Cannot lock target bitmap

no-reply@allegro.cc (damianday) — Sun, 04 Nov 2012 20:09:48 +0000

Hello I am trying to read pixels from the target bitmap (al_get_target_bitmap), by locking it.

Unfortunately locking always fails. So I tried cloning the target bitmap and then locking the clone, this also fails

I need to read the target bitmap before saving it to a file, is there any other way I can do this?

SOLVED - SEE POST FOR SOLUTION

no-reply@allegro.cc (Elias) — Sun, 04 Nov 2012 20:36:47 +0000

Post the code.

no-reply@allegro.cc (damianday) — Sun, 04 Nov 2012 20:47:19 +0000

#SelectExpand
  1procedure TGame.TakeScreenShot;
  2var
i, checksum: integer;
flname: string;
zcopy: PAnsiChar;
  6
bmp: PAllegroBitmap;
format: ALLEGRO_PIXEL_FORMAT;
lr: PAllegroLockedRegion;
 10begin
bmp := al_get_target_bitmap;
if bmp = nil then Exit;
format := al_get_bitmap_format(bmp);
 14
flname := 'Screenshot.bmp';
zcopy := PAnsiChar(AnsiString(flname));
 17
checksum := 0;
try
  lr := al_lock_bitmap(bmp, format, ALLEGRO_LOCK_READONLY);
  if lr = nil then Exit;
 22
  // Note: Never reach this point lr is always nil..
  for i := (SCREENHEIGHT - 200) to SCREENHEIGHT - 1 do begin
    // TODO: calculate checksum with lr.data
     
  end;
finally
  al_unlock_bitmap(bmp);
end;
 31
al_save_bitmap(zcopy, bmp);
 33end;

no-reply@allegro.cc (Elias) — Sun, 04 Nov 2012 20:51:51 +0000

Can you lock other bitmaps?

no-reply@allegro.cc (damianday) — Sun, 04 Nov 2012 21:00:01 +0000

Yes I've just tested and there is no problems with
locking any bitmaps.

On a side note I'm using Allegro 5.0.7 until 5.1 is stable.

Curiously this case works fine in a simple test project, but not in my main, I think it could be a bug in my code somewhere..

Ok I've tracked it down to:

#SelectExpand
 1al_set_new_display_option(ALLEGRO_DISPLAY_OPTIONS.ALLEGRO_SAMPLE_BUFFERS, 1, ALLEGRO_SUGGEST);
 2al_set_new_display_option(ALLEGRO_DISPLAY_OPTIONS.ALLEGRO_SAMPLES, 8, ALLEGRO_SUGGEST);

Commenting these lines out makes it go away

no-reply@allegro.cc (Elias) — Sun, 04 Nov 2012 21:21:27 +0000

Intersting to know. Are you using DirectX? I don't think multi-sampling should prevent reading the backbuffer, in general.

no-reply@allegro.cc (damianday) — Sun, 04 Nov 2012 21:24:36 +0000

I'm just using vanilla Allegro code on Windows 7 X64,
I assume by default it uses OpenGL.

It seems the code below creates the bug in question.

#SelectExpand
 1al_set_new_display_option(ALLEGRO_DISPLAY_OPTIONS.ALLEGRO_SAMPLE_BUFFERS, 1, ALLEGRO_SUGGEST);
 2  al_set_new_display_option(ALLEGRO_DISPLAY_OPTIONS.ALLEGRO_SAMPLES, 8, ALLEGRO_SUGGEST);

no-reply@allegro.cc (J-Gamer) — Sun, 04 Nov 2012 21:40:57 +0000

Allegro uses DirectX by default on Windows.

no-reply@allegro.cc (damianday) — Sun, 04 Nov 2012 21:43:45 +0000

Is there a way to override this behaviour and make it use opengl?
nvm found it al_set_new_display_flags(ALLEGRO_OPENGL);