From 3c19ce0666c27feb0a0de48a7c32b61a4d918765 Mon Sep 17 00:00:00 2001
From: Sam Lantinga <slouken@libsdl.org>
Date: Sun, 7 Jul 2013 18:23:04 -0700
Subject: [PATCH] Fixed bug 1943 - Wrong handling of legacy 32bpp BMP files

Kang Seonghoon

While BMP format supports alpha channel, it is enabled only when the header is at least 56 bytes long (BITMAPV3INFOHEADER and later). For very common 40-byte-long header (BITMAPINFOHEADER) 32bpp format should be interpreted as BGRX format, but currently SDL interprets them as BGRA format and causes a significant compatibility problem as many 32bpp files use a padding byte of 0 ("transparent" in BGRA interpretation).

---

I fixed this by checking to see if the alpha channel is all 0, and if so, setting it opaque.
---
 src/video/SDL_bmp.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/src/video/SDL_bmp.c b/src/video/SDL_bmp.c
index 7ee65d85d..df43ed0ce 100644
--- a/src/video/SDL_bmp.c
+++ b/src/video/SDL_bmp.c
@@ -47,6 +47,35 @@
 #endif
 
 
+static void CorrectAlphaChannel(SDL_Surface *surface)
+{
+    /* Check to see if there is any alpha channel data */
+    SDL_bool hasAlpha = SDL_FALSE;
+#if SDL_BYTEORDER == SDL_BIG_ENDIAN
+    int alphaChannelOffset = 0;
+#else
+    int alphaChannelOffset = 3;
+#endif
+    Uint8 *alpha = ((Uint8*)surface->pixels) + alphaChannelOffset;
+    Uint8 *end = alpha + surface->h * surface->pitch;
+
+    while (alpha < end) {
+        if (*alpha != 0) {
+            hasAlpha = SDL_TRUE;
+            break;
+        }
+        alpha += 4;
+    }
+
+    if (!hasAlpha) {
+        alpha = ((Uint8*)surface->pixels) + alphaChannelOffset;
+        while (alpha < end) {
+            *alpha = SDL_ALPHA_OPAQUE;
+            alpha += 4;
+        }
+    }
+}
+
 SDL_Surface *
 SDL_LoadBMP_RW(SDL_RWops * src, int freesrc)
 {
@@ -64,6 +93,7 @@ SDL_LoadBMP_RW(SDL_RWops * src, int freesrc)
     Uint8 *top, *end;
     SDL_bool topDown;
     int ExpandBMP;
+    SDL_bool correctAlpha = SDL_FALSE;
 
     /* The Win32 BMP file header (14 bytes) */
     char magic[2];
@@ -182,6 +212,8 @@ SDL_LoadBMP_RW(SDL_RWops * src, int freesrc)
 #endif
                 break;
             case 32:
+                /* We don't know if this has alpha channel or not */
+                correctAlpha = SDL_TRUE;
                 Amask = 0xFF000000;
                 Rmask = 0x00FF0000;
                 Gmask = 0x0000FF00;
@@ -358,6 +390,9 @@ SDL_LoadBMP_RW(SDL_RWops * src, int freesrc)
             bits -= surface->pitch;
         }
     }
+    if (correctAlpha) {
+        CorrectAlphaChannel(surface);
+    }
   done:
     if (was_error) {
         if (src) {