From: Keith Nybakke (knybakke@nuhill.com)
Date: Tue May 20 2003 - 21:33:19 EEST
>Hello folks,
>
>I was wondering if any of you can tell me something (or where to
>look for information) about the .igs format when you look at it in a
>text editor.
>
>I have created some very simple parts in solidworks, exported them
>in iges, then looked at the text version of the file. Some of the
>data makes sense, but so much of it does not seem to correlate to
>anything in the part.
>
>Anyone out there know the how the format is laid out?
>
>Thanks in advance for any help you can offer.
>
>Kevin Fortin
>Prototypes, Etc.
>303-375-0350
IGES File Structure Overview
"The fundamental unit of data in the file is the entity. Entities are
categorized as geometry and non-geometry. Geometry entities represent
the definition of the physical shape and include points, curves,
surfaces, solids, and relations which are collections of similarly
structured entities. Non-geometry entities typically serve to enrich
the model by providing a viewing perspective in which a planar
drawing may be composed and by providing annotation and dimensioning
appropriate to the drawing. Non-geometry entities further serve to
provide specific attributes or characteristics for individual or
groups of entities and to provide definitions and instances for
groupings of entities. The definitions of these groupings may reside
in another file. Typical non-geometry entities for drawing
definition, annotation, and dimensioning are the view, drawing,
general note, witness line, and leader. Typical non-geometry entities
for attributes and groupings are the property and associativity
entities.
A file consists of 5 ... sections, Start, Global, Directory Entry,
Parameter Data, and Terminate. A file may include any number of
entities of any type as required to represent the product
definition. Each entity occurrence consists of a directory entry and
a parameter data entry. The directory entry provides an index and
includes descriptive attributes about the data. The parameter data
provides the specific entity definition. The directory data are
organized in fixed fields and are consistent for all entities to
provide simple access to frequently used descriptive data. The
parameter data are entity-specific and are variable in length and
format. The directory data and parameter data for all entities in
the file are organized into separate sections, with pointers
providing bi-directional links between the directory entry and
parameter data for each entity. The Specification provides for
groupings whose definitions will be found in a file other than the
one in which they are used."
Quoted from Initial Graphics Exchange Specification (IGES) Version
4.0 , NBSIR 88-3813, June 1988, page 3.
The above is the technical view. A more colloquial view is that an
iges file has a header that tells the unit of measure, the scale and
other information about the file. Then there is short piece of data
that says "Okay, start here!" The third section is line after line of
information that describes all the individual entities in the file.
These entities are identified by a three digit code, such as 126 for
a B-Spline curve. The fourth section is a set of parameters that tell
the receiving system how to assemble the data into drawing views,
trimmed 3D surfaces, etc. Finally, there is a small section that says
, "Okay, stop now!"
Current version of IGES is 5.3
Hope this helps.
Keith
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