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1. Introduction to WebCGM

1.1 About WebCGM

WebCGM^TM is a profile of the ISO Computer Graphics Metafile standard (ISO/IEC 8632:1992), tailored to the requirements for scalable 2D vector graphics in electronic documents on the World Wide Web.

The WebCGM profile is a subset of the ISO standard, and a set of specifications targeted especially at the effective application of the ISO standard to representation of 2D graphical content within Web documents.

CGM has been an ISO standard since 1987, and CGM has been a registered media type (image/cgm) for the Internet and the World Wide Web since December 1995.

1.2 WebCGM, Profile Rules, and ISPs

WebCGM is a conforming profile of CGM under the stipulations of CGM:1992 Amendment 2, "Rules for Profiles", and it utilizes the Profile Proforma (PPF) of the amendment for representation of the element-by-element content details. Registration of WebCGM as an International Standardized Profile (ISP) is anticipated.

WebCGM is an "intelligent graphics" profile, which means that in addition to graphical content based on CGM Versions 1-3, the profile includes non-graphical content based on CGM Version 4, Application Structures. The non-graphical content allows the definition of hierarchies of application objects, as well as the association of metadata, such as link specifications and layer definitions, with the objects.

1.3 WebCGM Requirements

The content of WebCGM is chosen to satisfy the requirements articulated in the document "W3C Scalable Graphics Requirements", http://www.w3.org/Graphics/ScalableReq ([grfreq]), and follows the specifications and requirements defined in "Use of CGM as a Scalable Graphics Format", http://www.w3.org/TR/NOTE-cgm ([cgmreq]). The content of WebCGM was significantly influenced by technical recommendations from the CGM Open consortium (see http://www.cgmopen.org/).

The selection criteria for the graphical content of WebCGM included: it should have high expressive power; and, it should be both widely implemented, and implementable with a reasonable level of effort.

The selection criteria for the intelligence content of WebCGM - structuring and metadata elements - were derived principally from the above-mentioned W3C requirements document, [cgmreq].

1.4 WebCGM and other Profiles

The detailed profile specifications of WebCGM are based on those of the ATA (Air Transport Association) CGM profile, "GRexchange 2.4" ([grex24]). This profile is recognized as the best defined and most widely implemented in industry, and it has the valuable attribute that there is already an associated product certification testing service.

The details in the PPF of the ATA profile were taken as the starting point, and modified only where they did not meet the requirements articulated in the W3C requirements documents. The graphical content of WebCGM is a simplified, but powerful subset of the [grex24]. The intelligence content is a superset - minimal but sufficient to meet the articulated requirements.

1.5 Conformance of WebCGM

One of the primary benefits of any CGM profile is the ability to insure interoperabiltiy through the use of validation tools against CGM instances and certification services for applications. Once an application has been certified through a testing service, behavior of that application is predictable under the constraints of the profile. Several validation and certification tools and services exist which can be leveraged for WebCGM. Among them are:

• Validation tools exist for metafile instances.
• Test suites and certification services exist for interpreters for the closely related ATA profile, and a good number of them viewer, printer, and interpreter products have been certified.
• Test suites and certification services for generators are being devised for the closely related ATA profile.

1.6 Editions of WebCGM

CGM:1992 AM1, Rules for Profiles, prescribes that profiles shall maintain revision control by using a standard "ProfileEd" keyword. Instances of the profile carry this edition information in their identification section.

This release of WebCGM is Edition ("ProfileEd") 1.0.

Future editions are anticipated. For example, the inclusion of some of the graphical content of CGM:1992, which has been deferred from WebCGM 1.0 but which is useful in high-end graphic arts applications, is a likely addition for WebCGM 2.0. The progress of the XML linking standard, XLL, is being followed closely as well.

1.7 Roadmap to the Document

WebCGM is written in four major sections:

• This section, containing introductory and overview material, which is informative but not normative.
• A "WebCGM Concepts" section, informative but not normative.
• Detailed descriptive material on the V4 content of WebCGM, including a formal Content Model which can be used for V4 content validation. This section and the PPF contain all of the normative content of WebCGM.
• The Profile Proforma (PPF), comprising an extensive table which addresses every element of the CGM standard, per CGM Amd.2, Rules for Profiles.

1.8 References

All valid profiles of CGM conform to the ISO CGM standard. As are all profiles, WebCGM is defined by reference to the ISO standard:

ISO/IEC 8632:1992(E)

Information technology - Computer Graphics - Metafile for the storage and transfer of picture description information

Part1: Functional specification

Part 2: Character encoding

Part 3: Binary encoding

Part 4: Clear text encoding

ISO/IEC 8632-Amd 1:1994(E)

Information technology - Computer Graphics - Metafile for the storage and transfer of picture description information

Rules for profiles

Part1: Functional specifications

Part 2: Character encoding

Part 3: Binary encoding

Part 4: Clear text encoding

ISO/IEC 8632-Amd2:1995(E)

Information technology - Computer Graphics - Metafile for the storage and transfer of picture description information

Application structuring extensions

Part1: Functional specifications

Part 2: Character encoding

Part 3: Binary encoding

Part 4: Clear text encoding

Copies of the standard may be obtained from ISO:

The ISO CGM standard is in republication, to roll the base standard, amendments, and defect corrections together into a single document. It is expected that a Web (HTML) version of CGM:1998 will be posted and available before the end of 1998.

Other normative references of this profile include:

[RFC-1738] - Uniform Resource Locators, 1994, URL: http://www.w3.org/Addressing/rfc1738.txt

[RFC-1808] - Relative Uniform Resource Locators, 1995, URL: http://www.w3.org/Addressing/rfc1808.txt

[RFC-1951] - Deutsch, P., "DEFLATE Compressed Data Format Specification version 1.3", RFC1951, Aladdin Enterprises, May 1996. URL: http://www.w3.org/Graphics/PNG/RFC-1951.html

ISO/IEC 10646-1:1993, AM2:1996, "Information technology - Universal Multiple-Octet Coded Character Set (UCS) - Part 1: Architecture and Basic Multilingual Plane, AMENDMENT 2: UCS Transformation Format 8 (UTF-8)", URL: ftp://ftp.informatik.uni-erlangen.de/pub/doc/ISO/charsets/ISO-10646-UTF-8.html
or http://sunsite.auc.dk/RFC/rfc/rfc2279.html

For more information on the CGM standard itself, the CGM Open Web site has both bibliographic references, and short articles on CGM topics.

CGM Open - http://cgmopen.org/ Finally, the following World Wide Web sites have more information on CGM:

ISO - http://www.cwi.nl/JTC1SC24/

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