// Trim off some of the image.
void Image::trim(Uint16 left, Uint16 top, Uint16 right, Uint16 bottom) 
throw (SDL_Exception)
{
  // Check that the image isn't empty.
  if (m_surface == NULL)
  {
    throw SDL_Exception("Can't trim an empty image.");
  }
  
  // Create an empty image of the correct size.
  SDL_Surface* trimmed = SDL_CreateRGBSurface(m_surface->flags, 
  right - left, bottom - top, m_surface->format->BitsPerPixel, 
  m_surface->format->Rmask, m_surface->format->Gmask, m_surface->format->Bmask, 
  m_surface->format->Amask);
  
  if (trimmed == NULL)
  {
    throw SDL_Exception("Couldn't create empty surface.");
  }
  
  Uint32 flags;
  Uint8 r, g, b, a;
  
  // Get the color key and alpha.
  getColorKey(r, g, b);
  a = getAlpha();
  flags = m_surface->flags;
  
  // Clear color key and alpha.
  clearColorKey();
  clearAlpha();
  
  // Find the area to save.
  SDL_Rect srcRect;
  srcRect.x = left;
  srcRect.y = top;
  srcRect.w = trimmed->w;
  srcRect.h = trimmed->h;
  
  // Draw the area to the trimmed surface.
  SDL_BlitSurface(m_surface, &srcRect, trimmed, NULL);
  
  // Set the alpha and color key of the trimmed image.
  SDL_SetAlpha(trimmed, flags, a);
  SDL_SetColorKey(trimmed, flags, SDL_MapRGB(trimmed->format, r, g, b));
  
  // Set the image to the trimmed one.
  SDL_FreeSurface(m_surface);
  m_surface = trimmed;
}