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
AGOCG CGM Support
A.H.Francis@open.ac.uk
