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Fixed bug 1943 - Wrong handling of legacy 32bpp BMP files

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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.
This commit is contained in:
Sam Lantinga 2013-07-07 18:23:04 -07:00
parent 0ed61eba4b
commit 435103b3d9
1 changed files with 35 additions and 0 deletions
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35
src/video/SDL_bmp.c
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@ -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) {