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CGM: A Non-Proprietary, Editable 2D Graphics Format That Handles Vector, Raster, and Text Data

The Computer Graphics Metafile (CGM) file format is used to make 2D graphics viewable across a wide range of applications such as technical illustrations (usually for manuals), cartography, Geographic Information Systems (GIS), and presentations, While IGES fifes are used for engineering data interchange between different CAD systems, CGM files are often created to become the final output that end users will look at. An Auto-trol Technology white paper states that "CGM's primary publishing function is that it performs a reliable job of exchanging complex editable (composite) artwork views throughout their respective lifecycles; no other exchange standard adequately performs this function."

CGM is a formal ISO standard born from a 1979 SIGGRAPH specification, It received a major boost when it was endorsed by the U.S. military's paperless logistics support and engineering specification (CALS/CE) from the 1980's. It was subsequently adopted by private industry as CALS/CE began to wane. Several industries and consortiums have standardized on CGM for the technical manuals used by their personnel. These manuals cover topics such as product operations, maintenance, and repair. Traditionally these manuals have been printed books but increasingly they are the newer interactive electronic technical manuals (IETMS). Specific industries and consortiums include commercial aerospace, semiconductor manufacturing, telecommunications, and rail transport.

Other fields use CGM for different purposes. The petroleum industry uses CGM for mapping, lithology cross-sections, seismic traces, and well logs. The U.S. military uses CGM for simple redlining on top of raster (map) data In any case, end-users view the CGM graphic data with easy-to-use viewing or browsing programs.

Applications of CGM

Some CAD programs can output their files as CGM V1 files and oilier common programs call view them (Word97, Visio, or web browsers with plug-ins). Specialized programs are required to actually edit the geometry in a CGM file and/or add special functionality such as hyperlinks.

Here is a general example. A final version of a CAD file is exported either as IGES or CGM. An illustrator then opens the file with a technical illustration program to enhance its appearance for non-CAD users. The file may then be exported as a CGM for layout in a technical manual (printed or electronic) or posted to a website (internet, intranet, or extranet). Web browsers currently require add-on software to view CGM files.

Alternatively, a CGM with hyperlinks, hotspots, scripts, and animation can be created from existing IGES or CGM files using InterCAP's ActiveCGM Author program. Hotspots display text when a cursor stays over an onscreen area, Ron Wiehler, the marketing director at InterCAP, described one example where a customer with 800,000 raster parts drawings is using ActiveCGM Author "to automatically link the callouts on the drawings to parts data to create an online parts catalog."

Multiple CGM Formats and Profiles

CGM's situation is a bit complicated, however, since there are four versions. Three of them are actively being used today, and a Web version of the fourth is expected to become an Internet standard late this fall. As a result, CGM files created by some programs cannot be read by others. While this has not significantly hurt CGM's adoption and use by the previously mentioned industries where the details are clearly understood, it certainly has slowed CGM's adoption as a standard display format for the rest of the world.

According to CGM guru Lofton Henderson of Inso's Henderson Software unit, what drives the uptake of any data interchange format is product certification and testing by the National Institute of Standards and Technology (NIST). Programs whose implementations of a format pass, are usable by other programs that also pass. Henderson Software provided CGM technology as an OEM business for 12 years before it was acquired in November 1997 by Inso to work with Inso's SGML technology.

Currently, the three real-world versions of CGM are versions V1, V3, and V4. Version 1 is a basic vector drawing format and the one with the greatest interoperability since it has been around the longest. NIST has been testing anti certifying implementations of it for three years. CGM VI files can be seen using viewing programs (AutoVue), word processors (Word 97), diagramming software (Visio Technical,), virtual workbooks (Engineering Office) and Web browsers with add-on software such as lnterCAP's free ActiveCGM Browser. CAD programs that export their drawings as CGM V1 include CATIA, I-DEAS, MicroStation, Pro/ENGINEER, Solid Edge, and Unigraphics but not SolidWorks, AutoCAD, or Mechanical Desktop.

Using Internet Explorer with ActiveCGM Browser we were able to open a CGM V 1 file from Solid Edge and mark it up using redlines and text (which were saved as an associated file to the CGM). These markups were visible to others using the same software but we often had problems just opening CGM files using the browser and the acid on software We also had CGM viewing problems with Word 97, and CAD programs that can output CGM usually cannot view CGM files.

Version 3 allows raster data (maps) to be integrated with vector data (CAD images and text). CGM V3 also includes gradient colors, text on curved paths, and typesetting. It also allows symbol libraries to be defined externally thereby allowing industries to standardize on a set of symbols according to a white paper written by Henderson and InterCAP's CTO, John Gebhardt. CGM V3 compliance testing will start for the first time this fall.

Moving to a Web Environment

Version 4 enables CGM to go beyond graphics to provide additional functions such as hyperlinking, hotspots, scripts, and animations. CGM V4 provides a syntax for defining these functions via "tagging" but it does not spell out the specific definitions to be used. This is exactly like SGML, the big brother from which HTML was derived. While this arrangement allows vendors and industries to define their own tags, it creates interoperability problems.

CGM V4 capable applications from different vendors often cannot read each others' files. While the graphic data could be saved in a lower version of CGM and exported to the other system, all of the additional functionality would be lost. In order to work with a CGM containing hyperlinks that were created with InterCAP's ActiveCGM Author, an end-user would need to use a Web browser with the ActiveCGM Browser add-on software. An add-on is called a plug-in for Netscape's Navigator, and it is called an ActiveX control for Microsoft's Internet Explorer.

Interoperability Issues

CGM file interoperability is enabled by establishing what are called public profiles. Henderson explained that a profile is a CGM subset which is defined [or a specific area but still provides interoperability and conformance testing For CGM V3 Henderson noted that there are three distinct profiles: technical publishing, petroleum, and defense mapping. The various technical documentation profiles (ATA, J2008, RIF, and CALS Rev. B) are practically identical. The petroleum profile differs from the technical documentation profile by using more colorful data representations, raster capabilities to include digital photo taken by satellite and numerous vector primitives Defense mapping uses relatively few vector primitives for redlining on top of raster data.

Today, there are two profiles for CGM V4, one of which is for the commercial airlines (ATA), but its adoption has been slow.

The second CGM V4 profile is a generic, Internet friendly version called WebCGM. WebCGM uses a subset of CGM V3 graphics (vector and raster) while also using a few CGM V4 capabilities for hyperlinking and querying. For this version to really take off, it has to be approved by the World Wide Web Consortium (W3C). The Open CGM consortium is in the final stages of defining the draft they will submit to the W3C this fall.

When approval occurs later this year it will pave the way for Netscape and Microsoft to enable their Web browsers to natively read tile format just as they read GIF and JPEG raster formats today. Until then, CGM users will need to obtain CGM add on software for their browsers The CGM Open consortium is considering making such software freely available.

Even with the latest round of Web browsers and CGM add-on software, don't expect to start opening existing CGM V4 files.

"Of the three principal sources of CGM V4 technology (InterCAP, lTEDO/IsoDraw, and HSI/Inso), I don't expect that any legacy V4 metafiles will work in WebCGM directly," Henderson explained. "However, I do expect by the end of the year that all V4 technology vendors will offer WebCGM output modes, and there will likely be tools (filters) to convert proprietary legacy V4 to compliant WebCGM."

Reprinted from Engineering Automation Report, August 1998, Volume Seven, Issue Eight.
© 1998 Technology Automation Services.

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