Possible OBJ export bug?

Hi, I am currently working on a NURBS ray tracer and I use Rhino to export STEP, IGES and other files to OBJ, which I have chosen for model import, because it is very easy to parse, unlike STEP or IGES.

The problem is that certain surfaces seem to look different when I render them with my ray tracer or even when I use NURBS package in Octave to visualize it.

This is what I see in Rhino (a sphere):

This is what I render with my ray tracer:

This is what Octave outputs:
surface.pdf (344.0 KB)

The OBJ file and Octave .m file:
rhino.zip (1.0 KB)

The bspline surface is rational so your app needs to pay attention to the weights on the vertexes.


You were right, it works with some changes to the evaluation algorithm. Thanks and sorry for thinking the bug was in Rhino.

I think I have found an actual bug this time. With some IGES models when I import them into Rhino they look good, but when I export them to OBJ and then reopen in Rhino as OBJ, some surfaces aren’t looking right.

For example:

export settings:

IGES model:
f-16-novo_igs.zip (3.6 MB)

Exported OBJ:
f-16-novo.zip (2.5 MB)