Purpose

Typical Applications

Architecture

Status and Planned Extensions

Advantages of Current Specification

Implementation Issues

Purpose

The Computer Graphics Metafile standard is a published International Standard (ISO/IEC 8632) and has been adopted by the American National Standard Institute (ANSI). The CGM standard is being developed and maintained through a coordinated effort of ISO SC24 and ANSI X3H3.

The overall intent of the CGM standard is to provide the lowest level of drawing functionality required to capture and reproduce a picture, in a way that is portable across applications and devices. The CGM standard specifies two-dimensional graphics data interchange in a file format that can be created independently of device requirements and translated into formats needed by specific output devices, graphics systems, and computer systems.

A metafile is a device-independent, application-independent storage format for the exchange of the data that makes up a picture ("picture data"). The metafile definition in ISO/IEC 8632 includes a definition of output primitives and attributes that may be used to compose an illustration, but in an intentionally under-specified semantics (meaning). This was done to accommodate a wide range of existing systems, and to make the standard more adaptable to the requirements of diverse applications and users. The application software wishes to store a picture on a metafile. To do this, it has to write all the information required to a file. The software that performs these writing actions is called the generator. The software that reads a metafile back into an application is called an interpreter.

A profile addresses implementation conformance requirements for the generator and interpreter. For generators, the graphical characteristics of the picture are mapped onto a set of CGM elements which define the pictures within the accuracy and latitude defined by the implementation requirements in the profile. For interpreters, the graphical characteristics of the CGM elements are rendered into a graphical image or picture within the latitude defined by the implementation requirements in the profile. Without a profile, one can only address the syntactical correctness of the data stream. With a profile, one can address and test that the picture is correct. A profile provides a way of standardizing and publicly specifying the options that a vendor supports and how they are to be supported.

The three CGM encodings meet different needs, but all may be interchanged without loss of information. The binary encoding facilitates rapid graphic data processing. The character encoding is compact and transportable. The clear text encoding is human readable and editable. Only the binary format is approved for use by MIL-D-28003.

ISO/IEC 8632 CGM is an upwardly compatible standard format, developed in three versions that offer steps in capability. Version 1 includes elements of ISO 8632

Typical Applications

CGM is intended for use in computer graphics applications in the following situations:

• A graphics metafile is maintained at a central facility for a decentralized system that employs graphics devices of different makes and models that must utilize the data.
• A graphics metafile is required to preserve picture data when conversion or migration from one graphics system to another is necessary and the two systems are not necessarily compatible.
• A graphics metafile is intended for information interchange between a source system and a target system that are not necessarily compatible.

ISO/IEC 8632 is the recommended standard to:

• View the image on a wide variety of devices, with different characteristics (such as color and resolution), where the set of devices may not even be known at the time the metafile is generated;
• Enhance the picture before viewing the final image;
• Compose or overlay several drawings into a single picture for viewing.

Architecture

The CGM standard (ISO 8632-1987), "Computer Graphics - Metafile for the Storage and Transfer of Picture Description Information is composed of 4 parts. MIL-D-28003A utilizes Part 1 and Part 3 of the standard's four part architecture.

• Part 1 - Functional Specification - defines the functions of the CGM, independent of any encoding. It also includes responses for the standard and design requirements and design criteria.
• Part 2 - Character Encoding - defines an encoding of the Part 1 functionality in a format that conforms to ISO code extension rules. It is intended to provide an encoding of minimum size, and may be used for transfer of pictures through networks that cannot support binary transfer of data.
• Part 3 - Binary Encoding - defines an encoding of the Part 1 functionality that is intended to not require any other effort to generate and interpret on many systems.
• Part 4 - Clear Text Encoding - specifies an encoding of the Part 1 functionality that can be created, viewed, and edited with standard text editors. This encoding is appropriate for networked systems that support only text file transfer.

Status and Planned Extensions

Work is in progress to produce several International Standardization Profiles (ISP) for CGM. The profiles will be based on ISO 8632:1992 and Amendment 1 which specifies profile rules and a model profile. It is proposed that there will be three profiles based on current profiles (including MIL-D-28003 the CALS CGM Application Profile [AP]), which range from basic scientific and technical graphics to advanced presentation and visualization.

The 1992 edition of the international standard for CGM provides critical capabilities for the CALS environment, which include:

• Advanced two-dimensional graphics (curves; fine control of line appearance; composite line primitives; user defined line types, hatch styles and marker types; additional standardized hatch styles; arbitrary text path; filing mechanism; and general linear transformations).
• Improved text and font support.
• Arbitrary boundaries for clipping and shielding.
• Additional color models beyond RGB (used in printing).
• Additional raster graphics (scanned image) capabilities.
• Symbols: external reference to "standard" libraries of named symbols.

The purpose of this edition of the standard is to extend CGM to fulfill requirements of engineering drawings, the preparation of graphic arts quality presentation materials, cartography, and technical publishing. To a large degree, this work was directly in response to CALS requirements.

Of particular interest to the CALS environment is the work under way within the CGM standards organization (X3H3) to provide "intelligent graphics". "Intelligent Graphics" is defined as adding information to graphics that is not graphical information, but that attaches application intelligence or semantics to the graphics. Requirements are association of comments with graphics elements, association of comments with element groups (hierarchical), native format editing, version control, and text to graphics links. This requirement was originally introduced by the "electronic Review" work of the CALS Industry Steering Group (ISG) Electronic Publications Committee, where SGML-tagged documents are used to provide a commenting capability. The CGM additions will allow for SGML tags to be attached to objects within the CGM file.

Note that the addition of tiled raster capabilities, based on the Tiled Raster Interchange Format (TRIF), allows for the encoding of large raster images within a CGM file. Possible future extensions to the international standard that could be of considerable interest to CALS include the formulation of an object structured grammar. This has been requested from, and will be of major use to, CGM users in commercial aviation (intelligent graphics); CALS electronic review (review comments in graphics and stronger links to text); and hypermedia (smart objects in graphics databases).

Advantages of Current Specification

The CGM contains device-independent, digitally-encoded vector and raster graphics data. CGM files are easily transferred and displayed on a wide variety of hardcopy devices (dot-matrix, ink-jet, electrostatic, and laser printers, pen plotters, and film cameras). CGM files can also be easily previewed on an extensive range of softcopy terminals. In comparison to Raster, CGM is easily modifiable, generally of much smaller size, and not dependent upon resolution of the output device. In comparison to IGES (2D data), CGM is faster to interpret and display, and again more compact. The selection of which of the CALS graphic standards (raster, IGES, or CGM) that best fits the application, should be the result of the thorough examination of the processes involved in the application.

Implementation Issues

Standards testing, conformance testing and interoperability testing are essential steps towards achieving successful interoperability. Standards testing examines the design, construction, and details of the standard, and tests to see if it is correct and well-defined. Interoperability testing demonstrates that a given pair of systems (or products) can interoperate. In other words, standards testing assures that the standard is well-defined; conformance testing ensures that the product is "built correctly"; and interoperability testing ensures that there are no hidden problems in system interfaces resulting from laxity in defining the specification.