A5 Displays (Resize/Stretching)

no-reply@allegro.cc (jason perkins) — Tue, 10 May 2011 23:20:14 +0000

Hi, I'm just wondering how to go about getting everything I've drawn onto a display's back buffer to line up after a resize event.

I'm a little un-sure how this works, but it seems to me that during a resize the buffer is stretched to match this display's new width/height (This is what I'd like to happen), but after the event completes the buffer is also re-sized so I end up with empty space on the right and bottom sides.

What I'm trying to do is, when the window is re-sized have whatever's drawn on the back buffer stretch to match the display.

I think this is all the relevant code:

#SelectExpand
  1const int SCREEN_W = 640;
  2const int SCREEN_H = 480;
  3
  4al_set_new_display_flags(ALLEGRO_RESIZABLE);
  5display = al_create_display(SCREEN_W, SCREEN_H);
  6
  7if(!display)
  8{
  9  fprintf(stderr, "could not create the display!\n");
 10    return -1;
 11}
 12
 13al_register_event_source(event_queue, al_get_display_event_source(display));
 14
 15       // main loop
 16{...
 17  al_set_target_bitmap(al_get_backbuffer(ab.getDisplay()));
 18
 19  while (!doExit)
 20  {
 21    ALLEGRO_EVENT ev;
 22    al_wait_for_event(ab.getEventQueue(), &ev);
 23    if (ev.type == ALLEGRO_EVENT_TIMER)
 24    {
 25      im.checkInput();
 26      god.getInput();
 27      al_clear_to_color(al_map_rgb(0,0,0));
 28      god.draw();
 29
 30      al_flip_display();  
 31    }
 32
 33    if(ev.type == ALLEGRO_EVENT_DISPLAY_RESIZE)
 34      al_acknowledge_resize(ab.getDisplay());
 35  }
 36}

Everything I draw throughout the project is based on SCREEN_W/SCREEN_H, so it would be a lot easier to leave them as is, and just alter the way the buffer is drawn on to the display. Sorry for needing spoon feeding.

no-reply@allegro.cc (SiegeLord) — Tue, 10 May 2011 23:28:06 +0000

If you want stretching, then you can use transformations. For your purposes I think this will be sufficient:

if(ev.type == ALLEGRO_EVENT_DISPLAY_RESIZE)
{
  al_acknowledge_resize(ab.getDisplay());
  ALLEGRO_TRANSFORMATION trans;
  al_identity_transform(&trans);
  al_scale_transform(al_get_display_width(ab.getDisplay()) / SCREEN_W, al_get_display_height(ab.getDisplay()) / SCREEN_H);
  al_use_transform(&trans);
}

EDIT: Fixed fractions...

no-reply@allegro.cc (jason perkins) — Wed, 11 May 2011 00:21:58 +0000

Thanks, I feel kind of bad. Now that transforms exist there's tonnes of questions with this exact answer. Thanks for the quick reply

no-reply@allegro.cc (weapon_S) — Thu, 12 May 2011 10:05:06 +0000

SiegeLord, you also forgot the first argument to al_scale_transform. I'm not familiar with this function, but when I read the code I thought there was a magic global or something?!
SCREEN_W and SCREEN_H are the original window size (from the start of your program).

no-reply@allegro.cc (SiegeLord) — Thu, 12 May 2011 20:15:52 +0000

The compiler will remind me (or anyone using that code) of what I forgotten