101,87 @@ public function getPixelColor(ImageInterface $image, $x, $y) {
   }
 
   /**
-   * Since PHP can't visually check that our images have been manipulated
-   * properly, build a list of expected color values for each of the corners and
-   * the expected height and widths for the final images.
+   * Data provider for ::testManipulations().
    */
-  public function testManipulations() {
-
-    // Test that the image factory is set to use the GD toolkit.
-    $this->assertEquals('gd', $this->imageFactory->getToolkitId(), 'The image factory is set to use the \'gd\' image toolkit.');
-
-    // Test the list of supported extensions.
-    $expected_extensions = ['png', 'gif', 'jpeg', 'jpg', 'jpe', 'webp'];
-    $supported_extensions = $this->imageFactory->getSupportedExtensions();
-    $this->assertEquals($expected_extensions, array_intersect($expected_extensions, $supported_extensions));
-
-    // Test that the supported extensions map to correct internal GD image
-    // types.
-    $expected_image_types = [
-      'png' => IMAGETYPE_PNG,
-      'gif' => IMAGETYPE_GIF,
-      'jpeg' => IMAGETYPE_JPEG,
-      'jpg' => IMAGETYPE_JPEG,
-      'jpe' => IMAGETYPE_JPEG,
-      'webp' => IMAGETYPE_WEBP,
-    ];
-    $image = $this->imageFactory->get();
-    foreach ($expected_image_types as $extension => $expected_image_type) {
-      $image_type = $image->getToolkit()->extensionToImageType($extension);
-      $this->assertSame($expected_image_type, $image_type);
-    }
-
+  public function providerTestImageFiles(): array {
     // Typically the corner colors will be unchanged. These colors are in the
     // order of top-left, top-right, bottom-right, bottom-left.
-    $default_corners = [$this->red, $this->green, $this->blue, $this->transparent];
-
-    // A list of files that will be tested.
-    $files = [
-      'image-test.png',
-      'image-test.gif',
-      'image-test-no-transparency.gif',
-      'image-test.jpg',
-      'img-test.webp',
-    ];
+    $default_corners = [static::RED, static::GREEN, static::BLUE, static::TRANSPARENT];
 
     // Setup a list of tests to perform on each type.
-    $operations = [
+    $test_cases = [
       'resize' => [
-        'function' => 'resize',
+        'operation' => 'resize',
         'arguments' => ['width' => 20, 'height' => 10],
         'width' => 20,
         'height' => 10,
         'corners' => $default_corners,
       ],
       'scale_x' => [
-        'function' => 'scale',
+        'operation' => 'scale',
         'arguments' => ['width' => 20],
         'width' => 20,
         'height' => 10,
         'corners' => $default_corners,
       ],
       'scale_y' => [
-        'function' => 'scale',
+        'operation' => 'scale',
         'arguments' => ['height' => 10],
         'width' => 20,
         'height' => 10,
         'corners' => $default_corners,
       ],
       'upscale_x' => [
-        'function' => 'scale',
+        'operation' => 'scale',
         'arguments' => ['width' => 80, 'upscale' => TRUE],
         'width' => 80,
         'height' => 40,
         'corners' => $default_corners,
       ],
       'upscale_y' => [
-        'function' => 'scale',
+        'operation' => 'scale',
         'arguments' => ['height' => 40, 'upscale' => TRUE],
         'width' => 80,
         'height' => 40,
         'corners' => $default_corners,
       ],
       'crop' => [
-        'function' => 'crop',
+        'operation' => 'crop',
         'arguments' => ['x' => 12, 'y' => 4, 'width' => 16, 'height' => 12],
         'width' => 16,
         'height' => 12,
-        'corners' => array_fill(0, 4, $this->white),
+        'corners' => array_fill(0, 4, static::WHITE),
       ],
       'scale_and_crop' => [
-        'function' => 'scale_and_crop',
+        'operation' => 'scale_and_crop',
         'arguments' => ['width' => 10, 'height' => 8],
         'width' => 10,
         'height' => 8,
-        'corners' => array_fill(0, 4, $this->black),
+        'corners' => array_fill(0, 4, static::BLACK),
       ],
       'convert_jpg' => [
-        'function' => 'convert',
+        'operation' => 'convert',
         'width' => 40,
         'height' => 20,
         'arguments' => ['extension' => 'jpeg'],
         'corners' => $default_corners,
       ],
       'convert_gif' => [
-        'function' => 'convert',
+        'operation' => 'convert',
         'width' => 40,
         'height' => 20,
         'arguments' => ['extension' => 'gif'],
         'corners' => $default_corners,
       ],
       'convert_png' => [
-        'function' => 'convert',
+        'operation' => 'convert',
         'width' => 40,
         'height' => 20,
         'arguments' => ['extension' => 'png'],
         'corners' => $default_corners,
       ],
       'convert_webp' => [
-        'function' => 'convert',
+        'operation' => 'convert',
         'width' => 40,
         'height' => 20,
         'arguments' => ['extension' => 'webp'],
@@ -224,49 +189,51 @@ public function testManipulations() {
       ],
     ];
 
