Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!zodiac!spark.Verity.COM!medmunds
From: medmunds@spark.Verity.COM (Mike Edmunds x7645)
Newsgroups: comp.windows.x
Subject: setting a clip region in a shared GC
Message-ID: <9050@zodiac.ADS.COM>
Date: 7 Sep 89 23:30:18 GMT
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Reply-To: medmunds@Verity.COM (Mike Edmunds x7645)
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Organization: Verity, Inc., Mtn. View, CA  94043, (415) 960-7600
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According to the Xt manual, the function XtGetGC returns a "read-only,
sharable GC."  A number of widgets that I have been looking at,
however, use XtGetGC to get sharable GC's and then set the clip
regions in these GC's within their expose methods.  It seems
reasonable that a widget would want to use the clip mask to avoid
excess redrawing.  Which (if any) of the following statements is
correct?

1. A widget may alter the clip region in a shared GC.

This is what Douglas Young does in his dial widget (see "X Window
Systems Programming and Applications with Xt," p. 321), but seems
contrary to the Xt manual (and to Young's own statement on p. 186).

2. A widget may briefly alter a shared GC, but must undo any changes
it makes immediately after using them.

This is what the Athena scrollbar widget appears to do (in my
unpatched MIT X11R3 sources).

3. A widget may never alter a shared GC.

This is what the manual says, but would mean that any widget wanting
to take advantage of the expose method's region parameter would have
to use only private GC's.  The unfortunate implication would be that
an application using a large number of these widgets would create an
equally large number of GC's differing only in their clipping regions.

Any information on this would be greatly appreciated.

- Mike Edmunds
  Verity, Inc.			(standard disclaimers)