Allegro 5 basic game loop

Allegro 5 basic game loop

LennyLen

Member #5,313

December 2004

I'd like to spend the next few weeks learning the basics of Allegro 5 (I think it's finally time to say goodbye to Allegro 4). I know A5 is event based unlike A4, so the basic game loop is going to be different.

I won't have much spare time between paid work and moving so I don't want to spend too much time reinventing wheels, and as I learn best from dissecting code rather than from tutorials, I'm hoping someone could give me an example of a basic event driven gameloop. Just something simple like looping a large background image over a smaller window would be good to get me started I think. I can add extra functionality (input etc, from there).

Thanks in advance.

edit:

This is the code I tried to do the above, but I seem to have missed something somewhere, as I'm trying to increment an x value which represents the position on the larger bitmap to draw from, but x never seems to increment. I tried it without using the timer and just increased x each loop and that worked, but I want it to run at a set rate.

#SelectExpand
  1#include "stdafx.h"
  2#include <allegro5\allegro.h>
  3#include <allegro5\allegro_image.h>
  4#include <allegro5/allegro_native_dialog.h>
  5
  6int main()
  7{
ALLEGRO_DISPLAY * display;
ALLEGRO_EVENT_QUEUE *queue;
ALLEGRO_BITMAP * nebula = NULL;
ALLEGRO_TIMER *timer;
 12
al_init();
display = al_create_display(1366, 768);
queue = al_create_event_queue();
timer = al_create_timer(1.0 / 100.0);
 17
al_install_keyboard();
al_register_event_source(queue, al_get_keyboard_event_source());
al_register_event_source(queue, al_get_display_event_source(display));
al_register_event_source(queue, al_get_timer_event_source(timer));
 22
al_init_image_addon();
nebula = al_load_bitmap("Assets\\Gfx\\Background\\Nebula Red.png");
if (nebula == NULL) {
  al_show_native_message_box(display, "Error", "", "Error loading bitmap", "", ALLEGRO_MESSAGEBOX_ERROR);
  al_destroy_display(display);
  al_uninstall_keyboard();
  exit(EXIT_FAILURE);
}
 31
bool done = false;
 33
int display_width = al_get_display_width(display);
int height = al_get_display_height(display);
int nebula_width = 4096;
float x = 0.0;
 38
while (!done) {
  if (display_width + x <= nebula_width) {
    al_draw_bitmap_region(nebula, x, 0, display_width, height, 0, 0, 0);
  }
  else {
    al_draw_bitmap_region(nebula, x, 0, nebula_width - x, height, 0, 0, 0);
    al_draw_bitmap_region(nebula, 0, 0, display_width - (nebula_width - x), height, nebula_width - x, 0, 0);
  }
  al_flip_display();
 48
 49
  if (x > nebula_width) x = 0.0;
 51
  ALLEGRO_EVENT event;
 53
  if (!al_is_event_queue_empty(queue)) {
    al_wait_for_event(queue, &event);
    if (event.type == ALLEGRO_EVENT_KEY_UP || event.type == ALLEGRO_EVENT_DISPLAY_CLOSE)
      done = true;
    if (event.type == ALLEGRO_EVENT_TIMER) {
      x += 0.05;
    }
  }
}
 63
al_destroy_display(display);
al_uninstall_keyboard();
al_destroy_bitmap(nebula);
 67
return 0;
 69}

Xpicolo

Member #16,781

December 2017

Where is al_start_timer ?

LennyLen

Member #5,313

December 2004

Xpicolo said:

Where is al_start_timer ?

I knew I'd missed something really obvious.

Neil Roy

Member #2,229

April 2002

I honestly can't recall the issues I had with Allegro 5 when i first switched. It was just a matter of getting used to the events, which was trivial.

The only thing that popped into my head was where I added in a function similar to A4's to check if a key was pressed, and wrote my own wait for keypress function. Otherwise... it's pretty simple and honestly quite nice. The benefits outweigh any nitpicks I ever had (like the sound system, and some minor joystick issues I forget about now).

This was handy as I was used to having a similar function in A4...

// Returns whether or not a specified key is being pressed
bool key_pressed(int key)
{
   ALLEGRO_KEYBOARD_STATE s;
   al_get_keyboard_state(&s);
   return al_key_down(&s, key);
}

This was a function I wrote for Deluxe Pacman 2 to wait for a keypress or xxxx amount of milliseconds before continuing (this is the trimmed down version)...

#SelectExpand
  1// Waits a specified time (in ms, 1000=1s) for a key to be pressed.
  2// Will also respond if a mouse or joystick button is clicked.
  3// If the time passes before a key/mouse/joystick is pressed, the function returns
  4// If the time specified is zero, the function will wait indefinitely.
  5bool wait_for_keypress(double wait_time)
  6{
  7   double ts = al_get_time(); // time stamp
  8   double elapsed_time;
  9
 10   al_flush_event_queue(event_queue);
 11
 12   bool done = false;
 13
 14   // Wait until the specified time passes or a key is pressed.
 15   do {
 16      al_wait_for_event(event_queue, &event);
 17
 18      if(event.type == ALLEGRO_EVENT_DISPLAY_CLOSE) {
 19         shut_down(); // free resources
 20         exit(0);
 21      }
 22
 23      else if(wait_time && event.type == ALLEGRO_EVENT_TIMER) {
 24         elapsed_time = al_get_time() - ts;
 25         if(elapsed_time >= wait_time) return false;
 26      }
 27
 28      // Wait for key to be pressed and released to continue
 29      else if(event.type == ALLEGRO_EVENT_KEY_UP) {
 30         if(event.keyboard.keycode == ALLEGRO_KEY_ESCAPE) return true;
 31         else done = true;
 32      }
 33
 34      // And if a key wasn't pressed, maybe a mouse button was...
 35      else if(event.type == ALLEGRO_EVENT_MOUSE_BUTTON_UP) {
 36         done = true;
 37      }
 38   }
 39   while(!done);
 40
 41   return false;
 42}