To: uunet!acds.physik.rwth-aachen.de!kuku AT uunet DOT UU DOT NET (Christoph Kukulies)
Cc: uunet!sun.soe.clarkson.edu!djgpp AT uunet DOT UU DOT NET
Subject: Re: windows/regions etc. 
Date: Tue, 21 Apr 92 10:04:47 PDT
From: Jonathan Ryshpan <hitachi!amito!jon AT uunet DOT UU DOT NET>
Status: O

>I've seen in DJ's graphics C++ code that there are things like regions
>and that stuff.
>
>Does anyone know where I can find the essential algorithm underlying
>modern Window systems, e.g. the X-Window System? I've heard that AT&T
>claims a patent on parts of it (Backing Store).

There are a *very* large number of algorithms involved -- blitting, save
unders, sorting, data communications, scheduling, etc.  A good place to
start is to examine the code for the X Window System, which is publicly
available.

The ATT patent involves save unders.  When a menu (or whatever) is
popped up, it destroys the pixels "under" it.  There are two ways to
restore the image whan the menu pops down:

(1) Request the program managing that part of the screen to restore it
    from the database, for example a word processor might restore the
    screen using the underlying text.

(2) When the menu pops up, the screen manager copies the pixels under
    the menu to a region off screen from which they can be restored when
    the menu pops down.

Method (2) is patented by ATT.  They may have agreed to release the
patent to the general public, since it is used in the X Window System.
Then on the other hand, maybe not.

If you think this patent is unreasonable, you may want to join the
League for Programming Freedom.  I can supply more information on
request.

Many Thanks:

Jonathan Ryshpan		<...!uunet!hitachi!jon>

	===============> I write for myself ONLY. <===============