Fix Imagick grayscale flattening saturated colors

src/Drivers/Imagick/Modifiers/GrayscaleModifier.php

@@ -4,6 +4,7 @@
 
 namespace Intervention\Image\Drivers\Imagick\Modifiers;
 
+use Imagick;
 use ImagickException;
 use Intervention\Image\Exceptions\ModifierException;
 use Intervention\Image\Interfaces\ImageInterface;
@@ -19,15 +20,24 @@ public function apply(ImageInterface $image): ImageInterface
     {
         foreach ($image as $frame) {
             try {
-                $result = $frame->native()->modulateImage(100, 0, 100);
+                // turn image to grayscale
+                $result = $frame->native()->transformImageColorspace(Imagick::COLORSPACE_GRAY);
                 if ($result === false) {
                     throw new ModifierException(
-                        'Failed to apply ' . self::class . ', unable to modulate image',
+                        'Failed to apply ' . self::class . ', unable to transform image to grayscale',
+                    );
+                }
+
+                // return to srgb colorspace with grayscale image
+                $result = $frame->native()->transformImageColorspace(Imagick::COLORSPACE_SRGB);
+                if ($result === false) {
+                    throw new ModifierException(
+                        'Failed to apply ' . self::class . ', unable to transform image to grayscale',
                     );
                 }
             } catch (ImagickException $e) {
                 throw new ModifierException(
-                    'Failed to apply ' . self::class . ', unable to modulate image',
+                    'Failed to apply ' . self::class . ', unable to transform image to grayscale',
                     previous: $e,
                 );
             }

tests/Unit/Drivers/Gd/Modifiers/GrayscaleModifierTest.php

@@ -6,6 +6,7 @@
 
 use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\Attributes\RequiresPhpExtension;
+use Intervention\Image\Colors\Rgb\Channels\Red;
 use Intervention\Image\Modifiers\GrayscaleModifier;
 use Intervention\Image\Tests\GdTestCase;
 
@@ -21,4 +22,18 @@ public function testColorChange(): void
         $image->modify(new GrayscaleModifier());
         $this->assertTrue($image->colorAt(0, 0)->isGrayscale());
     }
+
+    public function testSaturatedColorsKeepDistinctBrightness(): void
+    {
+        // blocks.png holds pure blue, green and red in three of its quadrants
+        $image = $this->readTestImage('blocks.png')->modify(new GrayscaleModifier());
+
+        $blue = $image->colorAt(160, 120)->channel(Red::class)->value();
+        $green = $image->colorAt(160, 360)->channel(Red::class)->value();
+        $red = $image->colorAt(480, 360)->channel(Red::class)->value();
+
+        // any luma based conversion keeps blue darkest and green brightest
+        $this->assertLessThan($red, $blue);
+        $this->assertLessThan($green, $red);
+    }
 }

tests/Unit/Drivers/Imagick/Modifiers/GrayscaleModifierTest.php

@@ -6,6 +6,7 @@
 
 use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\Attributes\RequiresPhpExtension;
+use Intervention\Image\Colors\Rgb\Channels\Red;
 use Intervention\Image\Modifiers\GrayscaleModifier;
 use Intervention\Image\Tests\ImagickTestCase;
 
@@ -21,4 +22,18 @@ public function testColorChange(): void
         $image->modify(new GrayscaleModifier());
         $this->assertTrue($image->colorAt(0, 0)->isGrayscale());
     }
+
+    public function testSaturatedColorsKeepDistinctBrightness(): void
+    {
+        // blocks.png holds pure blue, green and red in three of its quadrants
+        $image = $this->readTestImage('blocks.png')->modify(new GrayscaleModifier());
+
+        $blue = $image->colorAt(160, 120)->channel(Red::class)->value();
+        $green = $image->colorAt(160, 360)->channel(Red::class)->value();
+        $red = $image->colorAt(480, 360)->channel(Red::class)->value();
+
+        // any luma based conversion keeps blue darkest and green brightest
+        $this->assertLessThan($red, $blue);
+        $this->assertLessThan($green, $red);
+    }
 }