Xref: utzoo comp.windows.x:34018 comp.windows.open-look:869
Path: utzoo!attcan!uunet!mstan!chuck
From: chuck@Morgan.COM (Chuck Ocheret)
Newsgroups: comp.windows.x,comp.windows.open-look
Subject: Re: Xor'ing of colors problems
Message-ID: <2846@s5.Morgan.COM>
Date: 28 Feb 91 00:52:11 GMT
References: <26@ftms.UUCP>
Organization: Morgan Stanley, & Co., Inc. / New York City, NY
Lines: 50

> We are using OpenLook under SunOS 4.1.  The programmer is using the GXxor
> function in the graphics context for Xlib calls.  We get the following results:
> 
>         draw color      background      result          expect
> 
>         black           black           white           black
>         white           white           tan(?)          black
>         red             red             white           black
>         red             blue            aquamarine(?)/  magenta
>                                         cyan
>         blue            blue            white           black
> 
> Can someone explain why we are not getting the expected results?  Is there
> a formula that can be used to determine ahead of time what the results will
> be?

Xoring pixels operates on bits, not on names.  Depending on your server and
the Visual you are using you cannot expect particular colors to have
particular pixel values.  Basically, colors can be allocated to arbitrary
cells in the colormap.  To accomplish what it looks like you are trying to
do you should be Xor drawing with your foreground pixel set to a value
which is an Xor of the displayed pixel and the desired color's pixel.

For example, let's say that you allocate the following colors on a 4 bit
deep display.

color name	pixel allocated(treat these as completely arbitrary)
----------	---------------
red		0x5
blue		0xf
magenta		0x8
green		0xa

Let's also say that most of your window has been drawn in blue and you
want to do Xor drawing (e.g. rubber band line) that appears magenta.
Don't try to draw in red 0x5 (Xor does not do color blending unless you
are real lucky with your color map).  If you were to Xor draw in red
the blue pixels will get changed from 0xf to 0xa = green.

Draw with the pixel value set to blue^magenta = 0xf^0x8 = 0x7 (^ is
Xor).  The funny thing is that you may never have allocated color cell
0x7.  When you Xor draw with 0x7 over the blue 0xf background, the pixels
are changed to magenta 0x8.

~chuck
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