Fix bug in CropModifier of Imagick driver (#1428)

The CropModifier produced strange artifacts if another resize step was performed after the modification.

This patch removes the copying of the alpha channel in the CropModifier and implements a different method.

See: https://github.com/Intervention/image/issues/1426

src/Drivers/Imagick/Modifiers/CropModifier.php

@@ -5,7 +5,7 @@ declare(strict_types=1);
 namespace Intervention\Image\Drivers\Imagick\Modifiers;
 
 use Imagick;
-use Intervention\Image\Drivers\Imagick\Driver;
+use ImagickPixel;
 use Intervention\Image\Interfaces\ImageInterface;
 use Intervention\Image\Interfaces\SpecializedInterface;
 use Intervention\Image\Modifiers\CropModifier as GenericCropModifier;
@@ -14,20 +14,30 @@ class CropModifier extends GenericCropModifier implements SpecializedInterface
 {
     public function apply(ImageInterface $image): ImageInterface
     {
-        $crop = $this->crop($image);
+        // decode background color
         $background = $this->driver()->colorProcessor($image->colorspace())->colorToNative(
             $this->driver()->handleInput($this->background)
         );
 
         // create empty container imagick to rebuild core
         $imagick = new Imagick();
+
+        // save resolution to add it later
         $resolution = $image->resolution()->perInch();
 
+        // define position of the image on the new canvas
+        $crop = $this->crop($image);
+        $position = [
+            ($crop->pivot()->x() + $this->offset_x) * -1,
+            ($crop->pivot()->y() + $this->offset_y) * -1,
+        ];
+
         foreach ($image as $frame) {
             // create new frame canvas with modifiers background
             $canvas = new Imagick();
             $canvas->newImage($crop->width(), $crop->height(), $background, 'png');
             $canvas->setImageResolution($resolution->x(), $resolution->y());
+            $canvas->setImageAlphaChannel(Imagick::ALPHACHANNEL_SET); // or ALPHACHANNEL_ACTIVATE?
 
             // set animation details
             if ($image->isAnimated()) {
@@ -36,31 +46,28 @@ class CropModifier extends GenericCropModifier implements SpecializedInterface
                 $canvas->setImageDispose($frame->native()->getImageDispose());
             }
 
-            // place original frame content onto the empty colored frame canvas
-            $canvas->compositeImage(
-                $frame->native(),
-                Imagick::COMPOSITE_DEFAULT,
-                ($crop->pivot()->x() + $this->offset_x) * -1,
-                ($crop->pivot()->y() + $this->offset_y) * -1,
+            // make the rectangular position of the original image transparent
+            // so that we can later place the original on top. this preserves
+            // the transparency of the original and shows the background color
+            // of the modifier in the other areas. if the original image has no
+            // transparent area the rectangular transparency will be covered by
+            // the original.
+            $clearer = new Imagick();
+            $clearer->newImage(
+                $frame->native()->getImageWidth(),
+                $frame->native()->getImageHeight(),
+                new ImagickPixel('black'),
             );
+            $canvas->compositeImage($clearer, Imagick::COMPOSITE_DSTOUT, ...$position);
 
-            // copy alpha channel if available
-            if ($frame->native()->getImageAlphaChannel()) {
-                $canvas->compositeImage(
-                    $frame->native(),
-                    version_compare(Driver::version(), '7.0.0', '>=') ?
-                        Imagick::COMPOSITE_COPYOPACITY :
-                        Imagick::COMPOSITE_DSTIN,
-                    ($crop->pivot()->x() + $this->offset_x) * -1,
-                    ($crop->pivot()->y() + $this->offset_y) * -1,
-                );
-            }
+            // place original frame content onto prepared frame canvas
+            $canvas->compositeImage($frame->native(), Imagick::COMPOSITE_DEFAULT, ...$position);
 
             // add newly built frame to container imagick
             $imagick->addImage($canvas);
         }
 
-        // replace imagick
+        // replace imagick in the original image
         $image->core()->setNative($imagick);
 
         return $image;

tests/Feature/Imagick/CropResizePngTest.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Intervention\Image\Tests\Feature\Imagick;
+
+use Intervention\Image\Tests\ImagickTestCase;
+
+class CropResizePngTest extends ImagickTestCase
+{
+    public function testCropResizePng(): void
+    {
+        $image = $this->readTestImage('tile.png');
+        $image->crop(100, 100);
+        $image->resize(200, 200);
+        $this->assertTransparency($image->pickColor(7, 22));
+        $this->assertTransparency($image->pickColor(22, 7));
+    }
+}