Path: utzoo!utgpu!watserv1!watmath!att!bellcore!rutgers!tut.cis.ohio-state.edu!snorkelwacker!bloom-beacon!LARRY.MCRCIM.MCGILL.EDU!mouse
From: mouse@LARRY.MCRCIM.MCGILL.EDU (der Mouse)
Newsgroups: comp.windows.x
Subject: Re:  Problem with GXxor on sun server
Message-ID: <9006160338.AA14478@Larry.McRCIM.McGill.EDU>
Date: 16 Jun 90 03:38:37 GMT
Sender: root@athena.mit.edu (Wizard A. Root)
Organization: The Internet
Lines: 40

[William Anderson writes that he's trying to port something to X11]

> This software writes messages to the screen and then unwrites them
> using the GXxor logical operation.

> On a monochrome display with foreground and background set using
> WhitePixel() and BlackPixel() respectively, foreground xor writes
> result in no visible text using XDrawString().  If I do a
> `XSetForeground(display,gc,backgnd)`, things work.

You're writing the wrong thing.  There is no guarantee that the
foreground pixel value is anything special; in particular, it may be 0.
Drawing 0s with GXxor won't do a whole lot, now will it?

Set the foreground value of the GC to the XOR of the foreground pixel
value and the background pixel value.  *Then* try drawing with GXxor.

You're assuming that the blank window background is pixel value 0 and
that the foreground is something else.  The situations where it works
for you now are precisely those where this is so.

On a color machine, for example, assume your background and foreground
colors are pixel values 9 and 10 respectively.  Then when you draw
something (text, line, whatever) with the foreground color and GXxor as
the function, the pixels will be set to 9 XOR 10 = 3, which is probably
not what you want.  (Pixel value 3 will likely come from someone else's
colormap cell, and thus will get displayed as an effectively random
color.)  Instead, store 9 XOR 10 = 3 as the foreground pixel value of
the GC.  Then when you draw, you will get pixels set to to 9 XOR 3 =
10, which is what you want.

The same thing is true on a one-bit screen; it's just that there are
only the two possible pixel values, which can easily confuse you
because it means that drawing with GXxor will always appear to work
with one of them as the GC's foreground color.

					der Mouse

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			new: mouse@larry.mcrcim.mcgill.edu