Preserve transparency in the Imagick grayscale effect

CHANGELOG.md

@@ -1,6 +1,7 @@
 # CHANGELOG
 
 ### NEXT (YYYY-MM-DD)
+- Fix the Imagick driver deactivating the alpha channel in `effects()->grayscale()` (transparent pixels were encoded as opaque gray); it now uses the alpha-preserving grayscale image type, like `usePalette()` (#880, @nlemoine)
 
 ### 1.5.3 (2026-06-03)
 - Fix the Imagick driver painting a "black box" when pasting an image with transparent areas at an alpha lower than 100; the opacity now scales the existing per-pixel alpha instead of overwriting it (#878, @nlemoine)

src/Imagick/Effects.php

@@ -95,7 +95,20 @@ public function grayscale()
     {
         static::getDriverInfo()->requireFeature(DriverInfo::FEATURE_GRAYSCALEEFFECT);
         try {
-            $this->imagick->setImageType(\Imagick::IMGTYPE_GRAYSCALE);
+            // IMGTYPE_GRAYSCALE is an alpha-less image type: ImageMagick deactivates the alpha
+            // channel when switching to it, so transparent pixels would be encoded as opaque
+            // gray. Use the alpha-preserving variant instead, as Image::setColorspace() already
+            // does (the constant was named IMGTYPE_GRAYSCALEMATTE before ImageMagick 7 / Imagick
+            // 3.4.3, and some combinations of Imagick and ImageMagick versions define neither,
+            // hence the hard-coded fallback value).
+            if (defined('\Imagick::IMGTYPE_GRAYSCALEALPHA')) {
+                $grayscaleType = \Imagick::IMGTYPE_GRAYSCALEALPHA;
+            } elseif (defined('\Imagick::IMGTYPE_GRAYSCALEMATTE')) {
+                $grayscaleType = \Imagick::IMGTYPE_GRAYSCALEMATTE;
+            } else {
+                $grayscaleType = 3;
+            }
+            $this->imagick->setImageType($grayscaleType);
         } catch (\ImagickException $e) {
             throw new RuntimeException('Failed to grayscale the image', $e->getCode(), $e);
         }

tests/tests/Effects/AbstractEffectsTest.php

@@ -13,6 +13,7 @@
 
 use Imagine\Driver\Info;
 use Imagine\Driver\InfoProvider;
+use Imagine\Exception\NotSupportedException;
 use Imagine\Image\Box;
 use Imagine\Image\Palette\RGB;
 use Imagine\Image\Point;
@@ -117,6 +118,35 @@ public function testGrayscale()
         $this->assertEquals($greyG, $greyB);
     }
 
+    public function testGrayscalePreservesTransparency()
+    {
+        if (!$this->getDriverInfo()->hasFeature(Info::FEATURE_GRAYSCALEEFFECT)) {
+            $this->isGoingToThrowException('Imagine\Exception\NotSupportedException');
+        } else {
+            try {
+                $this->getDriverInfo()->requireFeature(Info::FEATURE_TRANSPARENCY);
+            } catch (NotSupportedException $x) {
+                $this->markTestSkipped($x->getMessage());
+            }
+        }
+        $palette = new RGB();
+        $imagine = $this->getImagine();
+
+        // Grayscaling must desaturate the color channels without dropping the
+        // alpha channel (on the Imagick driver, IMGTYPE_GRAYSCALE used to
+        // deactivate it, so every transparent pixel was encoded as opaque gray).
+        // The image is saved and reloaded because the defect only shows at
+        // encode time: in-memory pixel reads still expose the stored alpha.
+        $image = $imagine->create(new Box(20, 20), $palette->color('f00', 0));
+        $image->effects()
+            ->grayscale();
+
+        $reloaded = $imagine->load($image->get('png'));
+        $pixel = $reloaded->getColorAt(new Point(10, 10));
+
+        $this->assertSame(0, $pixel->getAlpha());
+    }
+
     public function brightnessProvider()
     {
         $color = '#145af0';