Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!mintaka!bloom-beacon!LARRY.MCRCIM.MCGILL.EDU!mouse
From: mouse@LARRY.MCRCIM.MCGILL.EDU
Newsgroups: comp.windows.x
Subject: Re:  GXxor problems on Xqvss
Message-ID: <9011091619.AA28709@Larry.McRCIM.McGill.EDU>
Date: 9 Nov 90 16:19:05 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 35

> Some X programs (that use special GC's like xor) do not display
> correct on my Xqvss server.  When I change display to a Xsun server
> on a SPARC (also form the mit tape) but the client is still running
> on the same, original host, the programs display fine.

This could be the common XOR mistake.  Is the qvss a color server and
the SPARC a monochrome?  If so, the problem is probably programmers not
thinking about how GXxor works and carelessly drawing with the
foreground (or sometimes the background) color and function GXxor,
which will work correctly only when the background (resp. foreground)
color is 0.  On a monochrome display, this is true often enough that
the bug goes unnoticed, but it shows up big-time on color machines.

> - Xchess, [...].  (Xchess uses outline bitmaps, mask bitmaps and
>   GXcopy, GXor, GXandInverted, GXinvert contexts)

Aha.  On a color display you basically never want to use GXinvert (or
the related GXandInverted), not unless you have very carefully arranged
your colormap so that you're sure it does the right thing.  (The only
exception is when it's being used to copy bitmaps to other bitmaps
preparatory to drawing them on the screen.)

As for fixing it, I can't offer anything concrete without seeing the
code.  Basically, you have to go through and make certain that none of
the operations used in the GCs make any assumptions about the pixel
values corresponding to the colors used.  Anything but GXcopy is
dangerous on a color display; anything but GXcopy, and GXxor with
carefully-specified colors, is probably useless unless you have set up
your colors with XAllocColorPlanes (or have a decomposed-colormap
display) and have thought out precisely what each operation does.

					der Mouse

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