c++pixelbitsdl-2color-palette

SDL2 1-bit surface, assign one of two colors in palette to a pixel


in c++ SDL2 i would want to access pixels and draw on a 1-bit surface same way i do with a 8-bit surface but i'm missing something i don't see.

in 8-bit surface this works fine (the pixel take the expected color among palette->colors):

SDL_Surface * surface = SDL_CreateRGBSurfaceWithFormat(0, width, height, 8, SDL_PIXELFORMAT_INDEX8);
SDL_Color colors[2] = {{0, 0, 0, 255}, {255, 255, 255, 255}};
SDL_SetPaletteColors(surface->format->palette, colors, 0, 2);
Uint8* pixels = (Uint8*)surface->pixels;
    for(int i = 0; i < pixelsCount; i++){
        if(*geometrical condition*){
            pixels[i]=1;
        }else{
            pixels[i]=0;
        }
    }

with a 1-bit surface i get a non 0 exit because of assignment (last if):

SDL_Surface * surface = SDL_CreateRGBSurfaceWithFormat(0, width, height, 1, SDL_PIXELFORMAT_INDEX1MSB);
SDL_Color colors[2] = {{0, 0, 0, 255}, {255, 255, 255, 255}};
SDL_SetPaletteColors(surface->format->palette, colors, 0, 2);
Uint8* pixels = (Uint8*)surface->pixels;
    for(int i = 0; i < pixelsCount; i++){
        if(*geometrical condition*){
            pixels[i]=1;
        }else{
            pixels[i]=0;
        }
    }

What's the right pixel assignment in 1-bit case?


Solution

  • well, this worked for me, maybe it is not the best solution but:

    1. it works
    2. it can be didactic for someone else

    remind, 1-bitperpixel surface, SDL_PIXELFORMAT_INDEX1MSB.

    //coordinates
    int x, y;
    
    //more pixels than in SCREEN_WIDTH, not used here, but useful for /*geom. cond.*/
    int actualWidthPixels = surface1->pitch * CHAR_BIT;
    
    //follow the pixel
    int pixel{};
    
    //iterate each byte
    for( uint8_t* byte = pixels; byte - pixels < surface1->pitch * surface1->h; byte++)
    {
        //iterate each bit in byte
        for(int bit = 0; bit < CHAR_BIT; bit++)
        {
            x = pixel % (surface1->pitch * CHAR_BIT);
            y = (byte - pixels) / surface1->pitch;//pixels = (uint8_t*)s->pixels;
        
            if (/*geometrical condition*/)
            {
                //set the bit
                *byte |= (1 << 7-bit);
            }
            pixel++;
        }
    }