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From: karl@mote.umb.EDU ("Karl Berry.")
Newsgroups: comp.windows.x
Subject: Handling expose events.
Message-ID: <8908220058.AA08564@mote.umb.edu>
Date: 22 Aug 89 00:58:16 GMT
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 41

Two programs I've worked on recently (Metafont and Ghostscript) are
designed to work with many devices; to them, X is just another device. 
(Well, there are some differences between MF and Ghostscript as far as
the way X is used goes, but they aren't relevant for my purposes here.)

The interface between any device and the program is a set of small
procedures.  The program does all the real graphics work.

For both these programs, it is quite straightforward to implement the
interface procedures in Xlib; from what little I have read of the Xt
manual, Xlib seems far more appropriate.  It seems like overkill to
me to have a widget for these windows, when the only thing that is ever
done is drawing rectangles and lines (say).  Perhaps I'm wrong about
this; that's one of the things I'd like to know.

But there is a problem with using Xlib: handling expose events.  In
neither case does the program keep track of a bitmap representing the
current state of the window; it just makes marks as it goes along.  And
although I could write all the drawing routines to update a bitmap, that
is probably an unacceptable memory overhead, and it is certainly not
very clean.

What I would like to do is get X to tell me when part of the window is
obscured; that seems like the VisibilityNotify event.  But I also want
to get the image that was on the screen with XGetImage (or whatever; the
point is to get it from X), and thus I need the coordinates of what was
obscured.  It looked like (a) those coordinates are not available to me (I
suspect I'm wrong about this), and (b) XGetImage is not guaranteed to
work on obscured parts of windows, and I only get the event after the
window is obscured, so I'm up the creek...

The general question is: what is the best approach to this?  For
example, is it somehow possible to use Xt to handle the Expose events,
but deal with the drawing myself?

The specific question is: is it possible to handle expose events using
Xlib if a bitmap of the window's contents is not kept by the program?

Thanks for any help.

karl@umb.edu   ...!harvard!umb!karl