The CGM community has been discussing the concept of XML companion files for CGM files for a while. Now the first draft of a standardized XML companion file has been prepared within the ATA GREXCHANGE 2.9 profile by the ATA Graphics Working Group.

Both WebCGM and the ATA GREXCHANGE profile provide ways to specify objects inside a CGM file. These objects can be used for hotspotting and hyperlinking an illustration. However, very often users want to associate much more non-graphical data with such an object than what is allowed by the standard profiles. For such cases the concept of an XML companion file has been developed.

For a given CGM file a second XML file exists containing XML elements that match the objects found in the CGM file. Example: If the CGM file contains a grobject with the ID "myObject1", the corresponding entry in the XML companion file might look like this:

<grobject id="myObject1"
 screentip = "This is my first object"
 link = "http://www.itedo.com"
/>

At runtime, the information inside the XML companion file can be retrieved at any time. So if the mouse is clicked inside a grobject, the CGM viewer only needs to report the ID of the object to the outside. In a JavaScript or similar the ID can then be used to find the corresponding entry in the XML file.

As the XML content is managed outside of the CGM file, it is not restricted by any profile, so it is possible to store any kind of data in there, e.g. language dependent screentips, context-sensitive links etc.

The draft proposal inside GREXCHANGE 2.9 now tries to define a standardized XML companion file for usage within the ATA community. ATA GREXCHANGE 2.9 is expected to be released in 2004.

Links:
The ATA GREXCHANGE Profile: http://www.airlines.org
The WebCGM Profile: http://www.w3.org/TR/REC-WebCGM/

Further reading:
Various papers can be found in the CGM Open's collection, such as "Graphical hotspot definition - a common ATA/AECMA approach" by Dave Cruikshank and Peter Zimmermann.