Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!snorkelwacker!bloom-beacon!LARRY.MCRCIM.MCGILL.EDU!mouse
From: mouse@LARRY.MCRCIM.MCGILL.EDU (der Mouse)
Newsgroups: comp.windows.x
Subject: Re:  save_under & backing_store relations?
Message-ID: <9001061202.AA18171@Larry.McRCIM.McGill.EDU>
Date: 6 Jan 90 12:02:19 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 48

> From the Red Bible:

> "If save-under is True, the X server is advised that, when this
>  window is mapped, saving the contents of windows it obscures would
>  be beneficial."

> It seems that the good interpretation is:

> ... saving the *entire* contents of the windows it obscures ...
> [setting all the obscured windows' backing-store flags temporarily ON]

> and the bad interpretation was:

> ... saving the *partial* contents of the windows it obscures ...
> [saving the part of the screen under the save-under window]

> My question is: Why?

I don't *know*, not being responsible for the decision.  However, I
would guess that it's simply because doing the latter and getting it
right is very difficult.

[Terminology: call the window whose save-under hint is set the "menu"
 window; call the window being temporarily obscured the "obscured"
 window.  I will speak of "the" obscured window even though there may
 be more than one such.  An obscured window for which the server is
 already maintaining backing-store can of course be ignored.]

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.)

					der Mouse

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