Help with blender and alpha masking

Help with blender and alpha masking

Michael Weiss

Member #223

April 2000

I am new to the whole blender thing and have spent many hours trying
to figure out what I though would be a simple task.

I am trying to draw a bitmap onto the backbuffer and blend it with the
backbuffer.

This code from the examples works great:

al_set_blend_color(al_map_rgba_f(0.5, 0.5, 0.5, 0.5));
al_set_blender(ALLEGRO_ADD, ALLEGRO_CONST_COLOR, ALLEGRO_CONST_COLOR);

But my source bitmap has an alpha channel, and I can't figure out how
to get that respected...no matter how many different combination I try
I always get the zero pixels blended also.

I have tried al_set_separate_blender with many different combinations
and gotten nowhere.

I have read the docuemntation many times, and looked at other explanations
of blending of the web. But I can't seem to make it work.

Here is a simple test program:

#SelectExpand
  1// this draws a background of colored squares
  2   ALLEGRO_COLOR c[5];
  3   c[0] = al_map_rgb(255, 0, 0);
  4   c[1] = al_map_rgb(0, 255, 0);
  5   c[2] = al_map_rgb(0, 0, 255);
  6   c[3] = al_map_rgb(255, 255, 255);
  7   c[4] = al_map_rgb(0, 0, 0);
  8
  9   al_set_target_backbuffer(display);
 10   al_flip_display();
 11   al_clear_to_color(al_map_rgb(0,0,0));
 12
 13   int ci = 0;
 14   for (int y=0; y<10; y++)
 15      for (int x=0; x<10; x++)
 16      {
 17         al_draw_filled_rectangle(x*20, y*20, x*20+20, y*20+20, c[ci]);
 18         if (++ci>2) ci = 0;
 19      }
 20
 21
 22
 23// this is the bitmap I am drawing
 24   sprintf(msg, "Test") ;
 25   int sw = strlen(msg) * 8;
 26   ALLEGRO_BITMAP *temp = al_create_bitmap(sw, 8);
 27   al_set_target_bitmap(temp);
 28   al_clear_to_color(al_map_rgb(0,0,0));
 29   al_draw_text(font, ci[3], sw/2, 0, ALLEGRO_ALIGN_CENTRE, msg);
 30   al_convert_mask_to_alpha(temp, al_map_rgb(0, 0, 0));
 31
 32
 33// here is where I have tried just about every blender combination I can think of
 34
 35   al_set_blend_color(al_map_rgba_f(0.5, 0.5, 0.5, 0.5));
 36   al_set_blender(ALLEGRO_ADD, ALLEGRO_CONST_COLOR, ALLEGRO_CONST_COLOR);
 37
 38
 39
 40// this is where i actually draw  it
 41   al_set_target_backbuffer(display);
 42   al_draw_scaled_rotated_bitmap(temp, sw/2, 4, 88, 48, 4, 4, 0, 0);
 43   al_flip_display();
 44   al_destroy_bitmap(temp);

When I don't specify any blender I get a nice masked text over background.

When I try this:
al_set_blend_color(al_map_rgba_f(0.5, 0.5, 0.5, 0.5));
al_set_blender(ALLEGRO_ADD, ALLEGRO_CONST_COLOR, ALLEGRO_CONST_COLOR);

I get a nice blended source and dest, just like what I wanted,
except the alpha is not respected and I get a rectangle around the text.

I tried:
al_set_blend_color(al_map_rgba_f(0.5, 0.5, 0.5, 0));
al_set_blend_color(al_map_rgba_f(0.5, 0.5, 0.5, 1));
al_set_blend_color(al_map_rgb_f(0.5, 0.5, 0.5));
but they all made no difference.

I tried:
al_set_blend_color(al_map_rgb_f(0.5, 0.5, 0.5));
al_set_separate_blender(ALLEGRO_ADD, ALLEGRO_CONST_COLOR, ALLEGRO_CONST_COLOR, ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO);

al_set_blend_color(al_map_rgb_f(0.5, 0.5, 0.5));
al_set_separate_blender(ALLEGRO_ADD, ALLEGRO_CONST_COLOR, ALLEGRO_CONST_COLOR, ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_ZERO);

plus many more combinations, but I can never get the alpha mask to work...

