Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!olivea!mintaka!bloom-beacon!EXPO.LCS.MIT.EDU!keith
From: keith@EXPO.LCS.MIT.EDU (Keith Packard)
Newsgroups: comp.windows.x
Subject: Re: Save Under Bit Implementation
Message-ID: <9011061609.AA26329@xenon.lcs.mit.edu>
Date: 6 Nov 90 16:09:50 GMT
References: <8bBX6we00Vpc89mW1i@andrew.cmu.edu>
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 23


> First, lets say I'm running tvtwm, and I have a big virtual window.  I
> bring up a menu and pop, My server grows to 10Meg.

Gee, this doesn't happen on *my* server (which creates a pixmap large enough
only for the occluded region)...

>  On the other hand the 'Never give a
> backing store' could work, and would allow windows which really
> shouldn't get backing stores, like the tvtwm virtual root, to not get
> backing stores.

Curiously enough, the way to get this effect is to request backing store on
that window before it is mapped.  This way, the backing store code can keep
track of the fact that nothing is ever rendered to that window and that it
needn't save any information in it other than the background tile.  Any window
with backing store which hasn't been rendered to gets a special optimization
which avoids saving any real bits, instead saving only the background state
when the bits were occluded.  Unfortunately, the backing store code must
assume that any currently mapped window which gets backing store enabled has
useful bits currently displayed on the screen.  You could easily remove this
constraint, at the loss of some level of protocol conformance (if not in word,
than certainly in intent).