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Can CGM be used over the Internet?

Yes. CGM has been registered as an Internet Media Type (MIME Type) so CGM files can be sent over the net. CGM files have the type image/cgm. CGM is the International Standard for computer graphics interchange as specified in the Computer Graphics Metafile IS8632:1992.

CGM offers the Internet community the first opportunity for a vector-based (i.e. line-art) image type. Previously, the only valid image types were JPEG, and GIF, both raster or bitmap type image files. Vector images allow for greater detail and clarity at multiple zoom levels. Also, they are usually much more compact than the equivalent bitmap and so are quicker to download.

The CGM files must be binary encoded, this is the most common encoding, and must either conform to one of the 4 International Standardised Profiles (ISPs) (see CGM FAQ 6 in Graphics & Visualisation Issue 50) or state that they do not conform to any profile. Interchange can only be guaranteed for CGMs which conform to an ISP.

To take full advantage of this new media type mailers and browsers need to be aware of it. They can then either invoke an internal CGM interpreter or launch a helper application to interpret the CGM file.

As yet there is no core data type on the World Wide Web which handles vector images. Negotiation are taking place with the W3C consortium to have CGM accepted as a core data type for the web. It is hoped that CGM interpretation would then become a standard facility for web browsers.

Queries about CGM can be posted to the CHEST-CGM list at Mailbase.

Alan Francis