[SDL] Per-frame updates

[Todd Klaus]

I guess the fundamental thing is to decouple the screen updates from the rest of the game logic. 
That's what I was missing from the abstract examples in books, they usually just lump everything
together in one 'game logic' function, I was just assuming that the two were tied together. 
Thanks for the clarification.

Todd

--- Max Watson <max at blackholesun.com> wrote:
> On Friday 18 April 2003 12:37, Todd Klaus wrote:
> >
> > It seems to me (intuitively, I haven't written any games yet) that a better
> > way to do this would be to not have 'do_game_logic()' in the main loop at
> > all, but instead use a timer and have the game logic execute in the
> > callback for that timer.  However, I've seen a lot of anecdotal comments on
> > boards and in tutorials that say basically, 'don't use timers, they will
> > kill your frame rate!' Is there really a big downside to using timers this
> > way?  I can see that if you are developing a 3d FPS or flight sim you would
> > want to get as many frames as possible, and having the game logic called
> > every frame makes sense for those games, but why do it for 2d sprite games?
> 
> You are correct. There are a few reasons not to do your game logic inside the 
> timer callback. The one that always bites me is that SDL timer callbacks run 
> in a separate thread (under linux).  It is possible to use a timer to 
> regulate the game logic, what you have to do is decouple the game logic from 
> the display update. This allows the logic to run at a steady speed, and slow 
> computers can skip a screen update when necessary while fast computers can 
> get super high numbers of screen refreshes. The only real downside to this is 
> that very slow computers will skip every screen update if your game logic 
> takes too long. But we were trying to future-proof the game for faster 
> computers, right?
> The way I like to handle this is ripped straight from the allegro toolkit FAQ 
> http://alleg.sourceforge.net/faq.html (with a little modification to work 
> with SDL):
> 
> 
> Q: How can I make my game run at the same speed on any computer?
> 
> A:You need to make sure the game logic gets updated at a regular rate, but 
> skip the screen refresh every now and then if the computer is too slow to 
> keep up. This can be done by installing a timer handler that will increment a 
> global variable at your game logic speed, eg: 
> 
> Uint32 speed_counter = 0;
> Uint32 increment_speed_counter(Uint32 interval, void *param) {
> 	speed_counter++;
> }
> 
> void play_the_game() {
> 	SDL_AddTimer(1000/DESIRED_FPS, increment_speed_counter, NULL);
> 	while (!game_over) {
> 		while (speed_counter > 0) {
> 			update_game_logic();
> 			speed_counter--;
> 		}
> 		update_display();
> 	}
> }
> 
> --
> Hope that was useful,
> Max Watson <max at blackholesun.com>
> 
> 
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