Normals in GH vs. Surface Direction in Rhino

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Permalink Reply by Andrew Heumann on June 21, 2011 at 8:20pm

Try loading the surface as a Brep, and getting Brep.Faces[0].NormalAt(u,v).

 

Not sure why would be any different, but it seems to work...

Permalink Reply by Jon Sargent on June 22, 2011 at 7:56am

Yes, strange, but that does the trick.  Thanks, Andrew!

 

-Jon

Permalink Reply by David Rutten on June 22, 2011 at 11:26am

Hi Jon,

 

sorry, I don't get it. Is this a bug in Grasshopper or are you not getting the expected normals when you call NurbsSurface.NormalAt in a script component?

 

The reason there's a difference is that Breps (all surfaces in Rhino are internally stored as single-face breps) have a Reverse flag on them, whereas surfaces do not. So if you import a Brep which has been reversed, then downcast it to a surface and then call things like NormalAt, it won't reflect this reversed state.

 

This is a problem for plugin developers but not one that can be solved at the moment (SDK breakages).

 

The reason we do this is because it's quite difficult to really flip a surface. Setting a single boolean is quickly achieved and since we often need to flip surfaces when you join them together, we chose the quick'n'easy path.

 

All components in Grasshopper should pay attention to the Flipped state, if they don't, that's a bug I need to fix.

 

--

David Rutten

david@mcneel.com

Poprad, Slovakia

Permalink Reply by Jon Sargent on June 22, 2011 at 12:41pm

Hi David,

 

No -- it's not a bug in Grasshopper.  I cast the brep to a surface in a script component.  Now I know better!

 

Thanks,

 

Jon