SDL_DisplayFormat man page
SDL_DisplayFormat — Convert a surface to the display format
SDL_Surface *SDL_DisplayFormat(SDL_Surface *surface);
This function takes a surface and copies it to a new surface of the pixel format and colors of the video framebuffer, suitable for fast blitting onto the display surface. It calls SDL_ConvertSurface
If you want to take advantage of hardware colorkey or alpha blit acceleration, you should set the colorkey and alpha value before calling this function.
If you want an alpha channel, see SDL_DisplayFormatAlpha.
If the conversion fails or runs out of memory, it returns NULL
SDL_ConvertSurface, SDL_DisplayFormatAlpha SDL_SetAlpha, SDL_SetColorKey, SDL_Surface