IGES Right Cylindrical Surface non-parameterized problem

Although I am not developing with OpenCascade, I am writing a small IGES exporter for one of my own programs. I am using US Pro IGES 5.3 specification and the IGES output from FreeCAD (and various other programs) as a guide to finding my way around this neutral file format. However, I have run into problems with the representation of both the right circular cylindrical surface and the right circular conical surface when using the non-parametric forms. My files open perfectly in FreeCAD, CAD-Exchanger, and IGS viewer but fail to open in the AutoCAD online viewer. I have attached a sample file 'DEBUG_BREP_23.igs', and a small pdf 'BREP_CYLINDER_01b.pdf' which depicts the edge loops as I have implemented them. I am wondering if there is an error in my file which OpenCascade is silently correcting, but which AutoCAD is not. Thanks in advance for any help.

Nicholas Shea.




Roman Lygin's picture

Hi Nicholas,

Just to avoid possible confusion, CAD Exchanger does not use OCCT converters. All converters are our own.

Best regards,


nicholasshea_157942's picture

Hi Roman,

Yes, I am aware of this, although I do not know if IGS Viewer uses OpenCascade converters. Either way, it seems odd that AutoCAD, which is regarded as the industry standard, fails to render the lateral face loop as I have implemented it. As I have already explained, I based my code on the US Pro 5.3 specification and the output from FreeCAD. Have you any ideas what might be amiss? I have inquired with AutoCAD, (via their online forum), if the unparameterized versions of the both the Right Circular Conical Surface and Right Circular Cylindrical Surface are are supported by their online viewer, but have yet to receive an answer. I would very much appreciate some insight on this. I thought an OpenCascade developer might be aware of the issue, or at least know how to resolve it. By the way, after opening the 'DEBUG_BREP_23.igs' in FreeCAD, you have to click the 'Fit to View' icon for the conical surface to appear in the camera. But as you can see, it is converted perfectly.


Kirill Gavrilov's picture

I am wondering if there is an error in my file which OpenCascade is silently correcting, but which AutoCAD is not. 

The fixes applied by OCCT translator can be found within Verbose log in CAD Assistant or via extra commands in Draw Harness:

11:06 Total number of loaded entities 27.
       Count	IGES Reader Failures for 'DEBUG_BREP_23.igs'
       -----	-----------
           1	F: Global Section (Parameter %d) : Incorrect Date Format.
           1	W: Global Section (Parameter 23) : Version Flag parameter is incorrect (Integer between 1 et 11 was expected. Default value 3 taken : Version 2.0.
          Nb Total:2  for 2 items
11:06 Count	IGES Translator Warnings for 'DEBUG_BREP_23.igs'
       -----	-----------
           1	W:514 1   Shell   : Faces were incorrectly oriented in the shell, corrected
          Nb Total:1  for 1 items

nicholasshea_157942's picture

Kirill Gavrilov,

Many thanks for pointing this out. That is precisely the information I was looking for. This is very much appreciated. Thank you!


nicholasshea_157942's picture

Hi Kirill Gavrilov,

I have just downloaded and installed cad_assistant_1.4.2_2020-03-28_win64.exe and opened my sample IGES file. However, I do not get the same errors as in your screenshot; just a single error about an incorrect date format. How do I configure the program to report the extended errors? Do I have to build/download a Debug version? Thanks for your time,


nicholasshea_157942's picture

Sorry - I've just found the 'Verbose Messages' switch in the Configuration settings. I missed that.

nicholasshea_157942's picture

Addendum... After debugging the face orientations in the shell, I opened the file in CAD Assistant with Verbose logging turned on. It imported without errors. However, the file still refused to open in AutoCAD’s online viewer. I decided to export the file from CAD Assistant and try that one with AutoCAD instead – this resulted in a successful import. After studying the two files I found that CAD Assistant gave the Right Circular Conical Surface (entity 194) a parameterized form number 1, even though no UV data was supplied (there is no UV data in the original file because I am using the non-parameterized option). CAD Assistant also supplied the RefDir vector for the surface. US-Pro IGES 5.3 specification states that the non-parameterized version of this entity (form number 0) does not require the RefDir vector; clearly AutoCAD’s online viewer does require it, whether the surface is parameterized or not. Thank goodness for CAD Assistant, without which this issue would have taken even longer to resolve.