CGM FAQ

The Computer Graphics Metafile (CGM), is the graphics format of choice for Interactive Electronic Technical Manuals (IETM), Illustrated Parts Catalogs (IPC's). The CGM format is also widely used for CAD graphics data exchange and in print publishing systems.

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1. What is CGM?

The Computer Graphics Metafile (CGM) is the international standard for storage and exchange of two-dimensional graphical data. Initially developed as a vector format it was extended to include raster capabilities, providing a useful archive format when combined raster and vector images.

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2. What does "metafile" mean?

A metafile is a list of commands that can be executed to draw geometric components, such as polylines, polygons or text elements. The metafile can also include commands to control the style of these objects such as line color. Metafile properties can also include non-graphical information, for example, to identify objects useful when creating hotspots for interactive graphics. The CGM standard specifies which elements are allowed to occur in which positions in a metafile, therefore, the file has the useful capability of being validated.

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3. What is the difference between a Vector and a Raster image?

A vector graphic is rendered using mathematically defined objects such as polygons, lines, and text. A raster graphic is rendered using an array of dots also known as pixels.

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4. Is CGM a Raster or a Vector format?

CGM is a robust vector graphics format; it can also include raster images. A 'geometric description language' is an accurate definition of CGM. The CGM format defines pictures and graphical elements in high-level geometric terms, for example, lines, circles, arcs, ellipses, polygons, text strings, and cell arrays.

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5. What sort of standard is CGM?

CGM is a file format suitable for the storage and retrieval of image information. The file format consists of an ordered set of elements that may be used to define images in a neutral form. The non-proprietary characteristic of CGM provides the ability for interpretation by different systems, applications and devices irrespective of the architecture.

CGM became an international standard in 1987 as ISO/IEC 8632 and was adopted as a national standard in many countries. Following some published amendments providing increased functionality, a revised standard was published in 1992 as ISO 8632:1992 and again in 1999 as ISO 8632:1999

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6. Why are there different versions of CGM?

Different CGM files can conform to different versions of the Standard as it was being developed. There are three main versions of CGM in common usage.

Version 1 refers to CGM files that conform to the original, 1987, version of the published standard.
Version 3 refers to CGM files that conform to the 1992 version of the standard which includes advanced graphical elements such as splines and polybezier curves. It also enriched the graphical attributes to allow for greater definition of engineering drawings and technical illustrations. Version 3 is mainly used in Technical Illustration systems.
Version 4 refers to CGM files that conform to the 1999 version of the standard that added Application Structuring. This allows the CGM to contain non-graphical information in addition to graphical content. This allows CGM to be used for a wide range of application interactive graphics, hotspots and hyperlinking.

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7. What are CGM Profiles?

Profiles are a subset of the standard developed independently by standards groups within specific industries. The purpose of a profile is to improve the interoperability of CGM within a specific technical community. The most popular profiles are those developed by the aerospace, defense, petroleum, automotive, and rail industries. The World Wide Web Consortium (W3C) has also developed a specific profile for the use of CGM on the Web.

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8. What is WebCGM?

WebCGM is a profile of the CGM standard (ISO 8632:1999), it describes how to use CGM vector, raster, and hybrid graphics on the web. It was developed jointly by the W3C and CGM Open Consortium. WebCGM represents a consensus of users and vendors on the way to exchange dynamic hyperlinked CGM files over the Web. WebCGM provides a vendor-neutral standard for 2D graphics, enabling creators and consumers to exchange, view, and browse intelligent, vector, raster, or hybrid graphics reliably. The creation of graphics can be performed in multiple applications without the loss or distortion of information.

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9. What are the Industry Specific CGM Profiles?

The S1000D specification has for many years used CGM as its preferred 2D graphics format. The baseline profile of S1000D is WebCGM; the community added further restrictions; described as a cascading profile. The result is a profile tailored to the S1000D specification, and therefore users and software developers need to account for this to produce valid and compliant CGM files.

ATA CGM profile is part of the Air Transport Association (ATA) specification 2100 that governs technical documentation for the manufacture and operation of commercial airplanes. The GREXCHANGE profile is based on CGM Version 3, suitable for the exchange of technical manuals, publishing applications, and visualization. A second profile, IGEXCHANGE provides for the transfer of intelligence associated with graphical data, based on application structuring defined in CGM Version 4.

The CALS CGM profile is a Department of Defense (DoD) standard (MIL_PRF-28003 Rev B) entitled "Digital Representation for Communication of Illustration Data: CGM Application Profile." It is part of the CALS (Continuous Acquisition and Life-Cycle Support) family of standards.

CGM PIP is the petroleum industry profile. The petroleum exploration and production community developed the profile for the interchange of graphics between applications. The PIP profile basis is CGM Version 3 and CGM+, a format for the graphical representation of geophysical data.

J2008 is the automotive industry standard for the exchange of technical documentation it states that graphics should be in CGM format. It references the ATA 2100 CGM profiles.

RIF / EPECS is the Electronic Parts Catalog Exchange Standard of the Rail Industry Forum. It also references the ATA 2100 specification for the use of CGM as graphics exchange standard.