SDL_VideoModeOK returns 0 +if the requested mode is not supported under any bit depth, or returns the +bits-per-pixel of the closest available mode with the given width, height and requested surface flags (see SDL_SetVideoMode).
The bits-per-pixel value returned is only a suggested mode. You can usually request and bpp you want when setting the video mode and SDL will emulate that color depth with a shadow video surface.
The arguments to SDL_VideoModeOK are the same ones you +would pass to SDL_SetVideoMode
SDL_Surface *screen;
+Uint32 bpp;
+.
+.
+.
+printf("Checking mode 640x480@16bpp.\n");
+bpp=SDL_VideoModeOK(640, 480, 16, SDL_HWSURFACE);
+
+if(!bpp){
+ printf("Mode not available.\n");
+ exit(-1);
+}
+
+printf("SDL Recommends 640x480@%dbpp.\n", bpp);
+screen=SDL_SetVideoMode(640, 480, bpp, SDL_HWSURFACE);
+.
+.