Try this: everytime you are drawing to the memory bitmap, call al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO) first. Then reset the blending mode to default before drawing to the backbuffer.
This results in solid black background in both cases, as expected. It's the opposite of what I'm trying to do.
al_set_blender(ALLEGRO_ADD,ALLEGRO_ONE,ALLEGRO_ZERO) is supposed to set "no-blending" blending mode where the source bitmap is copied to destination verbatim. Thus I use it for the final blit to screen, but not for printing characters.
I don't get the difference in output on Linux (both cases indicate transparent background). Could you try this with OpenGL?
With OpenGL it works correctly in both cases! (Transparent background).
The only thing I can think of is that you're calling al_get_bitmap_format() so soon after setting the video mode that the video mode may not be completely set or initialized by the GPU by the time the CPU makes the call. An example of this is if you attempt to call al_create_display() a second time too soon after calling it once prior, you can cause some really nasty problems to occur. (At least on Windows.)
I've just tried waiting for 5 seconds (al_rest(5)) after creating display - does not fix it.
Kris Asick said:
If your memory bitmap is thus getting a format that lacks an alpha channel, it's effectively the same as using ALLEGRO_ZERO instead of ALLEGRO_INVERSE_ALPHA for the destination pixels.
I was under impression that I don't need an alpha channel in the memory bitmap - it's the font glyphs that should have the alpha channel - for which there is no API to control.
Kris Asick said:
...actually, that's a good point. You should add some code to save the result of al_get_bitmap_format() into a variable so we can see what the result is on your system.
23, which should correspond to ALLEGRO_PIXEL_FORMAT_XRGB_8888.
EDIT: Simplified test code - now saves image to file instead of blitting to screen.
#SelectExpand
1#include <allegro5/allegro.h>
2#include <allegro5/allegro_image.h>
3#include <allegro5/allegro_font.h>
4
5enum { W
= 320, H
= 200, OPENGL
= 0, DRAW_TO_MEMORY
= 1 };
6
7int main
()
8{
9 ALLEGRO_DISPLAY *display
;
10 ALLEGRO_BITMAP *bmp
;
11
12 al_init();
13 al_init_image_addon();
14 al_init_font_addon();
15 if (OPENGL
) { al_set_new_display_flags(ALLEGRO_OPENGL
); }
16 if (!(display
= al_create_display(W,H
))) { exit(1); }
17
18 if (DRAW_TO_MEMORY
)
19 {
20 al_set_new_bitmap_flags(ALLEGRO_MEMORY_BITMAP
);
21 al_set_new_bitmap_format(al_get_bitmap_format(al_get_backbuffer(display
)));
22 bmp
= al_create_bitmap(W,H
);
23 al_set_target_bitmap(bmp
);
24 }
25 else { bmp
= al_get_backbuffer(display
); }
26
27 al_clear_to_color(al_map_rgb(0,
0,
0));
28 ALLEGRO_FONT *font = al_create_builtin_font
();
29
30 for (int i
=0; i
<10000; i
++)
31 {
32 char s
[2] = {(rand()%
96+32),
0};
33 al_draw_text(font,
al_map_rgb(rand()&0xFF,
rand()&0xFF,
rand()&0xFF),
rand()%
(W-20
)+5,
rand()%
(H-20
)+5,
0,s
);
34 }
35
36 al_save_bitmap("test_a5.png",bmp
);
37}
With OPENGL = 1 all works fine (characters have transparent background) regardless of DRAW_TO_MEMORY setting. With DRAW_TO_MEMORY = 0 everything fine too.
With OPENGL = 0 and DRAW_TO_MEMORY = 1 it prints each character with solid black background (second screenshot in first post).
EDIT: Solved!
It seems I must create font before changing target bitmap to memory bitmap - then it produces correct result. Apparently al_create_builtin_font() is sensitive to pixel format of current target bitmap. I think the same applies to al_load_ttf_font(), because I first noticed this with TTF fonts.
If this is the intended behavior, perhaps it should be documented.
Thanks everyone! 
EDIT: Actually it responds to setting bitmap format and flags, not to setting target bitmap. It seems that any grabbed or loaded font (including built-in one) is generated with currently set bitmap format and flags. So there is an API for controlling alpha channel in fonts. Just the documentation is lacking.
In particular, current flags control whether the font is loaded into video memory, which can have big impact on performance.
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