-    // Systems using non-bundled GD2 don't have imagerotate. Test if available.
-    // @todo Remove the version check once
-    //   https://www.drupal.org/project/drupal/issues/2670966 is resolved.
-    if (function_exists('imagerotate') && (version_compare(phpversion(), '7.0.26') < 0)) {
-      $operations += [
+    // Systems using non-bundled GD2 may miss imagerotate(). Test if available.
+    if (function_exists('imagerotate')) {
+      $test_cases += [
         'rotate_5' => [
-          'function' => 'rotate',
+          'operation' => 'rotate',
           // Fuchsia background.
           'arguments' => ['degrees' => 5, 'background' => '#FF00FF'],
-          'width' => 41,
-          'height' => 23,
-          'corners' => array_fill(0, 4, $this->fuchsia),
+          // @todo Re-enable dimensions' check once
+          //   https://www.drupal.org/project/drupal/issues/2921123 is resolved.
+          // 'width' => 41,
+          // 'height' => 23,
+          'corners' => array_fill(0, 4, static::FUCHSIA),
+        ],
+        'rotate_transparent_5' => [
+          'operation' => 'rotate',
+          'arguments' => ['degrees' => 5],
+          // @todo Re-enable dimensions' check once
+          //   https://www.drupal.org/project/drupal/issues/2921123 is resolved.
+          // 'width' => 41,
+          // 'height' => 23,
+          'corners' => array_fill(0, 4, static::ROTATE_TRANSPARENT),
         ],
         'rotate_90' => [
-          'function' => 'rotate',
+          'operation' => 'rotate',
           // Fuchsia background.
           'arguments' => ['degrees' => 90, 'background' => '#FF00FF'],
           'width' => 20,
           'height' => 40,
-          'corners' => [$this->transparent, $this->red, $this->green, $this->blue],
-        ],
-        'rotate_transparent_5' => [
-          'function' => 'rotate',
-          'arguments' => ['degrees' => 5],
-          'width' => 41,
-          'height' => 23,
-          'corners' => array_fill(0, 4, $this->rotateTransparent),
+          'corners' => [static::TRANSPARENT, static::RED, static::GREEN, static::BLUE],
         ],
         'rotate_transparent_90' => [
-          'function' => 'rotate',
+          'operation' => 'rotate',
           'arguments' => ['degrees' => 90],
           'width' => 20,
           'height' => 40,
-          'corners' => [$this->transparent, $this->red, $this->green, $this->blue],
+          'corners' => [static::TRANSPARENT, static::RED, static::GREEN, static::BLUE],
         ],
       ];
     }
 
-    // Systems using non-bundled GD2 don't have imagefilter. Test if available.
+    // Systems using non-bundled GD2 may miss imagefilter(). Test if available.
     if (function_exists('imagefilter')) {
-      $operations += [
+      $test_cases += [
         'desaturate' => [
-          'function' => 'desaturate',
+          'operation' => 'desaturate',
           'arguments' => [],
           'height' => 20,
           'width' => 40,
@@ -283,148 +250,202 @@ public function testManipulations() {
       ];
     }
 
