Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!adobe!mccoy
From: mccoy@adobe.com (Bill McCoy)
Newsgroups: comp.lang.postscript
Subject: Re: Important safety tip
Keywords: Chernobyl fonts
Message-ID: <907@adobe.UUCP>
Date: 13 Jun 89 22:26:17 GMT
References: <13-Jun-89.091841@192.41.214.2> <906@adobe.UUCP>
Sender: news@adobe.COM
Reply-To: mccoy@adobe.COM (Bill McCoy)
Organization: Adobe Systems Incorporated, Mountain View
Lines: 20

For those curious as to just exactly *why* an encoding vector is
required for all user-defined fonts...

from the Red Book, 2nd Ed., p.105:

  When a PostScript program tries to print a character of a user-defined
  font **and the character is not already present in the font cache**, the
  font machinery... executes the font's BuildChar procedure.

Given a character code in a show string, the letterform corresponding to that 
character code is determined by using the character code as an index into the
encoding vector to obtain a character name. Hence even though a Type 3 font
may never use it, the encoding vector is required for the font machinery to 
determine if a character is present in the font cache.

Also, it's worthwhile to note that recent versions *do* enforce the Red Book
requirement that the encoding vector be an array of exactly 256 objects.

Bill McCoy
Adobe Systems, Inc.