The Initial Graphics Exchange Specification (IGES) is an American national standard defining a neutral format for the exchange of digital information among CAD systems.
"This Specification establishes information structures to be used for the digital representation and communication of product definition data.
The file format defined by this Specification treats the product definition as a file of entities. Each entity is represented in an application-independent format, to and from which the native representation of a specific CAD/CAM system can be mapped. The entity representations provided in this Specification include forms common to the CAD/CAM systems currently available and forms which support the system technologies currently emerging.
In Chapters 3 and 4, the product is described in terms of geometric and non-geometric information, with non-geometric information being divided into annotation, definition, and organization. The geometry category consists of elements such as points, curves, surfaces, and solids that model the product. The annotation category consists of those elements which are used to clarify or enhance the geometry, including dimensions, drafting notation, and text. The definition category identifies groupings of elements from geometric, annotation, or property data which are to be evaluated and manipulated as single items."
Quoted from Initial Graphics Exchange Specification (IGES) Version 4.0, NBSIR 88-3813, June 1988, pages 1-2.
"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.