-    // Prepare a directory for test file results.
-    $directory = Settings::get('file_public_path') . '/imagetest';
-    \Drupal::service('file_system')->prepareDirectory($directory, FileSystemInterface::CREATE_DIRECTORY);
-
-    foreach ($files as $file) {
-      foreach ($operations as $op => $values) {
-        // Load up a fresh image.
-        $image = $this->imageFactory->get('core/tests/fixtures/files/' . $file);
-        $toolkit = $image->getToolkit();
-        if (!$image->isValid()) {
-          $this->fail(new FormattableMarkup('Could not load image %file.', ['%file' => $file]));
-          continue 2;
-        }
-        $image_original_type = $image->getToolkit()->getType();
+    $ret = [];
+    foreach ([
+      'image-test.png',
+      'image-test.gif',
+      'image-test-no-transparency.gif',
+      'image-test.jpg',
+      'img-test.webp',
+    ] as $file_name) {
+      foreach ($test_cases as $test_case => $values) {
+        $operation = $values['operation'];
+        $arguments = $values['arguments'];
+        unset($values['operation'], $values['arguments']);
+        $ret[] = [$file_name, $test_case, $operation, $arguments, $values];
+      }
+    }
 
-        // All images should be converted to truecolor when loaded.
-        $image_truecolor = imageistruecolor($toolkit->getResource());
-        $this->assertTrue($image_truecolor, new FormattableMarkup('Image %file after load is a truecolor image.', ['%file' => $file]));
+    return $ret;
+  }
 
-        // Store the original GD resource.
-        $old_res = $toolkit->getResource();
+  /**
+   * Since PHP can't visually check that our images have been manipulated
+   * properly, build a list of expected color values for each of the corners and
+   * the expected height and widths for the final images.
+   *
+   * @dataProvider providerTestImageFiles
+   */
+  public function testManipulations(string $file_name, string $test_case, string $operation, array $arguments, array $expected): void {
+    // Load up a fresh image.
+    $image = $this->imageFactory->get('core/tests/fixtures/files/' . $file_name);
+    $toolkit = $image->getToolkit();
+    $this->assertTrue($image->isValid());
+    $image_original_type = $image->getToolkit()->getType();
 
-        // Perform our operation.
-        $image->apply($values['function'], $values['arguments']);
+    $this->assertTrue(imageistruecolor($toolkit->getResource()), "Image '$file_name' after load should be a truecolor image, but it is not.");
 
-        // If the operation replaced the resource, check that the old one has
-        // been destroyed.
-        $new_res = $toolkit->getResource();
-        if ($new_res !== $old_res) {
-          // @todo In https://www.drupal.org/node/3133236 convert this to
-          //   $this->assertIsNotResource($old_res).
-          $this->assertFalse(is_resource($old_res), new FormattableMarkup("'%operation' destroyed the original resource.", ['%operation' => $values['function']]));
-        }
+    // Perform our operation.
+    $image->apply($operation, $arguments);
 
-        // To keep from flooding the test with assert values, make a general
-        // value for whether each group of values fail.
-        $correct_dimensions_real = TRUE;
-        $correct_dimensions_object = TRUE;
+    // Flush Image object to disk storage.
+    $file_path = $this->directory . '/' . $test_case . image_type_to_extension($image->getToolkit()->getType());
+    $image->save($file_path);
 
-        if (imagesy($toolkit->getResource()) != $values['height'] || imagesx($toolkit->getResource()) != $values['width']) {
-          $correct_dimensions_real = FALSE;
-        }
+    // Check that the both the GD object and the Image object have an accurate
+    // record of the dimensions.
+    if (isset($expected['height']) && isset($expected['width'])) {
+      $this->assertSame($expected['height'], imagesy($toolkit->getResource()), "Image '$file_name' after '$test_case' should have a proper height.");
+      $this->assertSame($expected['width'], imagesx($toolkit->getResource()), "Image '$file_name' after '$test_case' should have a proper width.");
+      $this->assertSame($expected['height'], $image->getHeight(), "Image '$file_name' after '$test_case' should have a proper height.");
+      $this->assertSame($expected['width'], $image->getWidth(), "Image '$file_name' after '$test_case' should have a proper width.");
+    }
 
