How to convert Open CASCADE
to IGS?

In solid modeling and computer-aided design, boundary representation—often abbreviated as B-rep or BREP—is a method for representing shapes using the limits. A solid is represented as a collection of connected surface elements, the boundary between solid and non-solid.

Boundary representation of models are composed of two parts: topology and geometry (surfaces, curves and points). The main topological items are: faces, edges and vertices. A face is a bounded portion of a surface; an edge is a bounded piece of a curve and a vertex lies at a point. Other elements are the shell (a set of connected faces), the loop (a circuit of edges bounding a face) and loop-edge links (also known as winged edge links or half-edges) which are used to create the edge circuits. The edges are like the edges of a table, bounding a surface portion.

Compared to the constructive solid geometry (CSG) representation, which uses only primitive objects and Boolean operations to combine them, boundary representation is more flexible and has a much richer operation set. In addition to the Boolean operations, B-rep has extrusion (or sweeping), chamfer, blending, drafting, shelling, tweaking and other operations which make use of these.

An IGS extension (Initial Graphics Exchange) is a 2D or 3D CAD vector file that can store wireframes, surfaces, solids, etc. IGS is mainly used for exchanging CAD drawings based on the Initial IGES (Graphics Exchange Specification) format.

The IGS file structure is based on the ASCII text-based format, which is used in numerous CAD software applications for delivering high-end graphics.

Among CAD applications that support IGS extension are Autodesk 3ds Max, Autodesk Fusion 360, Dassault Systemes CATIA, and Autodesk Inventor.