From 235c87dc92c9bc9a1b5c30a3a52b8ca17653b7dd Mon Sep 17 00:00:00 2001
From: Sam Lantinga <slouken@libsdl.org>
Date: Wed, 25 Sep 2024 21:54:05 -0700
Subject: [PATCH] Don't use BlitARGBto555PixelAlpha() for
 SDL_PIXELFORMAT_ARGB1555

This didn't properly take into account destination alpha.

Fixes https://github.com/libsdl-org/SDL/issues/8401

(cherry picked from commit 1aea43846e0286d44d3249a02220dab222e38218)
(cherry picked from commit c9f3cbe02f1b81f6591608f0f19b35c7d3a1347c)
---
 src/video/SDL_blit_A.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/src/video/SDL_blit_A.c b/src/video/SDL_blit_A.c
index 2cf00e0a2c..96fec1b478 100644
--- a/src/video/SDL_blit_A.c
+++ b/src/video/SDL_blit_A.c
@@ -1222,10 +1222,9 @@ static void BlitARGBto565PixelAlpha(SDL_BlitInfo *info)
         DUFFS_LOOP4({
         Uint32 s = *srcp;
         unsigned alpha = s >> 27; /* downscale alpha to 5 bits */
-        /* FIXME: Here we special-case opaque alpha since the
+        /* Here we special-case opaque alpha since the
            compositioning used (>>8 instead of /255) doesn't handle
-           it correctly. Also special-case alpha=0 for speed?
-           Benchmark this! */
+           it correctly. */
         if (alpha) {
           if (alpha == (SDL_ALPHA_OPAQUE >> 3)) {
             *dstp = (Uint16)((s >> 8 & 0xf800) + (s >> 5 & 0x7e0) + (s >> 3  & 0x1f));
@@ -1235,8 +1234,7 @@ static void BlitARGBto565PixelAlpha(SDL_BlitInfo *info)
              * convert source and destination to G0RAB65565
              * and blend all components at the same time
              */
-            s = ((s & 0xfc00) << 11) + (s >> 8 & 0xf800)
-              + (s >> 3 & 0x1f);
+            s = ((s & 0xfc00) << 11) + (s >> 8 & 0xf800) + (s >> 3 & 0x1f);
             d = (d | d << 16) & 0x07e0f81f;
             d += (s - d) * alpha >> 5;
             d &= 0x07e0f81f;
@@ -1268,21 +1266,19 @@ static void BlitARGBto555PixelAlpha(SDL_BlitInfo *info)
         unsigned alpha;
         Uint32 s = *srcp;
         alpha = s >> 27; /* downscale alpha to 5 bits */
-        /* FIXME: Here we special-case opaque alpha since the
+        /* Here we special-case opaque alpha since the
            compositioning used (>>8 instead of /255) doesn't handle
-           it correctly. Also special-case alpha=0 for speed?
-           Benchmark this! */
+           it correctly. */
         if (alpha) {
           if (alpha == (SDL_ALPHA_OPAQUE >> 3)) {
             *dstp = (Uint16)((s >> 9 & 0x7c00) + (s >> 6 & 0x3e0) + (s >> 3  & 0x1f));
           } else {
             Uint32 d = *dstp;
             /*
-             * convert source and destination to G0RAB65565
+             * convert source and destination to G0RAB55555
              * and blend all components at the same time
              */
-            s = ((s & 0xf800) << 10) + (s >> 9 & 0x7c00)
-              + (s >> 3 & 0x1f);
+            s = ((s & 0xf800) << 10) + (s >> 9 & 0x7c00) + (s >> 3 & 0x1f);
             d = (d | d << 16) & 0x03e07c1f;
             d += (s - d) * alpha >> 5;
             d &= 0x03e07c1f;
@@ -1452,7 +1448,7 @@ SDL_BlitFunc SDL_CalculateBlitA(SDL_Surface *surface)
             if (sf->BytesPerPixel == 4 && sf->Amask == 0xff000000 && sf->Gmask == 0xff00 && ((sf->Rmask == 0xff && df->Rmask == 0x1f) || (sf->Bmask == 0xff && df->Bmask == 0x1f))) {
                 if (df->Gmask == 0x7e0) {
                     return BlitARGBto565PixelAlpha;
-                } else if (df->Gmask == 0x3e0) {
+                } else if (df->Gmask == 0x3e0 && !df->Amask) {
                     return BlitARGBto555PixelAlpha;
                 }
             }