From a4556a327e3cc93185852a89f4ccc42f3f282bb0 Mon Sep 17 00:00:00 2001
From: Matthias Schmidt <gravatronics@live.com>
Date: Sun, 6 Nov 2016 08:37:52 +0100
Subject: [PATCH] Fix transparency handling in `GDImageAdapter::overlayImage()`

---
 .../image/adapter/GDImageAdapter.class.php    | 69 ++++++++++++++++++-
 1 file changed, 68 insertions(+), 1 deletion(-)

diff --git a/wcfsetup/install/files/lib/system/image/adapter/GDImageAdapter.class.php b/wcfsetup/install/files/lib/system/image/adapter/GDImageAdapter.class.php
index 0410dc387e..10a8f12851 100644
--- a/wcfsetup/install/files/lib/system/image/adapter/GDImageAdapter.class.php
+++ b/wcfsetup/install/files/lib/system/image/adapter/GDImageAdapter.class.php
@@ -390,9 +390,76 @@ class GDImageAdapter implements IImageAdapter {
 		// fix PNG alpha channel handling
 		// see http://php.net/manual/en/function.imagecopymerge.php#92787
 		$cut = imagecreatetruecolor($overlayImage->getWidth(), $overlayImage->getHeight());
+		imagealphablending($cut, false);
+		imagesavealpha($cut, true);
+		
 		imagecopy($cut, $this->image, 0, 0, $x, $y, $overlayImage->getWidth(), $overlayImage->getHeight());
 		imagecopy($cut, $overlayImage->image, 0, 0, 0, 0, $overlayImage->getWidth(), $overlayImage->getHeight());
-		imagecopymerge($this->image, $cut, $x, $y, 0, 0, $overlayImage->getWidth(), $overlayImage->getHeight(), $opacity * 100);
+		
+		$this->imagecopymerge_alpha($this->image, $cut, $x, $y, 0, 0, $overlayImage->getWidth(), $overlayImage->getHeight(), $opacity * 100);
+	}
+	
+	/**
+	 * `imagecopymerge` implementation with alpha support.
+	 * 
+	 * @see	http://php.net/manual/en/function.imagecopymerge.php#88456
+	 * 
+	 * @param	resource	$dst_im		destination image resource
+	 * @param	resource	$src_im		source image resource
+	 * @param	integer		$dst_x		x-coordinate of destination point
+	 * @param	integer		$dst_y		y-coordinate of destination point
+	 * @param	integer		$src_x		x-coordinate of source point
+	 * @param	integer		$src_y		y-coordinate of source point
+	 * @param	integer		$src_w		source width
+	 * @param	integer		$src_h		source height
+	 * @param	integer		$pct		opacity percent
+	 * @return	boolean
+	 */
+	private function imagecopymerge_alpha($dst_im, $src_im, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h, $pct) {
+		if (!isset($pct)) {
+			return false;
+		}
+		$pct /= 100;
+		// Get image width and height 
+		$w = imagesx($src_im);
+		$h = imagesy($src_im);
+		// Turn alpha blending off 
+		imagealphablending($src_im, false);
+		// Find the most opaque pixel in the image (the one with the smallest alpha value) 
+		$minalpha = 127;
+		for ($x = 0; $x < $w; $x++) {
+			for ($y = 0; $y < $h; $y++) {
+				$alpha = (imagecolorat($src_im, $x, $y) >> 24) & 0xFF;
+				if ($alpha < $minalpha) {
+					$minalpha = $alpha;
+				}
+			}
+		}
+		// loop through image pixels and modify alpha for each 
+		for ($x = 0; $x < $w; $x++) {
+			for ($y = 0; $y < $h; $y++) {
+				// get current alpha value (represents the TANSPARENCY!) 
+				$colorxy = imagecolorat($src_im, $x, $y);
+				$alpha = ($colorxy >> 24) & 0xFF;
+				// calculate new alpha 
+				if ($minalpha !== 127) {
+					$alpha = 127 + 127 * $pct * ($alpha - 127) / (127 - $minalpha);
+				}
+				else {
+					$alpha += 127 * $pct;
+				}
+				// get the color index with new alpha 
+				$alphacolorxy = imagecolorallocatealpha($src_im, ($colorxy >> 16) & 0xFF, ($colorxy >> 8) & 0xFF, $colorxy & 0xFF, $alpha);
+				// set pixel with the new color + opacity 
+				if (!imagesetpixel($src_im, $x, $y, $alphacolorxy)) {
+					return false;
+				}
+			}
+		}
+		// The image copy 
+		imagecopy($dst_im, $src_im, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h);
+		
+		return true;
 	}
 	
 	/**
-- 
2.20.1