From: "Alex Volkov"

Date: Thu, 10 Nov 2005 21:53:40 -0500
Subject: [SDL] BUG[?]: 32bpp RGBA->RGB colorkey blit, no SDL_SRCALPHA

It seems there is either a documentation vs. reality mismatch or a real bug
in SDL_blit_N.c:BlitNtoNKey().
The exact blit in question is a 32bpp RGBA->RGB, where RGBA has SDL_COLORKEY
and *no* SDL_SRCALPHA flags. The doc in SDL_video.h states:
* RGBA->RGB:
*     SDL_SRCALPHA not set:
*       copy RGB.
*       if SDL_SRCCOLORKEY set, only copy the pixels matching the
*       RGB values of the source colour key, ignoring alpha in the
*       comparison.

BlitNtoNKey(), however, forgets to "ignore alpha in the comparison". The
documentation makes perfect sense, so I think it is the code that is faulty.

The attached patch corrects the code.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401547

src/video/SDL_blit_N.c

@@ -2194,6 +2194,10 @@ static void BlitNtoNKey(SDL_BlitInfo *info)
 	int srcbpp = srcfmt->BytesPerPixel;
 	int dstbpp = dstfmt->BytesPerPixel;
 	unsigned alpha = dstfmt->Amask ? srcfmt->alpha : 0;
+	Uint32 rgbmask = ~srcfmt->Amask;
+
+	/* Set up some basic variables */
+	ckey &= rgbmask;
 
 	while ( height-- ) {
 		DUFFS_LOOP(
@@ -2203,7 +2207,7 @@ static void BlitNtoNKey(SDL_BlitInfo *info)
 			unsigned sG;
 			unsigned sB;
 			RETRIEVE_RGB_PIXEL(src, srcbpp, Pixel);
-			if ( Pixel != ckey ) {
+			if ( (Pixel & rgbmask) != ckey ) {
 			        RGB_FROM_PIXEL(Pixel, srcfmt, sR, sG, sB);
 				ASSEMBLE_RGBA(dst, dstbpp, dstfmt,
 					      sR, sG, sB, alpha);