A shape modelling applications programming interface for the STEP standard

The international standard ISO 10303 (STEP) for the exchange of product models and associated data between different computer aided design (CAD) and other engineering systems was first issued in 1994. This paper reports on current work on extending the standard to enable the capture and transfer of parametrized CAD models with geometric constraints, a capability not provided in the initial release. This will allow the transmission of ‘behavioural’ information with the exchanged model. Two complementary approaches are being worked on. The first aims to add supplementary data to the types of explicit models that can currently be exchanged. The second is more radical; its objective is to transfer CAD models in procedural form, i.e. expressed in terms of the sequence of operations used to construct them. The paper concentrates on the second approach, which is characteristic of the primary model representation used by many modern CAD systems. It is shown that the requirements for a standard in this area are virtually identical with those for a standardized applications programming interface (API) for CAD modellers. Previous work in the latter area is surveyed, to determine whether there exists a suitable basis for the ISO 10303 work, and progress and technical problems are reviewed.