Handle transparncy correctly for watermark

The watermark functionality uses imagecopymerge for copying the
watermark into the image, this function loses the alpha information.

Therefor use imagecopy for 100% opacity and use imagefilter in other
cases.

Resolves: #10661

Author: elzekool

lib/internal/Magento/Framework/Image/Adapter/Gd2.php

@@ -404,7 +404,7 @@ public function rotate($angle)
      */
     public function watermark($imagePath, $positionX = 0, $positionY = 0, $opacity = 30, $tile = false)
     {
-        list($watermarkSrcWidth, $watermarkSrcHeight, $watermarkFileType, ) = $this->_getImageOptions($imagePath);
+        list($watermarkSrcWidth, $watermarkSrcHeight, $watermarkFileType,) = $this->_getImageOptions($imagePath);
         $this->_getFileAttributes();
         $watermark = call_user_func(
             $this->_getCallback('create', $watermarkFileType, 'Unsupported watermark image format.'),
@@ -465,7 +465,7 @@ public function watermark($imagePath, $positionX = 0, $positionY = 0, $opacity =
         } elseif ($this->getWatermarkPosition() == self::POSITION_CENTER) {
             $positionX = $this->_imageSrcWidth / 2 - imagesx($watermark) / 2;
             $positionY = $this->_imageSrcHeight / 2 - imagesy($watermark) / 2;
-            imagecopymerge(
+            $this->imagecopymergeWithAlphaFix(
                 $this->_imageHandler,
                 $watermark,
                 $positionX,
@@ -478,7 +478,7 @@ public function watermark($imagePath, $positionX = 0, $positionY = 0, $opacity =
             );
         } elseif ($this->getWatermarkPosition() == self::POSITION_TOP_RIGHT) {
             $positionX = $this->_imageSrcWidth - imagesx($watermark);
-            imagecopymerge(
+            $this->imagecopymergeWithAlphaFix(
                 $this->_imageHandler,
                 $watermark,
                 $positionX,
@@ -490,7 +490,7 @@ public function watermark($imagePath, $positionX = 0, $positionY = 0, $opacity =
                 $this->getWatermarkImageOpacity()
             );
         } elseif ($this->getWatermarkPosition() == self::POSITION_TOP_LEFT) {
-            imagecopymerge(
+            $this->imagecopymergeWithAlphaFix(
                 $this->_imageHandler,
                 $watermark,
                 $positionX,
@@ -504,7 +504,7 @@ public function watermark($imagePath, $positionX = 0, $positionY = 0, $opacity =
         } elseif ($this->getWatermarkPosition() == self::POSITION_BOTTOM_RIGHT) {
             $positionX = $this->_imageSrcWidth - imagesx($watermark);
             $positionY = $this->_imageSrcHeight - imagesy($watermark);
-            imagecopymerge(
+            $this->imagecopymergeWithAlphaFix(
                 $this->_imageHandler,
                 $watermark,
                 $positionX,
@@ -517,7 +517,7 @@ public function watermark($imagePath, $positionX = 0, $positionY = 0, $opacity =
             );
         } elseif ($this->getWatermarkPosition() == self::POSITION_BOTTOM_LEFT) {
             $positionY = $this->_imageSrcHeight - imagesy($watermark);
-            imagecopymerge(
+            $this->imagecopymergeWithAlphaFix(
                 $this->_imageHandler,
                 $watermark,
                 $positionX,
@@ -531,7 +531,7 @@ public function watermark($imagePath, $positionX = 0, $positionY = 0, $opacity =
         }
 
         if ($tile === false && $merged === false) {
-            imagecopymerge(
+            $this->imagecopymergeWithAlphaFix(
                 $this->_imageHandler,
                 $watermark,
                 $positionX,
@@ -547,7 +547,7 @@ public function watermark($imagePath, $positionX = 0, $positionY = 0, $opacity =
             $offsetY = $positionY;
             while ($offsetY <= $this->_imageSrcHeight + imagesy($watermark)) {
                 while ($offsetX <= $this->_imageSrcWidth + imagesx($watermark)) {
-                    imagecopymerge(
+                    $this->imagecopymergeWithAlphaFix(
                         $this->_imageHandler,
                         $watermark,
                         $offsetX,
@@ -778,4 +778,68 @@ protected function _createEmptyImage($width, $height)
         $this->imageDestroy();
         $this->_imageHandler = $image;
     }
+
+    /**
+     * Fix an issue with the usage of imagecopymerge where the alpha channel is lost
+     *
+     * @param resource $dst_im
+     * @param resource $src_im
+     * @param int $dst_x
+     * @param int $dst_y
+     * @param int $src_x
+     * @param int $src_y
+     * @param int $src_w
+     * @param int $src_h
+     * @param int $pct
+     *
+     * @return bool
+     */
+    private function imagecopymergeWithAlphaFix(
+        $dst_im,
+        $src_im,
+        $dst_x,
+        $dst_y,
+        $src_x,
+        $src_y,
+        $src_w,
+        $src_h,
+        $pct
+    ) {
+        if ($pct >= 100) {
+            return imagecopy($dst_im, $src_im, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h);
+        }
+
+        if ($pct < 0) {
+            return false;
+        }
+
+        $sizeX = imagesx($src_im);
+        $sizeY = imagesy($src_im);
+        if (false === $sizeX || false === $sizeY) {
+            return false;
+        }
+
+        $tmpImg = imagecreatetruecolor($src_w, $src_h);
+        if (false === $tmpImg) {
+            return false;
+        }
+
+        if (false === imagealphablending($tmpImg, false)) {
+            return false;
+        }
+
+        if (false === imagecopy($tmpImg, $src_im, 0, 0, 0, 0, $sizeX, $sizeY)) {
+            return false;
+        }
+
+        $transparancy = 127 - (($pct*127)/100);
+        if (false === imagefilter($tmpImg, IMG_FILTER_COLORIZE, 0, 0, 0, $transparancy)) {
+            return false;
+        }
+
+        $result = imagecopy($dst_im, $tmpImg, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h);
+        imagedestroy($tmpImg);
+
+        return $result;
+    }
 }