CGM FAQ 6
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
AGOCG CGM Support
A.H.FRANCIS@OPEN.AC.UK
