Alpha Channel

Alpha Channel

Mariusz

Member #1,634

November 2001

Can anyone help me out with alpha channel issue? I have created an image using PS5.5 and made an alpha channel for my selection. As a next step, I have saved this image as a Targa (TGA) format including alpha channel.

My question is how do I load this image back to my Allegro project? I want to display it over other backgrounds so it would create transparency in the areas where it should. I have seen some DirectX demos a while ago where a particle system would be using some alpha channels (it looked really nice.)

I would really appreciate if someone had a link or an example of how to accomplish alpha channel within the Allegro.

Best regards,
Marius

Steve Terry

Member #1,989

March 2002

bmp = load_bmp("blah.tga", NULL);
set_alpha_channel();
blit(bmp, buffer, 0, 0, 0, 0, bmp->w, bmp->h);

I mean it's not hard

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Oscar Giner

Member #2,207

April 2002

To load the image you must be sure that it's loaded in a 32bpp bitmap. For that, you have to disable color conversion:

set_color_conversion(COLORCONV_NONE);
image_with_alpha = load_bitmap("file.tga", NULL);
set_color_conversion(COLORCONV_TOTAL);

To blit you just have to enable alpha blending:

drawing_mode(DRAW_MODE_TRANS, NULL, 0, 0);
set_alpha_blender();
// blit the alpha image
solid_mode();

[edit]

Quote:

set_alpha_channel();

Steve, you just made that up, didn't you?

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Steve Terry

Member #1,989

March 2002

>_< I had alpha channel in my mind when I wrote that... it's a typo yes

Mariusz

Member #1,634

November 2001

Thank you guys so much! This is one of the best places to get some answers :-)

Steve Terry

Member #1,989

March 2002

You better believe it 8-)

Mariusz

Member #1,634

November 2001

Worked like a charm

Thomas Fjellstrom

Member #476

June 2000

WE rule, and GameDev drools

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Gideon Weems

Member #3,925

October 2003

Hahaha. Sorry to interrupt the scheduled victory dance, but what's the point of draw_trans_sprite() if a regular blit does the same thing?

Steve Terry said:

    bmp = load_bmp("blah.tga", NULL);
    set_alpha_channel();
    blit(bmp, buffer, 0, 0, 0, 0, bmp->w, bmp->h);

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Oscar Giner

Member #2,207

April 2002

I wouldn't look much to Steve's code

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Steve Terry

Member #1,989

March 2002

:'( :'( :'( :'( :'( :'( :'( :'( :'( :'( :'(
I wrote it in a hurry and wasn't thinking
/me goes back to RTFM :P

Oscar Giner

Member #2,207

April 2002

Actually, the docs are a bit vague about this:

drawing_mode said:

This only affects the geometric routines like putpixel, lines, rectangles, circles, polygons, floodfill, etc, not the text output, blitting, or sprite drawing functions.

but if you go to the blitting and sprites section, only draw_trans_sprite (and draw_lit_sprite) mentions it's affected by the current drawing mode.

IIRC blit is not affected by draw_mode, but the docs say the opposite :-/ (but it wouldn't have sense to have two functions, draw_sprite and draw_trans_sprite).