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.
--- a/src/video/SDL_blit_N.c Wed Mar 15 15:39:29 2006 +0000
+++ b/src/video/SDL_blit_N.c Wed Mar 15 15:43:15 2006 +0000
@@ -2194,6 +2194,10 @@
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 @@
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);