-        // Check that the image object has an accurate record of the dimensions.
-        if ($image->getWidth() != $values['width'] || $image->getHeight() != $values['height']) {
-          $correct_dimensions_object = FALSE;
+    // Now check each of the corners to ensure color correctness.
+    foreach ($expected['corners'] as $key => $expected_color) {
+      // The test gif that does not have transparency color set is a
+      // special case.
+      if ($file_name === 'image-test-no-transparency.gif') {
+        if ($test_case == 'desaturate') {
+          // For desaturating, keep the expected color from the test
+          // data, but set alpha channel to fully opaque.
+          $expected_color[3] = 0;
         }
-
-        $file_path = $directory . '/' . $op . image_type_to_extension($image->getToolkit()->getType());
-        $image->save($file_path);
-
-        $this->assertTrue($correct_dimensions_real, new FormattableMarkup('Image %file after %action action has proper dimensions.', ['%file' => $file, '%action' => $op]));
-        $this->assertTrue($correct_dimensions_object, new FormattableMarkup('Image %file object after %action action is reporting the proper height and width values.', ['%file' => $file, '%action' => $op]));
-
-        // JPEG colors will always be messed up due to compression. So we skip
-        // these tests if the original or the result is in jpeg format.
-        if ($image->getToolkit()->getType() != IMAGETYPE_JPEG && $image_original_type != IMAGETYPE_JPEG) {
-          // Now check each of the corners to ensure color correctness.
-          foreach ($values['corners'] as $key => $corner) {
-            // The test gif that does not have transparency color set is a
-            // special case.
-            if ($file === 'image-test-no-transparency.gif') {
-              if ($op == 'desaturate') {
-                // For desaturating, keep the expected color from the test
-                // data, but set alpha channel to fully opaque.
-                $corner[3] = 0;
-              }
-              elseif ($corner === $this->transparent) {
-                // Set expected pixel to yellow where the others have
-                // transparent.
-                $corner = $this->yellow;
-              }
-            }
-
-            // Get the location of the corner.
-            switch ($key) {
-              case 0:
-                $x = 0;
-                $y = 0;
-                break;
-
-              case 1:
-                $x = $image->getWidth() - 1;
-                $y = 0;
-                break;
-
-              case 2:
-                $x = $image->getWidth() - 1;
-                $y = $image->getHeight() - 1;
-                break;
-
-              case 3:
-                $x = 0;
-                $y = $image->getHeight() - 1;
-                break;
-            }
-            $color = $this->getPixelColor($image, $x, $y);
-            // We also skip the color test for transparency for gif <-> png
-            // conversion. The convert operation cannot handle that correctly.
-            if ($image->getToolkit()->getType() == $image_original_type || $corner != $this->transparent) {
-              $correct_colors = $this->colorsAreEqual($color, $corner);
-              $this->assertTrue($correct_colors, new FormattableMarkup('Image %file object after %action action has the correct color placement at corner %corner.',
-                ['%file' => $file, '%action' => $op, '%corner' => $key]));
-            }
-          }
+        elseif ($expected_color === static::TRANSPARENT) {
+          // Set expected pixel to yellow where the others have
+          // transparent.
+          $expected_color = static::YELLOW;
         }
+      }
 
