Allegro 5 game: game loop that runs at constant speed?

Question

What is the best way to code a game loop in Allegro 5 that always runs at the same speed, and that properly separates drawing logic from update logic? Should I use threads or not? Should I make use of the new Allegro event system?

Agave

Answer 1

Taken from the allegro wiki:

al_install_timer(1.0 / FPS);

...

while (1) {
        al_wait_for_event(queue, &event);

        /* handle input events */

        if (event.type == ALLEGRO_EVENT_TIMER) {
                handle_game_tick();
                need_redraw = true;
        }

        if (need_redraw && al_event_queue_is_empty(queue)) {
                render_last_frame();
                need_redraw = false;
        }
}

If you want frame skipping, skip the render_last_frame() command whenever you detect that you are lagging behind in frames (e.g. by using the al_current_time() function).

Bohner

Ah, so you should make use of events. Could threading potentially improve image stability? Are event queues thread-safe? – Agave

Threading won't help at all. Usually it can help if you make use of the exact time when you are rendering (again, al_current_time() is your friend). This will allow things like interpolate the exact position where to display something in relation to the game logic. You should also keep in mind that al_flip_display() will wait for vsync (if it is enabled, but having vsync on is a must for perfectly smooth animation anyway). And A5 is 100% thread save - event queues are especially so as they are one of the means to communicate across threads. – Bohner

Link to wiki page mentioned: wiki.allegro.cc/index.php?title=Porting_from_A4_to_A5 – Poachy

Answer 2

Here is a more complete version of Allefant's answer (follow the link for detailed line-by-line explanation):

#include <stdio.h>
#include <allegro5/allegro.h>

const float FPS = 60;

int main(int argc, char **argv)
{
   ALLEGRO_DISPLAY *display = NULL;
   ALLEGRO_EVENT_QUEUE *event_queue = NULL;
   ALLEGRO_TIMER *timer = NULL;
   bool redraw = true;

   if(!al_init()) {
      fprintf(stderr, "failed to initialize allegro!\n");
      return -1;
   }

   timer = al_create_timer(1.0 / FPS);
   if(!timer) {
      fprintf(stderr, "failed to create timer!\n");
      return -1;
   }

   display = al_create_display(640, 480);
   if(!display) {
      fprintf(stderr, "failed to create display!\n");
      al_destroy_timer(timer);
      return -1;
   }

   event_queue = al_create_event_queue();
   if(!event_queue) {
      fprintf(stderr, "failed to create event_queue!\n");
      al_destroy_display(display);
      al_destroy_timer(timer);
      return -1;
   }

   al_register_event_source(event_queue, al_get_display_event_source(display));

   al_register_event_source(event_queue, al_get_timer_event_source(timer));

   al_clear_to_color(al_map_rgb(0,0,0));

   al_flip_display();

   al_start_timer(timer);

   while(1)
   {
      ALLEGRO_EVENT ev;
      al_wait_for_event(event_queue, &ev);

      if(ev.type == ALLEGRO_EVENT_TIMER) {
         redraw = true;
      }
      else if(ev.type == ALLEGRO_EVENT_DISPLAY_CLOSE) {
         break;
      }

      if(redraw && al_event_queue_is_empty(event_queue)) {
         redraw = false;
         al_clear_to_color(al_map_rgb(0,0,0));
         al_flip_display();
      }
   }

   al_destroy_timer(timer);
   al_destroy_display(display);
   al_destroy_event_queue(event_queue);

   return 0;
}

Poachy