2.5. Metafiles.   

Graphics metafiles have been used for a long time for storing and transmitting pictures. They are a mechanism for the transfer and storage of graphics data that is both device and application independent.

A minimal implementation of a graphics metafile must allow the recording of all functions necessary to describe pictures independently of its creation environment. The pictures must be storable on different media, transportable between different graphics systems and displayable on different graphics devices.

Many metafile formats, such as Graphics Kernel System (GKS), restrict access to the metafile to sequential only. This allows for greater flexibility.

Typically a metafile contains two or three dimensional pictures each represented by a series of data records or items that are generated as a result of calls to the particular graphics routines being used.

ISO 8632 is the Computer Graphics Metafile (CGM) and is a machine and operating system independent interchange format that provides elements to represent geometric graphics (e.g. polygons, circles) and raster graphics (e.g. pixel arrays).

Sponsoring body and standard details 


   ISO/IEC JTC1/SC24

   ISO/IEC 8632:1992 Information technology - Computer graphics - Metafile

   for the storage and transfer of picture description information

      Part 1: Functional Specification

      Part 2: Character encoding

      Part 3: Binary encoding

      Part 4: Clear text encoding

This standard defines a file format for 2D vector graphics. Part 1 defines the graphic elements (lines, filled polygons, text, colors, ...) that may appear in a CGM and the other parts define 3 different encodings for these graphic elements: 

Character encoding:  compact ASCII encoding, useful if CGM files have

     to be transported over not binary-transparent channels

     (e.g. e-mail, character set converter)

Binary encoding:     this is the most often implemented CGM encoding,

     because it is both efficient and easy to implement.

Clear text encoding: a human readable textual encoding.

Many graphics packages are today capable of generating and/or interpreting CGM files. With the publication of the new version (1992) the functionality has been broadened by many features previously only provided by page description languages (PDLs) like PostScript. The 1992 version of CGM provides features not even available in PostScript level 2, e.g. the type of colour interpolation for area fill which is very common in publishing applications. The early problems in computer graphics standards with fonts have also been greatly improved in CGM by the inclusion of a font name list which allows the use of professional fonts within graphics. These developments make CGM a very good candidate for graphics exchange within the publishing industry.

For more information see http://speckle.ncsl.nist.gov/~lsr/lsr.htm as well as http://speckle.ncsl.nist.gov/~lsr/cgm.htm and http://speckle.ncsl.nist.gov/~lsr/cgm_std.htm

The ISO standard for CGM is available as well as other standards concerned with graphics.