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Fixed bug 3352 - Adding alpha mask support to SDL_SaveBMP_RW

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Simon Hug

The current SDL_SaveBMP_RW function that saves surfaces to a BMP uses an old bitmap header which doesn't officially support alpha channels. Applications just ignore the byte where the alpha is stored. This can easily be extended by using a newer header version and setting the alpha mask.

The attached patch has these changes:

- Extending the description of the function in the SDL_surface.h header with the supported formats.
- Refining when surfaces get stored to a 32-bit BMP. (Must have bit depth of 8 or higher and must have an alpha mask or colorkey.)
- Fixing a small bug that saves 24-bit BGR surfaces with a colorkey in a 24-bit BMP.
- Adding code that switches to the bitmap header version 4 if the surface has an alpha mask or colorkey. (I chose version 4 because Microsoft didn't lose its documentation behind a file cabinet like they did with version 3.)
- Adding a hint that can disable the use of the version 4 header. This is for people that need the legacy header or like the old behavior better. (I'm not sure about the hint name, though. May need changing if there are any rules to that.)
This commit is contained in:
Sam Lantinga 2016-10-01 11:29:13 -07:00
parent e8128466a6
commit b8f456007d
3 changed files with 84 additions and 4 deletions
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63
src/video/SDL_bmp.c
View file

@ -32,6 +32,7 @@
This code currently supports Win32 DIBs in uncompressed 8 and 24 bpp.
*/
#include "SDL_hints.h"
#include "SDL_video.h"
#include "SDL_assert.h"
#include "SDL_endian.h"
@ -47,6 +48,11 @@
#define BI_BITFIELDS 3
#endif
/* Logical color space values for BMP files */
#ifndef LCS_WINDOWS_COLOR_SPACE
/* 0x57696E20 == "Win " */
#define LCS_WINDOWS_COLOR_SPACE 0x57696E20
#endif
static void CorrectAlphaChannel(SDL_Surface *surface)
{
@ -457,6 +463,8 @@ SDL_SaveBMP_RW(SDL_Surface * saveme, SDL_RWops * dst, int freedst)
int i, pad;
SDL_Surface *surface;
Uint8 *bits;
SDL_bool save32bit = SDL_FALSE;
SDL_bool saveLegacyBMP = SDL_FALSE;
/* The Win32 BMP file header (14 bytes) */
char magic[2] = { 'B', 'M' };
@ -478,14 +486,24 @@ SDL_SaveBMP_RW(SDL_Surface * saveme, SDL_RWops * dst, int freedst)
Uint32 biClrUsed;
Uint32 biClrImportant;
/* The additional header members from the Win32 BITMAPV4HEADER struct (108 bytes in total) */
Uint32 bV4RedMask = 0;
Uint32 bV4GreenMask = 0;
Uint32 bV4BlueMask = 0;
Uint32 bV4AlphaMask = 0;
Uint32 bV4CSType = 0;
Sint32 bV4Endpoints[3 * 3] = {0};
Uint32 bV4GammaRed = 0;
Uint32 bV4GammaGreen = 0;
Uint32 bV4GammaBlue = 0;
/* Make sure we have somewhere to save */
surface = NULL;
if (dst) {
SDL_bool save32bit = SDL_FALSE;
#ifdef SAVE_32BIT_BMP
/* We can save alpha information in a 32-bit BMP */
if (saveme->map->info.flags & SDL_COPY_COLORKEY ||
saveme->format->Amask) {
if (saveme->format->BitsPerPixel >= 8 && (saveme->format->Amask ||
saveme->map->info.flags & SDL_COPY_COLORKEY)) {
save32bit = SDL_TRUE;
}
#endif /* SAVE_32BIT_BMP */
@ -497,7 +515,7 @@ SDL_SaveBMP_RW(SDL_Surface * saveme, SDL_RWops * dst, int freedst)
SDL_SetError("%d bpp BMP files not supported",
saveme->format->BitsPerPixel);
}
} else if ((saveme->format->BitsPerPixel == 24) &&
} else if ((saveme->format->BitsPerPixel == 24) && !save32bit &&
#if SDL_BYTEORDER == SDL_LIL_ENDIAN
(saveme->format->Rmask == 0x00FF0000) &&
(saveme->format->Gmask == 0x0000FF00) &&
@ -537,6 +555,13 @@ SDL_SaveBMP_RW(SDL_Surface * saveme, SDL_RWops * dst, int freedst)
return -1;
}
if (save32bit) {
const char *hint = SDL_GetHint(SDL_HINT_BMP_SAVE_LEGACY_FORMAT);
if (hint != NULL && (hint[0] == '1' && hint[1] == 0)) {
saveLegacyBMP = SDL_TRUE;
}
}
if (surface && (SDL_LockSurface(surface) == 0)) {
const int bw = surface->w * surface->format->BytesPerPixel;
@ -572,6 +597,21 @@ SDL_SaveBMP_RW(SDL_Surface * saveme, SDL_RWops * dst, int freedst)
}
biClrImportant = 0;
/* Set the BMP info values for the version 4 header */
if (save32bit && !saveLegacyBMP) {
biSize = 108;
biCompression = BI_BITFIELDS;
/* The BMP format is always little endian, these masks stay the same */
bV4RedMask = 0x00ff0000;
bV4GreenMask = 0x0000ff00;
bV4BlueMask = 0x000000ff;
bV4AlphaMask = 0xff000000;
bV4CSType = LCS_WINDOWS_COLOR_SPACE;
bV4GammaRed = 0;
bV4GammaGreen = 0;
bV4GammaBlue = 0;
}
/* Write the BMP info values */
SDL_WriteLE32(dst, biSize);
SDL_WriteLE32(dst, biWidth);
@ -585,6 +625,21 @@ SDL_SaveBMP_RW(SDL_Surface * saveme, SDL_RWops * dst, int freedst)
SDL_WriteLE32(dst, biClrUsed);
SDL_WriteLE32(dst, biClrImportant);
/* Write the BMP info values for the version 4 header */
if (save32bit && !saveLegacyBMP) {
SDL_WriteLE32(dst, bV4RedMask);
SDL_WriteLE32(dst, bV4GreenMask);
SDL_WriteLE32(dst, bV4BlueMask);
SDL_WriteLE32(dst, bV4AlphaMask);
SDL_WriteLE32(dst, bV4CSType);
for (i = 0; i < 3 * 3; i++) {
SDL_WriteLE32(dst, bV4Endpoints[i]);
}
SDL_WriteLE32(dst, bV4GammaRed);
SDL_WriteLE32(dst, bV4GammaGreen);
SDL_WriteLE32(dst, bV4GammaBlue);
}
/* Write the palette (in BGR color order) */
if (surface->format->palette) {
SDL_Color *colors;