Problem related to STEP import/export and visualization of 360° B-spline surface

After exporting a B-spline surface to STEP and importing it back to OCC, the surface is not displayed correctly in shaded mode (see image in the appendix). Wireframe mode is displayed correctly, also iso-lines can be shown, so the geometry is basically there, but shading fails. As far as I know such problems often point to some geometrical problems.

The problem occurs only after the surface was sewed together with other surfaces to form a solid.
However, according to DRAW's "checkshape" this solid is valid.
Furthermore, the problem does not occur if the surface spans less than 360°.

The problem occurs in OCC 6.5.1 as well as in OCC 6.3.1. Other versions, I dind't test.

With other export formats (BREP, IGES, STL) there is no problem.

Timo Roth's picture

I append some brep files:
spiral_original_surface.brep the surface before sewing (no problem)
spiral_solid.brep the sewed solid
spiral_solid_face.brep the surface after sewing
spiral_solid_359_degree the sewed solid spanning only 359 degree (no problem)

In order to reproduce the problem with DRAW:
pload ALL
restore spiral_solid.brep s
stepwrite 0 s s.stp
stepread s.stp r *
vdisplay r_1
vsetdispmode r_1 1

Timo Roth's picture

Is it an OCC bug?

Kind regards,

Forum supervisor's picture

Dear Timo,
I would like to inform you that the posted problem is checked and reproduced.
The corresponding issue with ID = 22805 has been registered.
Later you can know if the issue is resolved by checking references to the specified ID in OCCT Release Notes. The analysis of the issue will take some time depending on our technical capability and availability of resources.
If you can't wait and the problem is urgent for you, you may contact us via Contact Form
We will try to find a solution/workaround acceptable for you.


Timo Roth's picture


Timo Roth's picture

The problem has been fixed in OCCT 6.5.3.