Fix handling of watermark with alpha for Imagick

When adding a watermark that already has an alpha channel this was reset
because the overal opacity was set for the image. Instead we should
multiply the original alpha with the requested opacity

Author: elzekool

lib/internal/Magento/Framework/Image/Adapter/ImageMagick.php

@@ -269,15 +269,17 @@ public function watermark($imagePath, $positionX = 0, $positionY = 0, $opacity =
             );
         }
 
-        if (method_exists($watermark, 'setImageOpacity')) {
-            // available from imagick 6.3.1
-            $watermark->setImageOpacity($opacity);
-        } else {
-            // go to each pixel and make it transparent
-            $watermark->paintTransparentImage($watermark->getImagePixelColor(0, 0), 1, 65530);
-            $watermark->evaluateImage(\Imagick::EVALUATE_SUBTRACT, 1 - $opacity, \Imagick::CHANNEL_ALPHA);
+        if (method_exists($watermark, 'getImageAlphaChannel')) {
+            // available from imagick 6.4.0
+            if ($watermark->getImageAlphaChannel() == 0) {
+                $watermark->setImageAlphaChannel(\Imagick::ALPHACHANNEL_OPAQUE);
+            }
         }
 
+        $compositeChannels = \Imagick::CHANNEL_ALL;
+        $watermark->evaluateImage(\Imagick::EVALUATE_MULTIPLY, $opacity, \Imagick::CHANNEL_OPACITY);
+        $compositeChannels &= ~(\Imagick::CHANNEL_OPACITY);
+
         switch ($this->getWatermarkPosition()) {
             case self::POSITION_STRETCH:
                 $watermark->sampleImage($this->_imageSrcWidth, $this->_imageSrcHeight);
@@ -309,14 +311,14 @@ public function watermark($imagePath, $positionX = 0, $positionY = 0, $opacity =
                 $offsetY = $positionY;
                 while ($offsetY <= $this->_imageSrcHeight + $watermark->getImageHeight()) {
                     while ($offsetX <= $this->_imageSrcWidth + $watermark->getImageWidth()) {
-                        $this->_imageHandler->compositeImage($watermark, \Imagick::COMPOSITE_OVER, $offsetX, $offsetY);
+                        $this->_imageHandler->compositeImage($watermark, \Imagick::COMPOSITE_OVER, $offsetX, $offsetY, $compositeChannels);
                         $offsetX += $watermark->getImageWidth();
                     }
                     $offsetX = $positionX;
                     $offsetY += $watermark->getImageHeight();
                 }
             } else {
-                $this->_imageHandler->compositeImage($watermark, \Imagick::COMPOSITE_OVER, $positionX, $positionY);
+                $this->_imageHandler->compositeImage($watermark, \Imagick::COMPOSITE_OVER, $positionX, $positionY, $compositeChannels);
             }
         } catch (\ImagickException $e) {
             throw new \Exception('Unable to create watermark.', $e->getCode(), $e);