Importing from Rhino

Hi, Sorry if this is redundant question, I may be asking wrong which is why I see no similar topics.

I sometime model in Rhino because its easier for me to create architectural objects with measurements. I then export them to .obj and open them in Blender for animating and rendering. However, I noticed that if I try to add a bevel, subdivide, or edge split modifier I get unexpected results ranging from nothing happening to disjointed or distorted geometry.

I’ve seen this happen with other imported .obj’s (but no all).

Does anyone know why a simple cube imported does not behave like a cube created in blender? Is there a work around or perhaps settings I’m unaware of that need to be applied before exporting or during importing…maybe a different file format all together?

Thanks
JS

Rhino should export those models with custom vertex normals to ensure correct shading even with the (usually totally horrible) geometry that its mesher produces. Any change to the imported geometry will most likely break those vertex normals and end up in a garbled mess.

Another issue could be that Rhino maybe does not weld the exported meshes along hard edges. So, there is nothing to bevel and the mesh falls apart under Subsurf. Just guessing here - any chance you could supply an example .blend file?

In general: If you want to have bevels or fillets, best add them in Rhino and leave the mesh alone in Blender.

test.blend (541 KB)

Thanks for the quick response. Here’s a screen shot of the Rhino export options and a blend file with three different welded objects. I appreciate your insight.


Just as I thought:

a.
The mesh is imported with custom vertex normals.

https://dl.dropboxusercontent.com/u/27650383/27-07-2017%2006-44-28.png

b.
When set to unwelded, each and every polygon is compelety separate. To fix this, hit W > Remove Doubles. Then you will most likely have to clear the custom vertex normals (button shown under a.) and play with the AutoSmooth settings.

With the default (unmodified) export settings, polygon islands are split along hard edges. Solution as above.

The welded export setting is closest to Blender’s default - everything is connected - only that Rhino additionally marks edges as sharp.

c.
The geometry is not exactly optimal (e. g. triangulated), which can cause issues with bevel and subdivision surfaces. I find surprising that seemingly Rhino doesn’t let you choose the density of the exported geometry: Custom vertex normals only take you so far when it comes to smoothness…? This isn’t important for exporting “cubic” meshes, though.

Anyway, I still think that adding bevels and fillets in Rhino and leaving the mesh unaltered in Blender is the best way to go. Or go with the “welded” export settings and use a Bevel modifier with “Angle” limit method.

Wow, thanks. I would have never figured this out. Rhino’s chamfer/fillet falls apart if you make complex / compound shapes so I wanted to use Blenders modifiers which work great on blender objects. Just wanted to extend that functionality to .obj’s from Rhino. I enjoy using Rhino’s NURBS modeler for certain type of work (mostly work with specific dimensions) and the generated shapes are amazing from grasshopper.