-        // Check that saved image reloads without raising PHP errors.
-        $image_reloaded = $this->imageFactory->get($file_path);
-        $resource = $image_reloaded->getToolkit()->getResource();
+      // Get the location of the corner.
+      switch ($key) {
+        case 0:
+          $x = 0;
+          $y = 0;
+          break;
+
+        case 1:
+          $x = $image->getWidth() - 1;
+          $y = 0;
+          break;
+
+        case 2:
+          $x = $image->getWidth() - 1;
+          $y = $image->getHeight() - 1;
+          break;
+
+        case 3:
+          $x = 0;
+          $y = $image->getHeight() - 1;
+          break;
       }
-    }
+      $actual_color = $this->getPixelColor($image, $x, $y);
 
-    // Test creation of image from scratch, and saving to storage.
-    foreach ([IMAGETYPE_PNG, IMAGETYPE_GIF, IMAGETYPE_JPEG, IMAGETYPE_WEBP] as $type) {
-      $image = $this->imageFactory->get();
-      $image->createNew(50, 20, image_type_to_extension($type, FALSE), '#ffff00');
-      $file = 'from_null' . image_type_to_extension($type);
-      $file_path = $directory . '/' . $file;
-      $this->assertEquals(50, $image->getWidth(), new FormattableMarkup('Image file %file has the correct width.', ['%file' => $file]));
-      $this->assertEquals(20, $image->getHeight(), new FormattableMarkup('Image file %file has the correct height.', ['%file' => $file]));
-      $this->assertEquals(image_type_to_mime_type($type), $image->getMimeType(), new FormattableMarkup('Image file %file has the correct MIME type.', ['%file' => $file]));
-      $this->assertTrue($image->save($file_path), new FormattableMarkup('Image %file created anew from a null image was saved.', ['%file' => $file]));
-
-      // Reload saved image.
-      $image_reloaded = $this->imageFactory->get($file_path);
-      if (!$image_reloaded->isValid()) {
-        $this->fail(new FormattableMarkup('Could not load image %file.', ['%file' => $file]));
+      // If image cannot handle transparent colors, skip the pixel color test.
+      if ($actual_color[3] === 0 && $expected_color[3] === 127) {
         continue;
       }
-      $this->assertEquals(50, $image_reloaded->getWidth(), new FormattableMarkup('Image file %file has the correct width.', ['%file' => $file]));
-      $this->assertEquals(20, $image_reloaded->getHeight(), new FormattableMarkup('Image file %file has the correct height.', ['%file' => $file]));
-      $this->assertEquals(image_type_to_mime_type($type), $image_reloaded->getMimeType(), new FormattableMarkup('Image file %file has the correct MIME type.', ['%file' => $file]));
-      if ($image_reloaded->getToolkit()->getType() == IMAGETYPE_GIF) {
-        $this->assertEquals('#ffff00', $image_reloaded->getToolkit()->getTransparentColor(), new FormattableMarkup('Image file %file has the correct transparent color channel set.', ['%file' => $file]));
-      }
-      else {
-        $this->assertNull($image_reloaded->getToolkit()->getTransparentColor(), new FormattableMarkup('Image file %file has no color channel set.', ['%file' => $file]));
-      }
+
+      // JPEG has small differences in color after processing.
+      $tolerance = $image_original_type === IMAGETYPE_JPEG ? 3 : 0;
+
+      $this->assertColorsAreEqual($expected_color, $actual_color, $tolerance, "Image '$file_name' object after '$test_case' action has the correct color placement at corner '$key'");
+    }
+
+    // Check that saved image reloads without raising PHP errors.
+    $image_reloaded = $this->imageFactory->get($file_path);
+    if (PHP_VERSION_ID >= 80000) {
+      $this->assertInstanceOf(\GDImage::class, $image_reloaded->getToolkit()->getResource());
+    }
+    else {
+      $this->assertIsResource($image_reloaded->getToolkit()->getResource());
+      $this->assertSame(get_resource_type($image_reloaded->getToolkit()->getResource()), 'gd');
+    }
+  }
+
+  /**
+   * @covers ::getSupportedExtensions
+   * @covers ::extensionToImageType
+   */
+  public function testSupportedExtensions(): void {
+    // Test the list of supported extensions.
+    $expected_extensions = ['png', 'gif', 'jpeg', 'jpg', 'jpe', 'webp'];
+    $this->assertEqualsCanonicalizing($expected_extensions, $this->imageFactory->getSupportedExtensions());
+
+    // Test that the supported extensions map to correct internal GD image
+    // types.
+    $expected_image_types = [
+      'png' => IMAGETYPE_PNG,
+      'gif' => IMAGETYPE_GIF,
+      'jpeg' => IMAGETYPE_JPEG,
+      'jpg' => IMAGETYPE_JPEG,
+      'jpe' => IMAGETYPE_JPEG,
+      'webp' => IMAGETYPE_WEBP,
+    ];
+    $image = $this->imageFactory->get();
+    foreach ($expected_image_types as $extension => $expected_image_type) {
+      $this->assertSame($expected_image_type, $image->getToolkit()->extensionToImageType($extension));
     }
+  }
+
+  /**
+   * Data provider for ::testCreateImageFromScratch().
+   */
+  public function providerSupportedImageTypes(): array {
+    return [
+      [IMAGETYPE_PNG],
+      [IMAGETYPE_GIF],
+      [IMAGETYPE_JPEG],
+      [IMAGETYPE_WEBP],
+    ];
+  }
 
