Import From IGES


The official IGES specification might not be the best starting point for understanding how Open CASCADE transfers IGES data into its own internal data structures. The following resources might help you to introduce yourself to IGES

IGES is, in it's most literal form, a nonhierarchical data format. The whole description is based on a flat list of Entities. An Entity is comparable to an object in an object-oriented data model. An Entity has an ID, a type, meta data and data. There are special subclasses of entities:

  • properties assign string and numerical data to other entities. They are used to enrich the data and meta data of Entities
  • associativities assign entities to other entities. They are used to define hierarchies

IGES files describe Entities using two different sets of data

  • a directory, which lists all Entities, their meta data as well as pointers to the Entities' data
  • a data block which contains the literal content of Entities

Open CASCADE's IGES model

Open CASCADE provides a (full?) implementation of the IGES entity types. For every type of Entity in the IGES specification, there's a class in Open CASCADE. The reader parses the IGES file and initializes those classes. Direct references between entities are evaluated (meaning: each entity index is replaced by a direct handle to the respective instance of the Open CASCADE IGES class).


Based on this in-memory-representation of the IGES file, Open CASCADE now converts the geometrical Entities into Open CASCADE data structures from the set of Geom_ and Geom2d_ classes. Topological Entities are converted to topolocial data structures etc.