Possible OBJ export bug?

Rhino Rhino for Windows
1

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:

result.png512×512 6.89 KB

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

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

2

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

Tim

3

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

result.png512×512 4.57 KB

4

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:

bug.png773×955 189 KB

export settings:

settings.png316×739 10.6 KB

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

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