Flickering border artifacts when using "al_set_clipping_rectangle"

no-reply@allegro.cc (David Couzelis) — Sat, 23 Dec 2017 03:17:29 +0000

I have my video game display coded so that "ALLEGRO_FULLSCREEN_WINDOW" mode will do the largest integer scale possible, then use "al_set_clipping_rectangle" to preserve the aspect ratio.

I expect the "borders" of the clipping rectangle to be black, but instead, they flicker with what looks like screen artifacts. (See attachments)

Here is an excerpt from my display code:

#SelectExpand
  1al_set_new_display_flags(ALLEGRO_FULLSCREEN_WINDOW);
  2
  3/* Initialize the one and only global display for the game */
  4display = al_create_display(width * scale, height * scale);
  5
  6al_clear_to_color(al_map_rgb(0, 0, 0));
  7
  8/* Scale and center the display as big as possible on this screen */                                               
  9set_display_transform(scale, offset_x, offset_y, fullscreen);
 10                                                                                                                       
 11/* Crop the drawing area, to not accidentally draw in the black borders */                                         
 12al_set_clipping_rectangle(offset_x, offset_y, display_width * scale, display_height * scale);

Full code here:

https://github.com/drcouzelis/colorwandcastle/blob/master/src/display.c

Is there something I can do to prevent the seizure-inducing flickering around the display area?

My computer:
Arch Linux (current)
Radeon HD 5450, with open source "radeon" driver

no-reply@allegro.cc (Chris Katko) — Sat, 23 Dec 2017 06:01:01 +0000

Did you forget the attachment?

no-reply@allegro.cc (David Couzelis) — Sat, 23 Dec 2017 08:16:52 +0000

Haha

Here are the attachments.

no-reply@allegro.cc (Elias) — Sat, 23 Dec 2017 09:31:57 +0000

Can you post the source code of set_display_transform?

Also note that the parameters to al_create_display are ignored when you use ALLEGRO_FULLSCREEN_WINDOW.

no-reply@allegro.cc (Edgar Reynaldo) — Sat, 23 Dec 2017 09:38:57 +0000

I can't say what you're doing for sure, but it sounds like you're leaving video memory uninitialized, with undefined contents. Are you clearing the screen?

no-reply@allegro.cc (David Couzelis) — Sat, 23 Dec 2017 22:03:15 +0000

Elias said:

Can you post the source code of set_display_transform?

The full code is here: https://github.com/drcouzelis/colorwandcastle/blob/master/src/display.c

Here it is again, for convenience:

#SelectExpand
  1static void set_display_transform(int scale, float offset_x, float offset_y, bool fullscreen)
  2{
  3    ALLEGRO_TRANSFORM trans;
  4
  5    /* Confirm that the display has been created */
  6    assert(al_get_target_bitmap());
  7
  8    /**
  9     * Scale the coordinates to match the actual size of the display.
 10     * It will be performed on the current target bitmap.
 11     */
 12    al_identity_transform(&trans);
 13    al_scale_transform(&trans, scale, scale);
 14    if (fullscreen) {
 15        al_translate_transform(&trans, offset_x, offset_y);
 16    }
 17    al_use_transform(&trans);
 18}

Edgar Reynaldo said:

Are you clearing the screen?

I think so...

#SelectExpand
 1al_clear_to_color(al_map_rgb(0, 0, 0));
 2draw(data); /* DRAW */
 3al_flip_display();
 4redraw = false;

Full source: https://github.com/drcouzelis/colorwandcastle/blob/master/src/run.c

no-reply@allegro.cc (Edgar Reynaldo) — Wed, 03 Jan 2018 00:18:46 +0000

Bumping this thread so it doesn't get lost.

Can you provide a minimal working (failing) example that does this?