Path: utzoo!utgpu!news-server.csri.toronto.edu!torsqnt!tmsoft!robohack!druid!darcy
From: darcy@druid.uucp (D'Arcy J.M. Cain)
Newsgroups: comp.graphics
Subject: Re: File Formats
Message-ID: <1990Jul12.183149.3002@druid.uucp>
Date: 12 Jul 90 18:31:49 GMT
References: <8437@arctic.nprdc.arpa>
Reply-To: darcy@druid.UUCP (D'Arcy J.M. Cain)
Organization: D'Arcy Cain Consulting, West Hill, Ontario
Lines: 47

In article <8437@arctic.nprdc.arpa> armfield@nprdc.navy.mil
(Sylvia Armfield) writes:
>   I still haven't figured out how to identify the following file formats:
>           CGM
>
There is more than one type of CGM file.  I can help you out with the clear
text encoding and the binary encoding.  The other encoding is character.  It
is described in ISO 8632-2.  In any case all CGM files start with the same
thing, the BEGIN METAFILE statement.  The only difference is in how this
element is encoded.

In clear text encoding the element is simply the ASCII string "BEGMF".  In
binary encoding you have to read in the first two bytes as a word.  The
MSB is followed in the file by the LSB.  This word is bitmapped to include
the following information:
   15 - 12:  Element class
   11 - 5:   Element ID
   4 - 0:    Parameter list length

BEGIN METAFILE is a "Delimiter Element" making it class 0.  The element
ID within that class is 1.  The parameter list length is variable so it
must be ANDed out when comparing.  The bit pattern is then:

       0 0 0 0 0 0 0 0 0 0 1 x x x x x

To check it simply AND the word with 0xffe0 and compare it with 0x0020.  In
reading the standard I get the impression that it is actually legal to add
padding characters (nulls) to the beginning of the file.  I rather doubt that
anyone would actually do this but it may be appropriate to read in words till
a non zero word is read and compare this word.  You can read in full words
since all elements are constrained to start on a word boundary.

If you want the full description of the CGM format it is available from
ANSI as the following ISO standards:

  ISO 8632-1     Functional specification
  ISO 8632-2     Character encoding
  ISO 8632-3     Binary encoding
  ISO 8632-4     Clear text encoding

HTH

-- 
D'Arcy J.M. Cain (darcy@druid)     |   Government:
D'Arcy Cain Consulting             |   Organized crime with an attitude
West Hill, Ontario, Canada         |
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