Can anyone with more experience with blenders help me?

Do I somehow have to do this with 2 steps?

My overall goal is simply to have a partially transparent bitmap superimposed over
another bitmap (the backbuffer), (kind like a water mark), but I want it masked so the
zero pixels are not blended with the background.

Thanks

Dizzy Egg

Member #10,824

March 2009

Try:

al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO);

OR

al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ONE);

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Michael Weiss

Member #223

April 2000

Hi Dizzy Egg,

al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO);
- no blending and no alpha mask

al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ONE);
- no blending, but alpha mask works (same as no blender at all)

I also tried:

al_set_separate_blender(ALLEGRO_ADD, ALLEGRO_ZERO, ALLEGRO_ONE, ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO);
- no blending and no alpha mask

al_set_separate_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ONE, ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO);
- no blending, but alpha mask works

al_set_blend_color(al_map_rgb_f(0.5, 0.5, 0.5));
al_set_separate_blender(ALLEGRO_ADD, ALLEGRO_CONST_COLOR, ALLEGRO_CONST_COLOR, ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ONE);
- blending, but no alpha mask

al_set_blend_color(al_map_rgb_f(0.5, 0.5, 0.5));
al_set_separate_blender(ALLEGRO_ADD, ALLEGRO_CONST_COLOR, ALLEGRO_CONST_COLOR, ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO);
- blending, but no alpha mask

I want something that has blending and alpha mask...

Does the pixel format have anything to do with it?

temp pixel format:
ALLEGRO_PIXEL_FORMAT_ARGB_8888
backbuffer pixel format:
ALLEGRO_PIXEL_FORMAT_RGB_565

Thanks

Edgar Reynaldo

Major Reynaldo

May 2007

Try al_draw_tinted_bitmap with al_map_rgba_f(0.5,0.5,0.5,0.5).

Eagle and Allegro 5 binaries | Older Allegro 4 and 5 binaries | Allegro 5 compile guide
King Piccolo will make it so Then I could just flip it with some sweet, sweet matrix math.

Neil Roy

Member #2,229

April 2002

You need to use al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA); BEFORE you create your display, not afterwards.

Note: the options are just ones I use in my Deluxe Pacman 2 game, yours may differ.

---
“I love you too.” - last words of Wanda Roy

Edgar Reynaldo

Major Reynaldo

May 2007

Poppycock. You can change the blender anytime you want, except when drawing is held.

What you just quoted is the pre-multiplied alpha blender, which is the default.

Eagle and Allegro 5 binaries | Older Allegro 4 and 5 binaries | Allegro 5 compile guide
King Piccolo will make it so Then I could just flip it with some sweet, sweet matrix math.

Neil Roy

Member #2,229

April 2002

I just tested that, you're right, no longer needed. But I needed to add it at one point so I will assume something changed. Good to know anyhow, even with Edgar's usual charming, kind replies.

My only guess would be that it has to do with al_convert_mask_to_alpha(temp, al_map_rgb(0, 0, 0));. It's not a function I have ever used but what mask is it converting? And what alpha is it making it? Is it expecting the bitmap to already have a mask? (awaits another rude response from Edgar)

---
“I love you too.” - last words of Wanda Roy

Edgar Reynaldo

Major Reynaldo

May 2007

If I'm being fresh it's because I dislike bad advice. :/

al_convert_mask_to_alpha takes a mask color, compares it to every pixel, and writes zero alpha if it matches.

Eagle and Allegro 5 binaries | Older Allegro 4 and 5 binaries | Allegro 5 compile guide
King Piccolo will make it so Then I could just flip it with some sweet, sweet matrix math.

Michael Weiss

Member #223

April 2000

Thanks Edgar!

your suggestion:

Try al_draw_tinted_bitmap with al_map_rgba_f(0.5,0.5,0.5,0.5).

Did exactly what I was looking for.

I didn't even look at the tinted drawing functions.
If I would have I would have found your exact suggestion as an example.

Thanks again.