al_draw_rectangle does different output between linux and windows

al_draw_rectangle does different output between linux and windows

gameovera

Member #15,340

October 2013

Hi.

I compiled the below code for linux and windows.

[EDIT] #include backslash to slash

#SelectExpand
  1#include <allegro5/allegro.h>
  2#include <allegro5/allegro_image.h>
  3#include <allegro5/allegro_primitives.h>
  4
  5int main(int argc, char **argv)
  6{
  7  al_init();
  8  al_install_keyboard();
  9  al_init_image_addon();
 10  al_init_primitives_addon();
 11
 12  ALLEGRO_DISPLAY *disp = al_create_display(640, 480);
 13  ALLEGRO_EVENT_QUEUE *queue = al_create_event_queue();
 14
 15  al_register_event_source(queue, al_get_display_event_source(disp));
 16  al_register_event_source(queue, al_get_keyboard_event_source());
 17
 18  bool running = true;
 19  ALLEGRO_EVENT ev;
 20
 21  while (running) {
 22    while (al_get_next_event(queue, &ev)) {
 23      if (ev.type == ALLEGRO_EVENT_DISPLAY_CLOSE) running = false;
 24      if (ev.type == ALLEGRO_EVENT_KEY_DOWN) running = false;
 25    }
 26
 27    al_clear_to_color(al_map_rgb(0, 0, 0));
 28    al_draw_rectangle(0, 0, 32, 32, al_map_rgb(255, 0, 0), 1);
 29    al_flip_display();
 30    al_rest(1.0/60.0);
 31  }
 32
 33  al_save_bitmap("disp.png", al_get_target_bitmap());
 34
 35  al_destroy_display(disp);
 36  al_destroy_event_queue(queue);
 37
 38  return 0;
 39}

result(scale 8x):
{"name":"608225","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/8\/3\/83c391ea44fb40f7610527e8b51ccfd8.png","w":647,"h":299,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/8\/3\/83c391ea44fb40f7610527e8b51ccfd8"}

How about it in your environment?

my environment:

- Allegro 5.0.10

- Ubuntu 13.0.10 on virtualbox 4.3.6
- Windows 7

- AMD HD 7800 series GPU with latest driver

Arthur Kalliokoski

Second in Command

February 2005

After converting the backslashes in the #includes to forward slashes, I get this

{"name":"608226","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/0\/9\/095d8329210338c3641e17da45d7ccaf.png","w":656,"h":624,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/0\/9\/095d8329210338c3641e17da45d7ccaf"}

Slackware 14.1
Allegro 5.1.7
Geforce GTX 650

They all watch too much MSNBC... they get ideas.

Elias

Member #358

May 2000

Try drawing the rectangle like this:

al_draw_rectangle(0.5, 0.5, 31.5, 31.5, al_map_rgb(255, 0, 0), 1);

The rectangle is not drawn by Allegro but directly sent to your graphics card. And for the GPU the positions 0/0 as well as 32/32 both lie on the intersection of 4 adjacent pixels. The kind of rectangle to draw is therefore ambiguous (and any possibility the GPU chooses is correct).

See here for more details: http://www.liballeg.org/a5docs/refman/primitives.html#pixel-precise-output

--
"Either help out or stop whining" - Evert

gameovera

Member #15,340

October 2013

Thanks Arthur Kalliokoski and Elias.

I assumed the rasterization of pixels boundary as independent GPU.