You're such a disappointment.

no-reply@allegro.cc (Chris Katko) — Mon, 23 May 2022 13:44:59 +0000

You, the Allegro developers have let me down.

j/k

Okay, so oddly enough right after talking about rarely finding bugs. I've got a curiosity that may be Allegro 5 related.

Saving a bitmap of the screen seems to change the brightness in Linux. It might not be respecting gamma or something.

{"name":"screen.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/c\/a\/ca53749d909c08577ef83569638f5bcc.png","w":1360,"h":720,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/c\/a\/ca53749d909c08577ef83569638f5bcc"}

The "space" background in that picture, on my screen, is black or almost black. But the screenshot I see uploaded to github is very very bright

http://www.allegro.cc/files/attachment/613209
https://www.allegro.cc/files/attachment/613209

al_save_bitmap("screen.png", al_get_backbuffer(al_display));

[edit] It's dumping the ALPHA CHANNEL of the screen. git issue now here:

https://github.com/liballeg/allegro5/issues/1337

no-reply@allegro.cc (Peter Hull) — Mon, 23 May 2022 15:24:51 +0000

Interesting. al_get_backbuffer returns the actual backbuffer so if there is an alpha channel then the bitmap will have it. And al_save_bitmap should save an alpha channel if there is one. So it's undesirable but I can't see how it could be otherwise. I suppose the solution is to make yourself a bitmap with an opaque black rectangle and blit the backbuffer onto it? Also this may even be quicker - the docs say that operations on the backbuffer may be unaccelerated, so blitting in one go to a memory bitmap may be better.

ps that's a groovy looking screenshot, what is it?

no-reply@allegro.cc (Chris Katko) — Tue, 24 May 2022 02:14:41 +0000

Still very prototype stage. But it's exploring a game akin to Asteroids and/or Gravity Well.

Public repo for the time, because there's nothing really proprietary in there yet. Just lots of sweet D code.

https://github.com/katastic/dgravity

I have Allegro dump a screenshot every time I load the game, so that github stores it. Then, I could, theoretically, span through hundreds of old commits, and get a history of screenshots as the game progresses.

no-reply@allegro.cc (Samuel Henderson) — Wed, 25 May 2022 23:48:36 +0000

Chris Katko said:

That is actually really smart! I may wind up doing something similar in future projects.

no-reply@allegro.cc (André Silva) — Sun, 29 May 2022 01:23:24 +0000

I can confirm I have this behavior as well. For some reason I struggle with understanding how blending works in general in Allegro, but it looks to me like I'm doing everything right, like I have for years. Sure enough, screenshot dumping never had this problem on my old laptop, but now that I'm on a new one, it does. I have no idea if this is a graphics card thing, OS thing, or Allegro version thing, and I'm not quite sure how I'd even start to debug this.

no-reply@allegro.cc (Chris Katko) — Sun, 29 May 2022 01:32:25 +0000

You can work around it, I've got a D language version in my github repo in file main.d above.

I mostly converted it to C/C++. (took all of like 30 seconds) Use whatever string (or c-string) you want for the path:

#SelectExpand
  1void al_save_screen(string path)
{
ALLEGRO_BITMAP* disp = al_get_backbuffer(al_display);
int w = al_get_bitmap_width(disp);
int h = al_get_bitmap_height(disp);
ALLEGRO_BITMAP* temp = al_create_bitmap(w, h);
al_lock_bitmap(temp, al_get_bitmap_format(temp), ALLEGRO_LOCK_WRITEONLY);
al_lock_bitmap(disp, al_get_bitmap_format(temp), ALLEGRO_LOCK_READONLY); // makes HUGE difference (6.4 seconds vs 270 milliseconds)
al_set_target_bitmap(temp);
for(int j = 0; j < h; j++)
  for(int i = 0; i < w; i++)
    {
    ALLEGRO_COLOR pixel = al_get_pixel(disp, i, j);
    pixel.a = 1.0; // remove alpha
    al_put_pixel(i, j, pixel);
    }
al_unlock_bitmap(disp);
al_unlock_bitmap(temp);
al_save_bitmap(path, temp);
al_set_target_bitmap(al_get_backbuffer(al_display)); //set back to normal screen target
al_destroy_bitmap(temp);
}