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From: butt@umd5.umd.edu (Edgar Butt)
Newsgroups: comp.lang.postscript
Subject: Re: Postscript Bug
Message-ID: <4583@umd5.umd.edu>
Date: 6 Mar 89 21:03:01 GMT
References: <4571@umd5.umd.edu> <577@adobe.UUCP>
Reply-To: butt@umd5.umd.edu (Edgar Butt)
Organization: University of Maryland, College Park
Lines: 29

In article <577@adobe.UUCP>, greid@adobe.com (Glenn Reid) writes
in reply to <4571@umd5.umd.edu> butt@umd5.umd.edu (Edgar Butt):

> Both this message and one following it (to do with bitmap fonts)
> mentioned "bugs" in PostScript printers because output was off by a
> pixel somewhere.
> 
> Both of these effects are due to numerical methods.  In each, the scale
> factor was essentially 1/resolution, which loses much of the accuracy
> of the numeric operations.
...
> Remember that the PostScript language coordinate system is an "ideal",
> and is device-independent.  The same program can run on printers of
> arbitrary resolution, and if you try too hard to tailor it to a bitmap
> grid, you often get undesirable results.

The program I posted illustrated that on a particular implementation
of Postscript, namely, version 47.0 on a Laserwriter II NT, varying
the height (y-dimension) of a rectangle caused its horizontal position
(x-position) to change.  Whatever numerical method is used to map the
"ideal" coordinate system onto an actual device, it should be consistent
and rounding off a y-dimension should not affect x-positioning.
The other two incarnations of Postscript on which I ran the program
gave what I consider to be correct results.

I believe the program I posted demonstrated a bug, not a lack of
understanding.

Edgar Butt (301) 454-2946 butt@umd2.umd.edu