-    // Test failures of the 'create_new' operation.
+  /**
+   * Tests that GD functions for the image type are available.
+   *
+   * @dataProvider providerSupportedImageTypes
+   */
+  public function testGdFunctionsExist(int $type): void {
+    $extension = image_type_to_extension($type, FALSE);
+    $this->assertTrue(function_exists("imagecreatefrom$extension"), "imagecreatefrom$extension should exist.");
+    $this->assertTrue(function_exists("image$extension"), "image$extension should exist.");
+  }
+
+  /**
+   * Tests creation of image from scratch, and saving to storage.
+   *
+   * @dataProvider providerSupportedImageTypes
+   */
+  public function testCreateImageFromScratch(int $type): void {
+    // Build an image from scratch.
+    $image = $this->imageFactory->get();
+    $image->createNew(50, 20, image_type_to_extension($type, FALSE), '#ffff00');
+    $file = 'from_null' . image_type_to_extension($type);
+    $file_path = $this->directory . '/' . $file;
+    $this->assertSame(50, $image->getWidth());
+    $this->assertSame(20, $image->getHeight());
+    $this->assertSame(image_type_to_mime_type($type), $image->getMimeType());
+    $this->assertTrue($image->save($file_path), "Image '$file' should have been saved successfully, but it has not.");
+
+    // Reload and check saved image.
+    $image_reloaded = $this->imageFactory->get($file_path);
+    $this->assertTrue($image_reloaded->isValid());
+    $this->assertSame(50, $image_reloaded->getWidth());
+    $this->assertSame(20, $image_reloaded->getHeight());
+    $this->assertSame(image_type_to_mime_type($type), $image_reloaded->getMimeType());
+    if ($image_reloaded->getToolkit()->getType() == IMAGETYPE_GIF) {
+      $this->assertSame('#ffff00', $image_reloaded->getToolkit()->getTransparentColor(), "Image '$file' after reload should have color channel set to #ffff00, but it has not.");
+    }
+    else {
+      $this->assertNull($image_reloaded->getToolkit()->getTransparentColor(), "Image '$file' after reload should have no color channel set, but it has.");
+    }
+  }
+
+  /**
+   * Tests failures of the 'create_new' operation.
+   */
+  public function testCreateNewFailures(): void {
     $image = $this->imageFactory->get();
     $image->createNew(-50, 20);
     $this->assertFalse($image->isValid(), 'CreateNew with negative width fails.');
@@ -472,11 +493,7 @@ public function testResourceDestruction() {
   /**
    * Tests for GIF images with transparency.
    */
-  public function testGifTransparentImages() {
-    // Prepare a directory for test file results.
-    $directory = Settings::get('file_public_path') . '/imagetest';
-    \Drupal::service('file_system')->prepareDirectory($directory, FileSystemInterface::CREATE_DIRECTORY);
-
+  public function testGifTransparentImages(): void {
     // Test loading an indexed GIF image with transparent color set.
     // Color at top-right pixel should be fully transparent.
     $file = 'image-test-transparent-indexed.gif';
@@ -484,20 +501,20 @@ public function testGifTransparentImages() {
     $resource = $image->getToolkit()->getResource();
     $color_index = imagecolorat($resource, $image->getWidth() - 1, 0);
     $color = array_values(imagecolorsforindex($resource, $color_index));
-    $this->assertEquals($this->rotateTransparent, $color, "Image {$file} after load has full transparent color at corner 1.");
+    $this->assertEquals(static::ROTATE_TRANSPARENT, $color, "Image {$file} after load has full transparent color at corner 1.");
 
