Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uwm.edu!bionet!ames!ncar!ico!auto-trol!marbru
From: marbru@auto-trol.UUCP (Martin Brunecky)
Newsgroups: comp.windows.x
Subject: Re:  save_under & backing_store relations?
Message-ID: <591@auto-trol.UUCP>
Date: 8 Jan 90 15:27:10 GMT
References: <9001061202.AA18171@Larry.McRCIM.McGill.EDU>
Reply-To: marbru@auto-trol.com (Martin Brunecky)
Organization: Auto-trol Technology, Denver
Lines: 42

In article <9001061202.AA18171@Larry.McRCIM.McGill.EDU> mouse@LARRY.MCRCIM.MCGILL.EDU (der Mouse) writes:
>
>Suppose the temporarily hidden part of the obscured window is changed
>while the menu window is up.  The saved bits then have to be updated,
>which is very difficult to get right without maintaining a shadow copy,
>which is precisely what backing-store does.  You're losing because
>backing-store maintains a shadow copy of more than just the area hidden
>by the menu window.
>
>(Personally, I'd be just as happy to see a much simpler and almost as
>good version: save the bits as in the "bad" interpretation above, but
>if an obscured window is changed, clear its portion of the save array
>to its background color and then generate expose events for that window
>when the menu window goes away, just as if save-unders weren't being
>used.  Gains the speed advantage in the common case and is correct,
>even though slower, in the difficult case.  And it's not a memory pig
>in the case that bothered you.)
>
...
     and in my opinion, this would very well justify the distinction
     between "save_under" and "backing_store"; i.e.
   
     save_under   ... attempts to preserve obscured area as long as
                      there is no attmpt to change this area

     backing_store ...makes sure the server maintains the entire window
                      no matter how much of it is obscured or invisible

     [ however, the statement that the server can "give up" on any of
       the both features is still valid, i.e. even backing store does
       NOT guarantee window integrity under ALL circumstances ]

     The "lightweight" save-under implementation would serve it's principal
     purpose: reduce (not eliminate) the amount of expose event generated
     by popup shells (or any transient in nature windows).


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Martin Brunecky, Auto-trol Technology Corporation,
12500 North Washington Street, Denver, CO-80241-2404
(303) 252-2499                                        ncar!ico!auto-trol!marbru