rhino surface normals different from GH surfcae normals

i am having a issue with surface directions being different from the surface normals derived from GH.

the method i am using to determine surface normals (and ultimately to extrude) in GH is creating plane from each surface and plane offset to determine a point to point vector.

could this be a issue or bug with grasshopper or if not what could i do to make the GH surface direction match the Rhino surface direction.

attached are images of the GH normals and the Rhino normals.

Replies to This Discussion

Permalink Reply by David Rutten on August 4, 2011 at 3:31pm

Hi Tyheem,

normal directions for Breps are not stored on the surfaces themselves. Basically, to really flip the normal of a surface you need to do a lot of work, especially if the surface has trims. This is why there is a Boolean flag on a BrepFace that indicates whether the normals ought to be interpreted as flipped.

There were several bugs in Grasshopper where I forgot to look at this flag and subsequently flip the normal direction so it's possible you ran into one of the cases I haven't found and solved yet (which Grasshopper version are you using?).

However, when you cast a BrepFace to a Surface, you lose the 'flip' data and there's no way to get it back, so it might also be one of those cases.

Did I understand correctly you're feeding the surfaces into a Plane parameter in order to get a surface frame? If so, then this is not something I can really fix. You see, frames are based on the surface U and V directions, and all Planes in Rhino are right-handed. If you flip the surface without reversing U or V, then the only frame that still correctly represents U, V and Normal would be left-handed.

David Rutten

david@mcneel.com

Poprad, Slovakia

Permalink Reply by th_joiner on August 4, 2011 at 4:49pm

OK i think i understand.

How would i ensure that i will extrude a face in the similar to the direction of the surface predetermined surface directions?

Currently I am inputting the surface (which are individual surfaces) into the basic plane component which generate a plane, use plane offset to get the surfaces' +z direction, then using vector 2 pt to connect the plane origin to plane offset origin. this vector is to be used to extrude. is there a different way of doing this? what are some other methods of controlling an extrusion direction?

ultimately i am working on a custom greeble script which i will apply to a series of surfaces imported and converted from sketchup. it currently extrudes the wrong direction on certain surfaces.