From: "Alex Volkov"
authorSam Lantinga <slouken@libsdl.org>
Wed, 15 Mar 2006 15:43:15 +0000
changeset 154398f9b16f565c
parent 1542 a8bf1aa21020
child 1544 ab1e4c41ab71
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.
src/video/SDL_blit_N.c
     1.1 --- a/src/video/SDL_blit_N.c	Wed Mar 15 15:39:29 2006 +0000
     1.2 +++ b/src/video/SDL_blit_N.c	Wed Mar 15 15:43:15 2006 +0000
     1.3 @@ -2194,6 +2194,10 @@
     1.4  	int srcbpp = srcfmt->BytesPerPixel;
     1.5  	int dstbpp = dstfmt->BytesPerPixel;
     1.6  	unsigned alpha = dstfmt->Amask ? srcfmt->alpha : 0;
     1.7 +	Uint32 rgbmask = ~srcfmt->Amask;
     1.8 +
     1.9 +	/* Set up some basic variables */
    1.10 +	ckey &= rgbmask;
    1.11  
    1.12  	while ( height-- ) {
    1.13  		DUFFS_LOOP(
    1.14 @@ -2203,7 +2207,7 @@
    1.15  			unsigned sG;
    1.16  			unsigned sB;
    1.17  			RETRIEVE_RGB_PIXEL(src, srcbpp, Pixel);
    1.18 -			if ( Pixel != ckey ) {
    1.19 +			if ( (Pixel & rgbmask) != ckey ) {
    1.20  			        RGB_FROM_PIXEL(Pixel, srcfmt, sR, sG, sB);
    1.21  				ASSEMBLE_RGBA(dst, dstbpp, dstfmt,
    1.22  					      sR, sG, sB, alpha);