Path: utzoo!attcan!uunet!husc6!think!bloom-beacon!ZERMATT.LCS.MIT.EDU!RWS
From: RWS@ZERMATT.LCS.MIT.EDU (Robert Scheifler)
Newsgroups: comp.windows.x
Subject: what is backing store?
Message-ID: <19880706125708.8.RWS@KILLINGTON.LCS.MIT.EDU>
Date: 6 Jul 88 12:57:00 GMT
References: <149@henry.ece.utexas.edu>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 14

Backing store simply means having the server preserve the contents of a
window, even when all or part of the window is obscured by other windows
(and potentially even when the window is unmapped).  There are various
ways of implementing this, but basically it means the server will copy
(all or parts of) the window's contents off-screen before the window
becomes obscured, and will continue to perform graphics output to the
off-screen contents.  As regions of the window become unobscured, the
server copies the saved contents back onto the screen.  The advantage of
backing store is that the client doesn't have to repaint regions of the
window when they become unobscured.  This is a win if the contents of
the window are relatively complex and expensive to recompute/redraw.  As
to when it is appropriate to use it, I'm a mechanist, not a policist,
but one answer is "let the user decide", by providing settable
widget/program resources or command line options.