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Can graphical interchange with CGM be guaranteed?

The specification for CGM (Computer Graphics Metafile) IS8632:1992 describes conformance only in terms of syntactic conformance and encoding conformance for the CGM. It does not contain conformance criteria for generators and interpreters. Interchange could not be guaranteed because, for example, there is no limit to the number of points in a polygon. Thus, however many points in a polygon that an interpreter can handle, a valid CGM could exceed that limit and not be capable of being handled by that interpreter. Or a CGM might specify a font which the interpreter did not have.

The designers of CGM deliberately put no limits on these factors so that CGM would be flexible. However, this has caused problems for another of its design goals, interpretability. In order to be able to guarantee interchange, profiles have been developed. These profiles include limits, such as a maximum number of points in a polygon and a set of fonts, and specify the behaviour for generators and interpreters. CGMs can than be checked for conformance to the profile and generators and interpreters can be tested to check their ability to handle CGMs which conform to the profile. By using generators and interpreters which conform to a profile interchange can be guaranteed.

Amendment 1 to CGM specifies the rules for profiles and a Model Profile. Four International Standardised Profiles (ISPs) have now been registered with ISO. These four core profiles have been designed to satisfy most graphical requirements. BST is intended for business graphics and simple desk-top publishing. AST is intended for CAD, mapping and earth sciences. BPV is intended for graphic arts and high end desk top publishing. APV is intended for imaging and scientific visualisation.

They are specified in ISP 12071-1 Basic Scientific and Technical (BST), ISP 12071-2 Advanced Scientific and Technical (AST), ISP 12071-3 Basic Presentation and Visualisation (BPV)(Model Profile), ISP 12071-4 Advanced Presentation and Visualisation (APV).

Suppliers of applications which generate and interpret CGM files should be encouraged to conform to one or more of these ISPs.

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

Alan Francis