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

Question

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?

Answer 1

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++;
    }
}