Shrinking of surfaces after export from Rhino

Rhino Rhino for Windows
1

Hello Rhino community

I am splitting a sphere and the result of this split are two separate surface parts (cutted sphere part 1 and cutted sphere part 2):

1.jpg713×513 53.5 KB

2.jpg713×513 54.5 KB

I then export these two cutted sphere parts to an .obj file.

If I try to import this .obj file back to Rhino, the cutted sphere parts are shrank:

3.jpg713×513 89.4 KB

I am using meters as Units and I tried using both 0.01 and 0.001 as an Absolute tolerance but still got the same result.

Why does this shrinking of cutted sphere parts happen once they are exported to the .obj file?
Does this mean that Rhino’s obj exporter is corrupted?

To export the .obj file I used the following export settings:

4.jpg329×738 64.5 KB

I am using Rhino 5 32 bit (5.12.50810) on Windows.
I attached the Rhino .3dm file and the exported .obj file (in the .zip file).

I would be very grateful if I could get any kind of help on this issue.

shrank surfaces.3dm (795.6 KB)
cutted_sphere_parts12.zip (14.8 KB)

2

I opened your Rhino file and exported the cut pieces as .obj NURBS objects with the same result. When I exported the pieces as .obj mesh objects the size stayed the same. Definitely looks like a bug.

3

David thank you for the answer!
Indeed I got the same result: exporting an .obj file as Polygon mesh keeps the size of the cutted sphere parts intact.
But when they are exported as NURBS, they are shrank.

Is there a way to fix this bug?

4

If you shrink the underlaying Nurbs-Surface before exporting it, your problem is solved:
Command:
_ShrinkTrimmedSrf

5

Still looks like a bug to me. @pascal Anyone at McNeel looked at this thread?

6

Thank you Tom_ and davidcockey.

Once I shrink (use _ShrinkTrimmedSrf command) the cutted sphere parts, export them as an .obj file and then import the .obj file, everything looks ok.

But if I increase the area, or is it better to say: radius of the cutted sphere part, and shrink it again, export it, and then import it, some slight differences exist between the original cutted sphere part and the .obj imported one.
Here is a preview:

4a.jpg714×513 58.3 KB

5.jpg714×513 63.4 KB

It is interesting to notice that this kind of difference does not exist when sphere cutted parts are exported as .igs files:

6.jpg714×513 51.8 KB

So as David said, it’s definitively some sort of a bug with Rhino’s .obj exporter.
The problem is: I do not want to export my surface cutted part as .igs but as .obj file.

I attached the files with the new sphere cutted part below.
Thank you.
shrank surfaces2.3dm (990.3 KB)
cutted_sphere_part_obj.zip (26.8 KB)
cutted_sphere_part_igs.zip (83.4 KB)

7

It looks like either the importer or exporter is using the parameter space curves from the original sphere, which is a surface of revolution, to trim the NURBS surface. This is a bug. You can get around the problem by changing the surfaces to NURBS before exporting. One way to do this is to use the test command, testMakeValidForV2. It does not autocomplete, so you have to type it out all the way. I’ll make a bug report.

Thanks for pointing this out.

8

Thank you for the solution Chuck.
Thank you David and Tom for the help.