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From: prl@ethz.UUCP (Peter Lamb)
Newsgroups: comp.arch
Subject: Re: XOR cursors
Message-ID: <4899@ethz.UUCP>
Date: 27 Jun 90 07:59:46 GMT
References: <1990Jun20.011602.1010@xavax.com> <1990Jun26.182359.2989@utzoo.uucp>
Organization: ETH Zuerich
Lines: 29

In article <1990Jun26.182359.2989@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) writes:
> In article <555@dg.dg.com> mpogue@dg.dg.com writes:
> >    I think all it takes is an example of a commercially shipped system,
> >that can be shown to have used the XOR technique, before the claimed date
> >of Cadtrak's invention.
> 
> Independent invention, even prior independent invention, does not necessarily
> invalidate a patent.  Prior *publication* generally does, as it is presumed
> to make the idea well-known, but a handful of highly-proprietary systems
> shipped earlier might not qualify.

Newmann and Sproull talk about the use of reversible raster operations
for cursors in `Introduction to Computer Graphics'. The earliest copy I
have is 1979 (2nd edition), but there's a 1973 copyright (to McGraw-Hill)
in the cover sheets. This is presumabably the first edition.

The use of arithmetic modulo 2^n (ie XOR for one bitplane) is given
as a possible implementation of a reversible function, *as an exercise
for the student*. All examples shown use XOR.

Does anyone have a copy of the 1973 edition? Does it also have
this material? Our library only has the 2nd edition.


-- 
Peter Lamb
uucp:  uunet!mcsun!ethz!prl	eunet: prl@iis.ethz.ch	Tel:   +411 256 5241
Integrated Systems Laboratory
ETH-Zentrum, 8092 Zurich