SDL2 Transparent surface - c++

Question

I am trying to create a transparent surface, blit it bigger surface (size of my screen), then create a texture of it, copy to the renderer, and finaly render present.
I have seen many other forums saying I have to use SetBlendMode (for surface, texture, and renderer), ColorKey, SetSurfaceBlendMode. I tried them all but I can't seem to get it to work. I read that SDL_BlitScaled is not suitable for combining surfaces that have transparent pixels, but I am completely lost as to what I have to do instead.
What I noticed, when I use SDL_CreateRGBSurface (with an alpha value) to create a surface, the surface's PixelFormat is RGB888 instead of RGBA8888 (what I am expecting since I provide the alpha value). Using SDL_ConvertSurfaceFormat did not help.
Can somebody tell me what I am missing?
Complete code: removed, my appologies
Please note that I have removed the attempts to get transparency working
The renderer:
mRenderer = SDL_CreateRenderer(mWindow, -1, SDL_RENDERER_ACCELERATED);
My render loop:
void CApp::Render()
{
SDL_RenderClear(mRenderer);
mBackGround->Render(mRenderer);
mForeGround->Render(mRenderer);
SDL_RenderPresent(mRenderer);
}
The big surface I am blitting to:
mSurface = SDL_CreateRGBSurface(0, CApp::Window_W(), CApp::Window_H(), 32, 0, 0, 0, 0);
The code to get tiles from a spritesheet:
bool Map::GetTilesFromSpriteSheet(SDL_Surface *pSpriteSheet, int pTile_w, int pTile_h)
{
if(pSpriteSheet->w % pTile_w == 0 && pSpriteSheet->h % pTile_h == 0) {
SDL_Rect srcRect;
srcRect.w = pTile_w;
srcRect.h = pTile_h;
for(int y = 0; y < pSpriteSheet->h / pTile_h; y++) {
for(int x = 0; x < pSpriteSheet->w / pTile_w; x++) {
srcRect.x = x*pTile_w;
srcRect.y = y*pTile_h;
SDL_Surface* tempSurface = SDL_CreateRGBSurface(0, pTile_w, pTile_h, 32, 0, 0, 0, 0);
if(SDL_BlitSurface(pSpriteSheet, &srcRect, tempSurface, nullptr)==0) {
mTiles.push_back(tempSurface);
} else {
Log("Error extracting tile (%d,%d)(w,h): %s", x, y, SDL_GetError());
return false;
}
}
}
Log("Number of tiles: %d", static_cast<int>(mTiles.size()));
} else {
Log("Background spritesheet is incompatible with tile dimensions (%d,%d)(w,h).", pTile_w, pTile_h);
return false;
}
return true;
}
This is where I combine the tiles to create the big surface:
bool Map::GenerateMap(std::vector<std::vector<int>> &pMap, std::vector<SDL_Surface*> &pTiles, SDL_Surface* pDestination)
{
SDL_Rect rect;
rect.w = mDstTile_W;
rect.h = mDstTile_H;
SDL_Surface* transparent = SDL_CreateRGBSurface(0, mDstTile_W, mDstTile_H, 32, 0, 0, 0, 0);
for(int y = 0; y < static_cast<int>(pMap.size()); y++) {
for(int x = 0; x < static_cast<int>(pMap.at(static_cast<unsigned long>(y)).size()); x++) {
rect.x = x*mDstTile_W;
rect.y = y*mDstTile_H;
int index = static_cast<int>(pMap.at(static_cast<unsigned long>(y)).at(static_cast<unsigned long>(x)));
if(index < 0) {
if(SDL_BlitScaled(transparent, nullptr, pDestination, &rect) != 0) {
Log("Error blitting transparent surface to destination: %s", SDL_GetError());
}
} else if(SDL_BlitScaled(pTiles[static_cast<unsigned long>(index)],
nullptr, pDestination, &rect) != 0) {
Log("Error blitting surface to destination: %s", SDL_GetError());
return false;
}
}
}
SDL_FreeSurface(transparent);
return true;
}
And finaly, this is the code to render the big surface to the screen, first by creating a texture:
void ForeGround::Render(SDL_Renderer* pRenderer)
{
if(mTexture) SDL_DestroyTexture(mTexture);
mTexture = SDL_CreateTextureFromSurface(pRenderer, mSurface);
if(mTexture == nullptr) {
Log("Unable to create foreground texture: %s", SDL_GetError());
} else if (SDL_RenderCopy(pRenderer, mTexture, nullptr, nullptr)) {
Log("Unable to render foreground: %s", SDL_GetError());
}
}

Answer 1

As #keltar mentioned, I was not creating surfaces with an alpha value.
I had to change the SDL_CreateRGBSurface call. The code snippet below creates an empty, transparent surface where I can blit to if needed (for example, tiles from a spritesheet).
SDL_Surface* tempSurface = SDL_CreateRGBSurface(..., 32, 0xff, 0xff00, 0xff0000, 0xff000000);
The code snippet below creates a black, non-transparent surface.
SDL_Surface* tempSurface = SDL_CreateRGBSurface(..., 32, 0xff, 0xff00, 0xff0000, 0x00000000);
This was enough for me, I did not have to use SDL_SetRenderDrawBlendMode, SDL_SetSurfaceBlendMode, SDL_SetTextureBlendMode, or SDL_ConvertSurface. A quick look at the SDL wiki showed:
By default surfaces with an alpha mask are set up for blending as with
SDL_SetSurfaceBlendMode(surface, SDL_BLENDMODE_BLEND)
Which explained why I did not have to call any of those functions. A big shout-out to keltar for his quick answer!