SDL: Combination of Tile sets and layered surfaces does not work

Question

I would like to create a modularly designed game using SDL, but yet I fail to get my sprites displayed. Precisely, I am trying to implement tile sets, which are are bunch of equally sized sprites collected in one single PNG file (in my case).

My data structure should hold an array of sprites available in the tile set which can then be drawn with a method, like draw(Tile *, Position, Layer);.

As indicated, I want to feature multiple layers of surfaces to later on implement multiple independent background layers and a foreground layer. Similarly, I have an array of layers that are blitted onto my screen surface (created with SDL_SetVideoMode) in a pre-defined order.

However, I don't get what's going wrong in my code. While a tile set is loaded, I can successfully blit the currently loaded tile onto a layer surface, like in this snippet:

this->graphic = SDL_CreateRGBSurface(SDL_HWSURFACE | SDL_SRCALPHA,
    tile_widths, tile_heights, bit_depth,
    rmask, gmask, bmask, amask);

SDL_BlitSurface(tileset_graphic, &tile_position, this->graphic, nullptr);
SDL_BlitSurface(tileset_graphic, &tile_position, 
    ((VideoController::get_instance())->layers)[0], &tile_position);

In the first line, I try to blit the part of the tileset_graphic, that corresponds to a sprite, to an SDL_Surface * that is held by my Tile structure to be used later on.

However, I cannot use this surface to blit onto a layer. The second (test) statement just copies the considered region of the tileset_graphic to the most bottom of my layers. Furthermore, I have performed several test commands in my main method.

My findings during testing:

• I can blit a tileset_graphic piece to a layer and it is correctly shown (see above)

• I can blit a Tile directly onto the screen:

  SDL_BlitSurface(
    tile->graphic,
    nullptr,
    (VideoController::the_instance)->screen,
    &relative_position);
  

• However, I cannot blit a Tile onto a layer:

  SDL_BlitSurface(
    tile->graphic,
    nullptr,
    (VideoController::the_instance)->layers[0],
    &relative_position);
  

To be more precise, when I blit the whole tileset_graphic for testing and then blit a Tile onto a region that is already occupied due to this test, I can partly see my sprite. But then again, I have absolutely no clue why this is the case...

Summary:

I try to blit several SDL_Surfaces onto another, but seem to fail only by trying this (desired) chain of surfaces:

graphic --> layer --> screen

Does anyone have a clue what may go wrong, or is able to tell me which additional information is needed by you guys?

EDIT

I was able to find that blitting on the initialized (SDL_CreateRGBSurface), but still "untouched" layer surfaces seems to fail somehow. When I use SDL_FillRect on my layer surfaces before blitting a tile onto the layer, it is displayed correctly. However, I am losing transparency of layers this way...

Answer 1

I figured out the solution.

I had to explicitly reset the SDL_SRCALPHA flag of my graphic by doing

SDL_SetAlpha(this->graphic, 0, SDL_ALPHA_OPAQUE );