Flickering border artifacts when using "al_set_clipping

David Couzelis

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

Chris Katko

Did you forget the attachment?

David Couzelis

Haha

Here are the attachments.

Elias

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.

Edgar Reynaldo

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?

David Couzelis

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

Edgar Reynaldo

Bumping this thread so it doesn't get lost.

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

Thread #617185. Printed from Allegro.cc