From: gradha AT iname DOT com
Newsgroups: comp.os.msdos.djgpp
Subject: Re: ANDing bitmaps under allegro
Date: Wed, 3 Nov 1999 23:53:19 +0100
Organization: Telefonica Transmision de Datos
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References: <381F80A9 DOT 2DE040EB AT cs DOT iastate DOT edu> <tc6pv7 DOT n77 DOT ln AT pedos DOT es> <38206093 DOT A877DBEC AT cs DOT iastate DOT edu>
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

>> > I need to AND two bitmaps together, and assign the result to a new
>> > bitmap. Does anyone know how to do this?
>>
>> But what do you mean by ANDing?
>
> I am running at 8bpp. Basically, I have a bitmap containing an image,
> and then another with a silhouette of that image in white on black.

Hmmm... I think that's simply "masking". In palette modes, usually
black is color 0 and white 255 (or the other way round), so using a
bit and wouldn't be very good, IMHO.

I think it's better if you create a loop, which reads pixels from the
mask bitmap and converts them to grey (if necessary). If the grey you
get is more than 50% black, you ignore that pixel. If it's white, you
draw the pixel on a third bitmap.

In the end you will have a final bitmap with the first image masked by
the second one.

If you know that you are really using only two palette colors, and that
for example, black is 0 and white is 255, then it's just a loop doing
something like (pseudo-code):

for (y=0;y<bmp_source->h;y++)
{
	for (x=0;x<bmp_source->h;x++)
	{
		color = getpixel(mask_bmp,x,y);
		/* white could be 0, 255, or other value depending on your palette */
		if (color == white)
			putpixel(final_bmp,x,y,color);
		else
			putpixel(final_bmp,x,y,0);
	}
}

Grzegorz Adam Hankiewicz - gradha AT iname DOT com
Gogosoftware - http://welcome.to/gogosoftware/

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