From: Ryan Blazecka <eblazecka AT bc DOT sympatico DOT ca>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Problem with XOR (Allegro)
Date: Mon, 27 Oct 1997 17:24:22 -0800
Organization: BCTEL Advanced Communications
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Gautam N. Lad wrote:
> 
> Hi,
> Ok, I've a palette in a Datafile, which I load up when my program runs. 
> The colours are defined so I don't have to use numbers.   Like so:
> 
> #define YELLOW  5
> #define WHITE   4
> #define XOR_ON  xor_mode(-1);
> #define XOR_OFF xor_mode(0);
> 
> circle(screen, 100, 200, 5, YELLOW);
> 
> Now, I have a problem.  It seems that when I turn XOR enabled, the 
> colours value seem a bit differnet.  What I mean is, if I draw using 
> YELLOW, the colour WHITE (index in palette of 4 in my case) get's used. 
> Is this how XOR works?

Yes, this is exactly how it works. If xor mode is on, Allegro takes the
destination pixel and the source pixel and combines them with an XOR
operation, and puts that on the bitmap. In your case (XORing 5 with 0)
should return 5, so perhaps color #5 isn't actually yellow.

> I'm pretty sure, but I don't think it should change the colour I'm 
> drawing with.  Now, this is on a background colour (index value of 0 in 
> palette), with nothing on it.

Yes, it should. Except when you are drawing on color 0.

> It seems the colour being used is one less then what I want.

Maybe the indexes you are using are off by 1. What happens when you try
to draw with color 6?

> I do something like this:
> 
> XOR_ON;
>         circle(screen, 100, 200, 5, YELLOW);
> XOR_OFF;

If what you really want is just a yellow circle, then leave xor_mode
alone. In it's off state (the default), the color drawn it the one you
pass to the drawing functions (not to mention it's much faster when
drawing to the screen).

-- 
[  Ryan Blazecka  --  http://www.deninc.com/~corrosion/  ]
[ _corrosion_ AT geocities DOT com OR eblazecka AT bc DOT sympatico DOT ca ]