From db8ae756e9d21e78e143accdfb62e3913cd0dbdc Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <sezeroz@gmail.com>
Date: Sat, 24 Mar 2018 22:44:12 +0300
Subject: [PATCH] backport fix for bug #2976:

Fix RGBA<->RGBA blit that was broken with the optimization from bug #11
id.zeta@yahoo.com
---
 src/video/SDL_blit_N.c | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/src/video/SDL_blit_N.c b/src/video/SDL_blit_N.c
index ac029d1d3..146a10007 100644
--- a/src/video/SDL_blit_N.c
+++ b/src/video/SDL_blit_N.c
@@ -2039,6 +2039,30 @@ static void Blit4to4MaskAlpha(SDL_BlitInfo *info)
 	}
 }
 
+/* blits 32 bit RGBA<->RGBA with both surfaces having the same R,G,B,A fields */
+static void Blit4to4CopyAlpha(SDL_BlitInfo *info)
+{
+	int width = info->d_width;
+	int height = info->d_height;
+	Uint32 *src = (Uint32 *)info->s_pixels;
+	int srcskip = info->s_skip;
+	Uint32 *dst = (Uint32 *)info->d_pixels;
+	int dstskip = info->d_skip;
+
+	/* RGBA->RGBA, COPY_ALPHA */
+	while ( height-- ) {
+		DUFFS_LOOP(
+		{
+			*dst = *src;
+			++dst;
+			++src;
+		},
+		width);
+		src = (Uint32*)((Uint8*)src + srcskip);
+		dst = (Uint32*)((Uint8*)dst + dstskip);
+	}
+}
+
 static void BlitNtoN(SDL_BlitInfo *info)
 {
 	int width = info->d_width;
@@ -2469,12 +2493,21 @@ SDL_loblit SDL_CalculateBlitN(SDL_Surface *surface, int blit_index)
 		blitfun = table[which].blitfunc;
 
 		if(blitfun == BlitNtoN) {  /* default C fallback catch-all. Slow! */
-			/* Fastpath C fallback: 32bit RGB<->RGBA blit with matching RGB */
 			if ( srcfmt->BytesPerPixel == 4 && dstfmt->BytesPerPixel == 4 &&
 			     srcfmt->Rmask == dstfmt->Rmask &&
 			     srcfmt->Gmask == dstfmt->Gmask &&
 			     srcfmt->Bmask == dstfmt->Bmask ) {
-				blitfun = Blit4to4MaskAlpha;
+				if( a_need == COPY_ALPHA ) {
+				    if( srcfmt->Amask == dstfmt->Amask ) {
+				    /* Fastpath C fallback: 32bit RGBA<->RGBA blit with matching RGBA */
+					blitfun = Blit4to4CopyAlpha;
+				    } else {
+					blitfun = BlitNtoNCopyAlpha;
+				    }
+				} else {
+				    /* Fastpath C fallback: 32bit RGB<->RGBA blit with matching RGB */
+				    blitfun = Blit4to4MaskAlpha;
+				}
 			} else if ( a_need == COPY_ALPHA ) {
 			    blitfun = BlitNtoNCopyAlpha;
 			}