From fe8930b3a2bb53000da362ac94c494f40ae95fbb Mon Sep 17 00:00:00 2001
From: Sam Lantinga <slouken@libsdl.org>
Date: Sat, 10 Oct 2009 07:48:15 +0000
Subject: [PATCH] Fixed bug #627

Increased accuracy of alpha blend calculation
---
 src/video/SDL_blit.h   | 19 +++----------------
 src/video/SDL_blit_A.c |  8 ++++----
 2 files changed, 7 insertions(+), 20 deletions(-)

diff --git a/src/video/SDL_blit.h b/src/video/SDL_blit.h
index ab1c12753..fc20ba04a 100644
--- a/src/video/SDL_blit.h
+++ b/src/video/SDL_blit.h
@@ -384,22 +384,9 @@ do {									   \
 /* Blend the RGB values of two Pixels based on a source alpha value */
 #define ALPHA_BLEND(sR, sG, sB, A, dR, dG, dB)	\
 do {						\
-	dR = (((sR-dR)*(A))>>8)+dR;		\
-	dG = (((sG-dG)*(A))>>8)+dG;		\
-	dB = (((sB-dB)*(A))>>8)+dB;		\
-} while(0)
-
-/* Blend the RGB values of two Pixels based on a source alpha value */
-#define ACCURATE_ALPHA_BLEND(sR, sG, sB, sA, dR, dG, dB)	\
-do {						\
-    unsigned tR, tG, tB, tA; \
-    tA = 255 - sA; \
-    tR = 1 + (sR * sA) + (dR * tA); \
-    dR = (tR + (tR >> 8)) >> 8; \
-    tG = 1 + (sG * sA) + (dG * tA); \
-    dG = (tG + (tG >> 8)) >> 8; \
-    tB = 1 + (sB * sA) + (dB * tA); \
-    dB = (tB + (tB >> 8)) >> 8; \
+	dR = (((sR-dR)*(A)+255)>>8)+dR;		\
+	dG = (((sG-dG)*(A)+255)>>8)+dG;		\
+	dB = (((sB-dB)*(A)+255)>>8)+dB;		\
 } while(0)
 
 
diff --git a/src/video/SDL_blit_A.c b/src/video/SDL_blit_A.c
index b1237052e..4ecb5215e 100644
--- a/src/video/SDL_blit_A.c
+++ b/src/video/SDL_blit_A.c
@@ -861,7 +861,7 @@ static void Blit32to565PixelAlphaAltivec(SDL_BlitInfo *info)
                 dR = (dstpixel >> 8) & 0xf8; \
                 dG = (dstpixel >> 3) & 0xfc; \
                 dB = (dstpixel << 3) & 0xf8; \
-                ACCURATE_ALPHA_BLEND(sR, sG, sB, sA, dR, dG, dB); \
+                ALPHA_BLEND(sR, sG, sB, sA, dR, dG, dB); \
                 *((unsigned short *)dst) = ( \
                     ((dR & 0xf8) << 8) | ((dG & 0xfc) << 3) | (dB >> 3) \
                 ); \
@@ -994,7 +994,7 @@ static void Blit32to32SurfaceAlphaKeyAltivec(SDL_BlitInfo *info)
             if(sA && Pixel != ckey) { \
                 RGB_FROM_PIXEL(Pixel, srcfmt, sR, sG, sB); \
                 DISEMBLE_RGB(((Uint8 *)dstp), 4, dstfmt, Pixel, dR, dG, dB); \
-                ACCURATE_ALPHA_BLEND(sR, sG, sB, sA, dR, dG, dB); \
+                ALPHA_BLEND(sR, sG, sB, sA, dR, dG, dB); \
                 ASSEMBLE_RGBA(((Uint8 *)dstp), 4, dstfmt, dR, dG, dB, dA); \
             } \
             dstp++; \
@@ -1097,7 +1097,7 @@ static void Blit32to32PixelAlphaAltivec(SDL_BlitInfo *info)
             DISEMBLE_RGBA((Uint8 *)srcp, 4, srcfmt, Pixel, sR, sG, sB, sA); \
             if(sA) { \
               DISEMBLE_RGBA((Uint8 *)dstp, 4, dstfmt, Pixel, dR, dG, dB, dA); \
-              ACCURATE_ALPHA_BLEND(sR, sG, sB, sA, dR, dG, dB); \
+              ALPHA_BLEND(sR, sG, sB, sA, dR, dG, dB); \
               ASSEMBLE_RGBA((Uint8 *)dstp, 4, dstfmt, dR, dG, dB, dA); \
             } \
             ++srcp; \
@@ -1296,7 +1296,7 @@ static void Blit32to32SurfaceAlphaAltivec(SDL_BlitInfo *info)
             unsigned sR, sG, sB, dR, dG, dB; \
             DISEMBLE_RGB(((Uint8 *)srcp), 4, srcfmt, Pixel, sR, sG, sB); \
             DISEMBLE_RGB(((Uint8 *)dstp), 4, dstfmt, Pixel, dR, dG, dB); \
-            ACCURATE_ALPHA_BLEND(sR, sG, sB, sA, dR, dG, dB); \
+            ALPHA_BLEND(sR, sG, sB, sA, dR, dG, dB); \
             ASSEMBLE_RGBA(((Uint8 *)dstp), 4, dstfmt, dR, dG, dB, dA); \
             ++srcp; \
             ++dstp; \