How do I determine if an indexed mode SDL_Surface has transparency or not?

Question

I've got code I've been working with to load and convert SDL_Surfaces to OpenGL textures, however I've realized that they only work with RGB(A) surfaces. I need to extend the support onto indexed mode images (with or without transparency).

Initially, I was looking into ::SDL_SetColorKey(), however it seems to only work with SDL blits. I've read up on SDL_Surface, SDL_PixelFormat and SDL_Color, and then begun to sketch up the following (//# is pseudocode):

SDL_Surface *pSurf(::IMG_Load(image));
::SDL_LockSurface(pSurf);

Uint32 bytesPerPixel(pSurf->format->bytesPerPixel);
GLenum pixelFormat;

Uint8  *pixelData(pSurf->pixels);
bool   allocated(false); // pixelData isn't allocated

if(pSurf->format->palette != 0) // indexed mode image
{
  //# Determine transparency. // HOW?
  //# bytesPerPixel = 3 or 4 depending on transparency being present;
  //# pixelFormat = GL_RGB or GL_RGBA depending on bytesPerPixel;

  Uint32 blockSize(pSurf->w * pSurf->h * bytesPerPixel);
  pixelData = new Uint8[blockSize];
  allocated = true;

  //# traverse pSurf->pixels, look up pSurf->format->palette references and copy
  // colors into pixelData;
}
else
{
  //# Determine pixelFormat based on bytesPerPixel and pSurf->format->Rmask
  // (GL_RGB(A) or GL_BGR(A)).
}

//# Pass bytesPerPixel, pixelFormat and pixelData to OpenGL (generate texture,
// set texture parameters, glTexImage2D etc).

if(allocated)
{
  delete[] pixelData;
  pixelData = 0;
}

::SDL_UnlockSurface(pSurf);
::SDL_FreeSurface(pSurf);

So, the question is: how can I determine if the indexed mode image I'm passing to this routine has transparency?

Answer 1

The typical way it's done for indexed mode is either to have a full 32-bit RGBA palette, so you have 8 bits of alpha per indexed color slot. Or, you can just define a certain (range of) palette index as being transparent.

OpenGL supports the latter, through the GL_PIXEL_MAP_I_TO_A table accessed through glPixelMap(). See glPixelTransfer() for a description of the translation logic.