Support saving 1- and 4-bit MSB indexed surfaces as BMP

[madebr]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Description

• Only 1-bit and 4-bit MSB indexed surfaces are supported.
  1-bit LSB surfaces would have to be bit flipped, and 4-bit surfaces nibble swapped.
• While writing tests, I noticed SDL_ReadSurfacePixel does not support 1-bit, 2-bit or 4-bit indexed surfaces, so I fixed that as well.

Here's a little app showing what it generates:

#include <SDL3/SDL.h>

static void write_bmp_1bit(void)
{
    SDL_Surface *surface = SDL_CreateSurface(128, 128, SDL_PIXELFORMAT_INDEX1MSB);
    SDL_Palette *pal = SDL_CreateSurfacePalette(surface);

    const SDL_Color colors[2] = {
        {0xff, 0xff, 0xff, 0xff, },
        {0x00, 0x00, 0x00, 0xff, },
    };

    SDL_SetPaletteColors(pal, colors, 0, 2);

    for (int y = 0; y < surface->h; ++y) {
        Uint8 *row = (Uint8 *)surface->pixels + y * surface->pitch;
        for (int x = 0; x < surface->w; x += 8) {
            Uint8 p0 = (Uint8)((((x+0) + y) & 0xf) >= 8);
            Uint8 p1 = (Uint8)((((x+1) + y) & 0xf) >= 8);
            Uint8 p2 = (Uint8)((((x+2) + y) & 0xf) >= 8);
            Uint8 p3 = (Uint8)((((x+3) + y) & 0xf) >= 8);
            Uint8 p4 = (Uint8)((((x+4) + y) & 0xf) >= 8);
            Uint8 p5 = (Uint8)((((x+5) + y) & 0xf) >= 8);
            Uint8 p6 = (Uint8)((((x+6) + y) & 0xf) >= 8);
            Uint8 p7 = (Uint8)((((x+7) + y) & 0xf) >= 8);
            row[x / 8] = (p0 << 7) | (p1 << 6) | (p2 << 5) | (p3 << 4) | (p4 << 3) | (p5 << 2) | (p6 << 1) | (p7 << 0);
        }
    }

    const char *filename = "pattern_1bit.bmp";
    if (SDL_SaveBMP(surface, filename)) {
        SDL_Log("Wrote %s", filename);
    } else {
        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "SDL_SaveBMP failed (%s)", SDL_GetError());
    }
    SDL_DestroySurface(surface);
}
static void write_bmp_4bit(void)
{
    SDL_Surface *surface = SDL_CreateSurface(128, 128, SDL_PIXELFORMAT_INDEX4MSB);
    SDL_Palette *pal = SDL_CreateSurfacePalette(surface);

    const SDL_Color colors[16] = {
        {0xff, 0xff, 0xff, 0xff, },
        {0xee, 0xee, 0xee, 0xff, },
        {0xdd, 0xdd, 0xdd, 0xff, },
        {0xcc, 0xcc, 0xcc, 0xff, },
        {0xbb, 0xbb, 0xbb, 0xff, },
        {0xaa, 0xaa, 0xaa, 0xff, },
        {0x99, 0x99, 0x99, 0xff, },
        {0x88, 0x88, 0x88, 0xff, },
        {0x77, 0x77, 0x77, 0xff, },
        {0x66, 0x66, 0x66, 0xff, },
        {0x55, 0x55, 0x55, 0xff, },
        {0x44, 0x44, 0x44, 0xff, },
        {0x33, 0x33, 0x33, 0xff, },
        {0x22, 0x22, 0x22, 0xff, },
        {0x11, 0x11, 0x11, 0xff, },
        {0x00, 0x00, 0x00, 0xff, },
    };

    SDL_SetPaletteColors(pal, colors, 0, 16);

    for (int y = 0; y < surface->h; ++y) {
        Uint8 *row = (Uint8 *)surface->pixels + y * surface->pitch;
        for (int x = 0; x < surface->w; x += 2) {
            Uint8 p0 = (Uint8)(((x + 0) + y) & 0xf);
            Uint8 p1 = (Uint8)(((x + 1) + y) & 0xf);
            row[x / 2] = (p0 << 4) | (p1 << 0);
        }
    }

    const char *filename = "pattern_4bit.bmp";
    if (SDL_SaveBMP(surface, filename)) {
        SDL_Log("Wrote %s", filename);
    } else {
        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "SDL_SaveBMP failed (%s)", SDL_GetError());
    }
    SDL_DestroySurface(surface);
}

int main(void)
{
    SDL_Init(0);

    write_bmp_1bit();
    write_bmp_4bit();

    SDL_Quit();
    return 0;
}

1-bit BMP:
pattern_1bit.bmp

4-bit BMP:
pattern_4bit.bmp

Existing Issue(s)

[icculus]

Oh man, I literally ran into this today, good timing!!

[madebr]

I pushed a fix for widths that are a non-multiple of 2 (for 4MSB) and 8 (for 1MSB).