Fixed bug #75124 (gdImageGrayScale() may produce colors)

We have to make sure to avoid alpha-blending issues by explicitly
switching to `gdEffectReplace` and to restore the old value afterwards.

This is a port of <https://github.com/libgd/libgd/commit/a7a7ece>.

NEWS

@@ -8,6 +8,9 @@ PHP                                                                        NEWS
 - CURL:
   . Fixed bug #75093 (OpenSSL support not detected). (Remi)
 
+- GD:
+  . Fixed bug #75124 (gdImageGrayScale() may produce colors). (cmb)
+
 - Intl:
   . Fixed bug #75090 (IntlGregorianCalendar doesn't have constants from parent
     class). (tpunt)

ext/gd/libgd/gd_filter.c

@@ -53,12 +53,17 @@ int gdImageGrayScale(gdImagePtr src)
 	int new_pxl, pxl;
 	typedef int (*FuncPtr)(gdImagePtr, int, int);
 	FuncPtr f;
+	int alpha_blending;
+
 	f = GET_PIXEL_FUNCTION(src);
 
 	if (src==NULL) {
 		return 0;
 	}
 
+	alpha_blending = src->alphaBlendingFlag;
+	gdImageAlphaBlending(src, gdEffectReplace);
+
 	for (y=0; y<src->sy; ++y) {
 		for (x=0; x<src->sx; ++x) {
 			pxl = f (src, x, y);
@@ -75,6 +80,8 @@ int gdImageGrayScale(gdImagePtr src)
 			gdImageSetPixel (src, x, y, new_pxl);
 		}
 	}
+	gdImageAlphaBlending(src, alpha_blending);
+
 	return 1;
 }
 

ext/gd/tests/bug75124.phpt

@@ -0,0 +1,31 @@
+--TEST--
+Bug #75124 (gdImageGrayScale() may produce colors)
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('gd extension not available');
+if (!GD_BUNDLED && version_compare(GD_VERSION, '2.2.5', '<')) {
+    die('skip only for bundled libgd or external libgd >= 2.2.5');
+}
+?>
+--FILE--
+<?php
+$im = imagecreatefrompng(__DIR__ . '/bug75124.png');
+var_dump(imageistruecolor($im));
+imagefilter($im, IMG_FILTER_GRAYSCALE);
+for ($i = 0, $width = imagesx($im); $i < $width; $i ++) {
+    for ($j = 0, $height = imagesy($im); $j < $height; $j++) {
+        $color = imagecolorat($im, $i, $j);
+        $red = ($color >> 16) & 0xff;
+        $green = ($color >> 8) & 0xff;
+        $blue = $color & 0xff;
+        if ($red != $green || $green != $blue) {
+            echo "non grayscale pixel detected\n";
+            break 2;
+        }
+    }
+}
+?>
+===DONE===
+--EXPECT--
+bool(true)
+===DONE===