IGESControl_Writer - diffrence

What is the difference in IGEs output for
the 2 options - faces and BREP mode ?
If I make a box, and export it using 'faces', 6 IGES faces would be written. It is true ?
What is the significance of BREP mode then?

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IGESControl_Writer();

Purpose
Creates a writer object with the default unit (millimeters) and write mode (Face).

IGESControl_Writer
(const Standard_CString unit,
const Standard_Integer modecr = 0);

Purpose
Creates a writer with given values for units and for write mode.
unit may be any unit that is accepted by the IGES standard. By default, it is the millimeter.
modecr defines the write mode and may be:
- 0: Faces (default)
- 1: BRep.

Roman Lygin's picture

This mode affects how solids, shells and faces are written into IGES.
By default faces are written as separate Trimmed Surfaces (IGES type 144) and shells/solids are written as Groups (type 402). Thus, connectivity information between faces (see shared edges) is lost.

Since IGES 5.1 (Open CASCADE supports up to 5.3), it can support boundary representations. In this mode Open CASCADE translates faces into Faces (IGES type 510), shells and solids into their full IGES equivalents. In the past, many IGES readers could not (correctly) read such entities and therefore Open CASCADE used 5.0 style by default.

Hope this help.
Roman

PG's picture

Thanks , which option is better ?
A OCC model when exported using the default 'faces' option reported loss of data(missing faces) when read in another CADsystem but when the BREP option is used - no such information loss.
But the behaviour was inverse in another CADSystem - 'faces' option works fine but fails when 'brep' option used.
The IGES USer's PDF guide does not give any useful info
in this regard.
pl. guide
thanks
- PG