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From: asw@vax-d.rutherford.ac.UK (Antony Williams)
Newsgroups: comp.windows.x
Subject: Re: save-unders etc
Message-ID: <10804.8711111536@vd.rl.ac.uk>
Date: Wed, 11-Nov-87 10:36:59 EST
Article-I.D.: vd.10804.8711111536
Posted: Wed Nov 11 10:36:59 1987
Date-Received: Sat, 14-Nov-87 03:15:03 EST
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There has been quite a lot of discussion about save-unders.  Meanwhile,
there are proposals for double buffering, and no doubt there will
be implementations of retained windows appearing soon.

I would like to take this opportunity to point out that Brad Myers implemented
save-unders as well as (optionally) retained windows in his Sapphire
window manager.  He wrote about this in his chapter
"Issues in Window Management Design and Implementation"
in Methodology of Window Management (Springer 1986, ISBN 0-387-16116-3).
He pointed out that combining save-unders (he called them "courteous"
windows) with retained windows
     " ... does not work very well.  If you have windows
     that have off-screen memory and courteous windows overlapping
     each other, and you move one of the windows to the top, there is
     no order in which you can update the windows that will be correct.
     They all have to be updated first or you have to use separate buffers
     into which you move the information temporarily."

In Sapphire, Brad was trying to retain as much of the window images
as possible while conserving memory space and cpu time.

He also observed a great deal of complexity in moveing and growing
windows in an overlapped set.  I think this comes largely because
the image is partially on screen, and partially somewhere else,
and complexity is n^2.  If you have enough store to save a copy
of all of the pixels in one place (eg if retaining or double buffering)
life gets much easier.  Also note that if all windows are retained,
saving only the obscured regions (as on the BLIT)
saves *at most* one screenful of memory compared to saving whole windows.
For monochrome displays this is almost trivial,
and well worth the simplification in software.
   Tony
---------------------------------------------------------------------------
Tony Williams					|Informatics Division
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