Normal, Flip

Hi all,

In my project I would like to explode a 3D volume into its pieces (faces) set in XY plane (basic).

But I want to present in the 2D space the faces as they can be seen from the interior of the 3D model.

I thought it was just a matter of normal in the definition of my 3D model. I oriented all my face normals to the interior. Unfortunately this seems not enough ...

In grasshopper when I display the normals using Evaluate Surface component I see that vectors are still pointing to the exterior.

As one can see white arrow is the normal displayed by "Analyse direction" (Rhino command) and red vector is grasshopper one.

So my question is:

Why flipping a surface in Rhino is not enough to flip it in Grasshopper too?

Thanks in advance

Jerome

Replies to This Discussion

Permalink Reply by David Rutten on June 20, 2016 at 3:18am

It's possibly a bug in Grasshopper, but the normal direction of surfaces is complicated. The natural normal is due to a right-hand-rule based on surface U and V directions. Ie. if U goes along the index finger and V goes along the middle finger, then the thumb indicates the normal direction.

In order to properly flip a surface normal one must thus flip either the U or the V direction, or swap the U and V directions. This can be quite an involved operation from a geometric point of view that affects a lot of other surface properties in addition to just the normal direction.

This is why Brep faces have an additional boolean value associated with them that tells Rhino whether or not the normal of that face is flipped compared to the natural direction. This makes joining multiple surfaces into a single brep a lot quicker because Rhino doesn't have to restructure adjacent faces with opposing normals.

Any code that tries to get the normal direction must therefore look in two places to get it right, plus if at any time the geometry is converted back to a surface the flipping information is lost.

Permalink Reply by Nicolo Bencini on June 20, 2016 at 7:21am

If you have python installed I have found that using the reverseSurface command in the python editor seems to resolve the issue some times as it reverses the u or v direction. I have attached an example script.

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