Game Programming with the Simple DirectMedia Layer

The file bounce.cpp [available at ftp.linuxjournal.com/pub/lj/listings/issue110/6410.tgz] is a game written using SDL for input and graphics and SDL_ttf to load TrueType fonts. The game itself is a little over 1,300 lines of C++, and the complete package includes the source code, images, a TrueType font, a makefile, sdl-install.sh and the license files for the font and images used in the game. Finding fonts, graphics and sounds that you can use legally in your games can be more work than writing the game.

To get started learning SDL, download the Bounce source code from the Linux Journal FTP site and unpack it with tar -xzvf bounce.tar.gz. Then run make to build the program. Run the program by typing bounce at the command line. You can run it in full-screen mode by typing bounce -fullscreen. The plot of the game is that Earth has started wandering around the solar system and is in danger of falling into the Sun. Your job is to keep Earth out of the Sun by hitting it with the Moon. You score a point each time you hit the Earth with the Moon, and the game scores every time the Earth hits the Sun. The game is designed to show off features of SDL, not to be the most interesting game you've ever seen.

Figure 1. The Game Bounce

SDL must be initialized before any SDL functions are used by calling SDL_Init():

if (-1 == SDL_Init((SDL_INIT_VIDEO |
                    SDL_INIT_TIMER |
                    SDL_INIT_EVENTTHREAD)))
{
  ...
}

The parameter to SDL_Init() identifies the subsystems that need to be initialized. Here, I tell SDL to initialize the video, timer and subsystems and to use thread-based event processing. I also could have used the catch-all SDL_INIT_EVERYTHING, but you should initialize only the parts of SDL that your program uses. There is no reason to initialize the joystick or CD-ROM if you are not going to use them. You can initialize and shut down subsystems at any time by the use of the SDL_InitSubSystem() and SDL_QuitSubSystem() functions.

It is important to shut down SDL with a call to SDL_Quit() before your program shuts down. SDL_Quit() shuts down all SDL subsystems, frees all system resources used by SDL and restores the video mode. It is good practice to use atexit() to make sure that SDL_Quit() runs when your program terminates. Failure to call SDL_Quit() can leave your computer in a strange video mode.

When selecting a video mode, decide whether to run in a window or as a full-screen application. Then, choose the size of the window or screen. If you go with a window, decide whether the user can resize it. Then, choose how to adapt to the color depth of the screen. In Bounce I use something like:

options = SDL_ANYFORMAT |  SDL_FULLSCREEN;
screen = SDL_SetVideoMode(640, 480,
                          0,
                          options);

The first two parameters specify the width and height, in pixels, of the screen or window in which the program runs. To use a particular width and height in full-screen mode, the screen section of your XF86Config-4 (or XF86Config for some versions of X) file must list the specified size. If Bounce won't run in full-screen mode on your machine, it is most likely because you don't have a 640 × 480 mode set up in the screen section of your XF86Config-4 file.

The third parameter specifies the number of bits per pixel. Or, if it is set to 0 (zero), it tells SDL to use the current display depth. It is best to adapt the game to the current display depth rather than counting on every machine on which the code will ever run to support your desired pixel format.

The last parameter lets you give SDL detailed instructions on how to set up the video mode. There are nearly a dozen options from which to chose. In Bounce, I use SDL_ANYFORMAT to let SDL pick the best available mode. This option forces your code to adapt to whatever pixel depth you have, but using it can provide better performance at the cost of some extra coding. The SDL_FULLSCREEN option tells SDL to set a full-screen mode.

The value returned by SDL_SetVideoMode() is a pointer to an SDL_Surface structure. This structure describes the screen in great detail. If the pointer is NULL, the video mode you requested is not available. But, getting a non-NULL value doesn't mean you got everything you wanted. Check the flags field of this structure against the options you specified. I have found it is best to ask for little and work with what I receive, that way I avoid hard wiring my machine and OS restrictions in my code.

Now that the video mode is configured, use SDL_WM_SetCaption() to set the window title and icon name. This isn't necessary; it's one of those touches that make the program a little easier to use:

SDL_WM_SetCaption("Bounce", "Bounce")