Path: utzoo!attcan!uunet!husc6!bloom-beacon!HPFCLP.SDE.HP.COM!diamant
From: diamant@HPFCLP.SDE.HP.COM (John Diamant)
Newsgroups: comp.windows.x
Subject: Re: backing store
Message-ID: <8807051918.AA21487@hpfcjrd.sde.hp.com>
Date: 5 Jul 88 19:18:35 GMT
References: <19880705150956.9.RWS@KILLINGTON.LCS.MIT.EDU>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 34


> From:  Robert Scheifler 
> Subject:  Re: backing store

>     Date: Mon, 04 Jul 88 22:45:29 MDT
>     From: John Diamant 

> Servers do tell you up front (see Xlib's XDoesBackingStore and
> XDoesSaveUnders) whether they implement backing-store and save-unders at
> all.  This is probably an adequate indication for most clients; if a
> server says it implements the functionality, it probably will do what
> you ask for most windows.  You should assume the server will maintain
> contents until you get an exposure event that contradicts it, and only
> then start maintaining a client-side shadow if that will increase
> performance.

I'm not sure this is possible in some applications.  For instance, a
paint program may want to rely on the server maintaining its image,
and when it's ready to write the output, it could grab the server
(or whatever was required to ensure no window got placed on top of
it) and read the bits back off the screen.  If somewhere in the
middle of all this painting, the server suddenly decided it couldn't
handle the backing store, then the application is in trouble, since
it didn't maintain the information itself.  In the current scenario,
it would still be required to maintain a backup copy and duplicate
all the work of the server.  With a little more information from the
server, it wouldn't be required to do that (this all assumes the
paint program did not maintain a complete history of user commands
to reproduce the image some other way).

John Diamant
Software Development Environments
Hewlett Packard Co.             ARPA Internet: diamant@hpfclp.sde.hp.com
Fort Collins, CO                UUCP:  {ihnp4!hpfcla,hplabs}!hpfclp!diamant