Path: utzoo!attcan!uunet!ginosko!usc!apple!bloom-beacon!odi.COM!mlm
From: mlm@odi.COM
Newsgroups: comp.windows.x
Subject: backing store
Message-ID: <8909132320.AA00371@bopper.odi.com>
Date: 13 Sep 89 23:20:43 GMT
References: <8909132212.AA01424@expire.lcs.mit.edu>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 58


	Setting backing store
	to WhenMapped in my application's windows results in strange behavior,
	including displaying pieces of other windows.

    Sounds like your server is not working properly.  If you can reliably
    reproduce the problems, submit a detailed bug report.  I don't believe we've
    seen any other reports like this.

I'll perform some further experiments....

	First request: would someone who understands backing store in X
	please give the rest of us an explanation of what it does, how it
	works, and how to use it?

    Sounds like you already understand this.

I should have been more explicit: in addition to a clear general
description (and there several ways backing store can be implemented
in a window system, including having each window save the bits that IT
obscures), what I am really looking for is an "official" or
"recommended" way of selectively using backing store, both in raw Xlib
and in Xt (i.e., is one supposed to do anything other than act on the
XSetWindowProperties on XtWindow(widget)?).  Is it OK to turn backing
store on in the middle of things, perhaps even after the initial
expose event, or must it be set before the window is mapped?  What
about window parents/children -- must they all have backing_store to
get a desired effect, or is setting it for the application's
top-window sufficient?  I can't make sense out of backing_pixel,
documented in "X Window System" as "value to use in restoring planes",
page 32; shouldn't backing store record the value of each pixel in the
image being saved (i.e., the binary value at each position of each
plane?)?  Why is a pixel necessary, and what kind of value should it
be given?  If backing store is just a window property, then I suppose
one could write an application along the lines of xwininfo that turned
backing store on for a designated window (by name or by pointing);
anything wrong with that idea?

    On a color display, it can bloat your server size dramatically.  I used to
    use -wm, but I don't any more.  My server is fast enough ... :-)

Yes, several people pointed out the greatly increased penalty paid on
a color monitor, although it's perhaps on a color monitor that one
stands to gain the most from backing store.  Are there any
applications around that specify backing store because their response
to expose events is slow (or do applications like this generally
maintain their own internal pixmap for redisplay, scrolling, etc. --
another standard windowing technique that works in the absence of
backing store and also can provide superior scrolling behavior at the
cost of generating the entire image rather than just what fits within
the clipping region).

Thanks.

	Mitchell L Model		    Object Design, Inc.
	Director, HeadStart Program	    1 New England Executive Park
		  			    Burlington MA 01803
	mlm@odi.com, odi!mlm@uunet.uu.net   (617) 270-9797