README.TXT

README.TXT


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There are three CGMs (Computer Graphics Metafiles) in this collection. 
The contents of this CGM collection are: 

   ALLPRIMS.CGM  (a Clear Text metafile)
   CORVETTE.CGM  (a Binary metafile) 
   TECHDRAW.CGM  (a Binary metafile)

We have indicated the CGM encoding (there are three) with which each of
the files has been encoded.  Clear Text is human readable and can be
manipulated in normal text editors.  Binary is compact and efficient,
and the encoding most often used by implementors.  We have not included
a sample in the Character Encoding, as it is not much used in practice. 
Any CGM can be represented in any one of the three encodings with
equivalent graphical content, and there are tools available to convert
amongst the encodings. 

There are three other files associated with this collection.  

   ALLPRIMS.SUM
   CORVETTE.SUM
   TECHDRAW.SUM

These are summary reports produced by the MetaCheck (TM) product,
indicating at a summary level what each file contains and whether it is
a valid metafile (detailed disassemblies and trace reports are also
possible, but are too voluminous to be included here). 

A description of each file follows (a color rendition of each can be
found in the central color plates section of "The CGM Handbook",
L.Henderson and A.Mumford, Academic Press, 1993). 


CORVETTE.CGM
============

This file is a good example of typical CGM use in graphic arts
applications.  The picture is of a sleek and shiny Corvette sports car,
viewed headon and closeup, at sunset.  The foreground around the car and
receeding to the horizon is black.  Behind the car and above the horizon
is a sunset sky, shading from orange at the horizon, through pale green
and blue, to deep blue at the top.  Stars are beginning to appear in the
sky.  There is a row of palm trees on the horizon, in silouette but
catching the orange evening light.  The car is deep blue-black, with
parts of the hood and the windshield reflecting the colors of the
sunset.  The parking lights are on.  This file is a Binary-encoded CGM. 
It is drawn entirely using color filled polygons, rectangles, circles,
and ellipses, plus lines and text (and associated attributes). 
Extensive use is made of color.  The original source was a Genigraphics
graphics arts workstation, of the (former) Genigraphics Corp.  The
picture was produced circa 1988, and used in the CGM integration
demonstration (Integrate 88) at the NCGA 1988 Exposition. 


TECHDRAW.CGM
============

This file is a good example of the typical use of CGM in electronic
documents.  The Air Transport Association (ATA) -- commercial aircraft
manufacturers and the airlines themselves, as well as suppliers to the
industry -- has developed a program for delivery and use of technical
documents in revisable electronic form.  This is patterned after the
similar program in the US DoD, CALS (Computer-Aided Acquisition and
Logistic Support).  ATA documents comprise SGML for revisable text, and
CGM for revisable vector and raster graphics (TIFF has been included in
the past for purely raster graphics, but ATA is replacing its use by the
equally capable CGM:1992 Version 3 Tile Array element).  The picture is
a typical technical illustration from an aircraft maintenance manual,
including three views: the aircraft indicating assembly location; the
mounting area of a component panel; and, a complex illustration of the
subassembly itself (an electropneumatic drive unit for a flap).  The
picture is a black-and-white line drawing, using only line and text
elements (and a few associated attributes).  This file is a
Binary-encoded CGM.  The source is a requirements document for standard
graphics exchange, produced by the ATA Graphics Working Group, circa
1991. 


ALLPRIMS.CGM
============

This CGM file is interesting for two reasons: it contains at least one
instance of 18 of the 19 graphical primitives of Version 1 metafiles of
CGM:1992 -- Polyline, Polymarker, Text, Polygon Set, Cell Array,
Elliptical Arc Close, etc -- plus a good sampling of the 35 attribute
elements; and, it is in the Clear Text encoding so you can read the
contents and see what CGM structure and elements look like.  The one
omitted primitive is GDP (Generalized Drawing Primitive, which is a more
or less private extension element).  The picture is composed of a grid
of 18 large cells -- 3 rows of 6 cells each.  Each cell contains a
sample of one of the Version 1 graphical primitives, along with a label
identifying the primitive.  Color is used, different interior styles for
filled-area elements are used, wide and narrow lines are used, etc.  At
the bottom, under and contiguous to the grid of cells, is a low wide
block containing samples of the 5 standard CGM line types, and to the
right is a documentation block with the metafile "pedigree".  This file
was originally produced by the CTN, the CALS Test Network (CALS is the
DoD Computer-Aided Acquisition and Logistics Support program, whose
standards-based revisable electronic document program calls for CGM as
the preferred format for delivery of revisable electronic documents). 
CTN's role with CGM is to identify and test the standards concepts
themselves, perform informal interoperability events, etc.  CTN does not
do validation or certification testing for CGM and CGM implementations. 
NIST (the National Institute of Standards and Technology) is now
performing this role.