I have an issue to display sprites in SDL

Question

I am doing a platform game with SDL but I have a bug which I don't know how can I fix it.

I read a map that looks like this :

0=sprites/background.jpeg
1=sprites/dirt.jpeg

2
sprites/ww_left.png
sprites/ww_right.png

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

I display 1 and 2 by simply blitting my picture and I display 0 by taking a pixels from my picture.

These are my displaying functions : https://github.com/vladydan/Darien_Reborn/blob/master/srcs/aff_map.c

And my problem is that the background overwrite my character sprites. This is the display when I begin, it looks ok :

And this is what happens when I move too much :

If I continue to move to the right the display will be good again but there is always a moment where my sprite is overwritten. I know where does it come from but I have no idea how to fix it, do you have an idea ?

Answer 1

 while (++i != 16)
    {
      pos.x = 0;
      j = -1;
      while (++j != 15)
    {
      if (game->map.map[i][j] == 1)
        SDL_BlitSurface(game->map.sprites[1], NULL, game->sdl.screen,
                &pos);
      else if ((game->map.map[i][j] == 0 || game->map.map[i][j] == 2))
        aff_background(game, pos);
      if (game->map.map[i][j] == 2)
        aff_sprite(game, pos);
      pos.x += 100;
    }
      pos.y += 52;
    }
  SDL_Flip(game->sdl.screen);
  return (0);
}

Seems like you're just arbitrarily rendering in some kind of logically-controlled order. I think the best thing to do is to render in descending order in the index that you've given us, rendering each layer one by one, with the first layer as the background.

You might also want to try and use SDL_Texture as well to use hardware acceleration, which might also fix your problems with the rendering. Perhaps it could be a rendering mode problem as well.

Answer 2

Speculating: When you do direct pixel access, as in the function aff_background, you have to lock the surface, especially in hardware modes.

An easy way to check if this is your problem (not 100% conclusive), is to call SDL_SetVideoMode with the SDL_SWSURFACE flag instead. Even better check SDL_MUSTLOCK.

In general prefer blitting subimages to raw pixel writes. It's possible blits are queued or even done asynchronously, and the pixel writes interrupt the draw ordering.

Good luck.

Edit: After reading the other answer and going through your code a bit more, here's a hack that should give you the result you want,

while (++i != 16)
  {
    pos.x = 0;
    j = -1;
    while (++j != 15)
      {
        if (game->map.map[i][j] == 1)
          SDL_BlitSurface(game->map.sprites[1], NULL, game->sdl.screen,
                          &pos);
        else if ((game->map.map[i][j] == 0 || game->map.map[i][j] == 2))
          aff_background(game, pos);
        if (game->map.map[i][j] == 2)
          // record sprite starting position from map entry.
          sprite_start_pos = pos;
        pos.x += 100;
      }
    pos.y += 52;
  }
// write sprite image after everything else.
aff_sprite(game, sprite_start_pos);
SDL_Flip(game->sdl.screen);