     // Test deliberately creating a GIF image with no transparent color set.
     // Color at top-right pixel should be fully transparent while in memory,
     // fully opaque after flushing image to file.
     $file = 'image-test-no-transparent-color-set.gif';
-    $file_path = $directory . '/' . $file;
+    $file_path = $this->directory . '/' . $file;
     // Create image.
     $image = $this->imageFactory->get();
     $image->createNew(50, 20, 'gif', NULL);
     $resource = $image->getToolkit()->getResource();
     $color_index = imagecolorat($resource, $image->getWidth() - 1, 0);
     $color = array_values(imagecolorsforindex($resource, $color_index));
-    $this->assertEquals($this->rotateTransparent, $color, "New GIF image with no transparent color set after creation has full transparent color at corner 1.");
+    $this->assertEquals(static::ROTATE_TRANSPARENT, $color, "New GIF image with no transparent color set after creation has full transparent color at corner 1.");
     // Save image.
     $this->assertTrue($image->save($file_path), "New GIF image {$file} was saved.");
     // Reload image.
@@ -522,37 +539,20 @@ public function testGifTransparentImages() {
     // can be loaded correctly.
     $file = 'image-test-transparent-out-of-range.gif';
     $image = $this->imageFactory->get('core/tests/fixtures/files/' . $file);
-    $toolkit = $image->getToolkit();
-
-    if (!$image->isValid()) {
-      $this->fail(new FormattableMarkup('Could not load image %file.', ['%file' => $file]));
-    }
-    else {
-      // All images should be converted to truecolor when loaded.
-      $image_truecolor = imageistruecolor($toolkit->getResource());
-      $this->assertTrue($image_truecolor, new FormattableMarkup('Image %file after load is a truecolor image.', ['%file' => $file]));
-    }
+    $this->assertTrue($image->isValid(), "Image '$file' after load should be valid, but it is not.");
+    $this->assertTrue(imageistruecolor($image->getToolkit()->getResource()), "Image '$file' after load should be a truecolor image, but it is not.");
   }
 
   /**
    * Tests calling a missing image operation plugin.
    */
-  public function testMissingOperation() {
-
-    // Test that the image factory is set to use the GD toolkit.
-    $this->assertEquals('gd', $this->imageFactory->getToolkitId(), 'The image factory is set to use the \'gd\' image toolkit.');
-
-    // An image file that will be tested.
-    $file = 'image-test.png';
-
+  public function testMissingOperation(): void {
     // Load up a fresh image.
-    $image = $this->imageFactory->get('core/tests/fixtures/files/' . $file);
-    if (!$image->isValid()) {
-      $this->fail(new FormattableMarkup('Could not load image %file.', ['%file' => $file]));
-    }
+    $image = $this->imageFactory->get('core/tests/fixtures/files/image-test.png');
+    $this->assertTrue($image->isValid(), "Image 'image-test.png' after load should be valid, but it is not.");
 
     // Try perform a missing toolkit operation.
-    $this->assertFalse($image->apply('missing_op', []), 'Calling a missing image toolkit operation plugin fails.');
+    $this->assertFalse($image->apply('missing_op', []), 'Calling a missing image toolkit operation plugin should fail, but it did not.');
   }
 
 }