Path: utzoo!attcan!uunet!cs.utexas.edu!usc!orion.oac.uci.edu!ucivax!baxter
From: baxter@ics.uci.edu (Ira Baxter)
Newsgroups: comp.arch
Subject: Re: XOR cursors
Message-ID: <2688DC2A.26723@ics.uci.edu>
Date: 27 Jun 90 16:17:46 GMT
References: <1990Jun20.011602.1010@xavax.com> <1990Jun26.182359.2989@utzoo.uucp> <4899@ethz.UUCP>
Lines: 39

In <4899@ethz.UUCP> prl@ethz.UUCP (Peter Lamb) writes:

>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.

I do.  Raster ops aren't mentioned :-{  On the other hand, if N&S put
it into the 1979 edition, presumably there are technical articles
in which it was documented before it appeared in N&S; their
bibliography in the 1973 version is pretty extensive, so I
would expect a reference to such a technical article in the 1979
version.

I don't have the 1979 version, so I can't check this.

Another place to look is obviously in Foley& van Dam,
"Fundamentals of Computer Graphics", who also have an extensive
bibliography.
--
Ira Baxter