C# SDK Surface.NormalAt() and GH Evaluate surface component returns different result

[AndrewAhn (Andrew Jinho AHN)]

Normal directions are matched prior to evaluate surface but evaluate surface and SUrface.NormalAt() returns different result. As you can see, surface at index 16 is flipped to match the base geometry, but it seems that NormalAt() returns normal vector that is not flipped.
I have tried same operation with baking the surfaces and flipping the surface manually and also tried with joining them to polysurface(which also matches face normals) but results were same.
Am I doing something wrong?
Please see attached GH.

problem.gh (16.8 KB)

[DavidRutten]

In RhinoCommon the Surface type supports no trims, and no normal flipping. For that you need BrepFace, which is what a Grasshopper “surface” really is.

[AndrewAhn (Andrew Jinho AHN)]

Thank you David as always. I have one more question. What about NurbsSurface? What are the differences between NurbSurface and BrepFace?

[wim (Wim Dekeyser)]

Hi - I’m not sure where you got that term from but a “NurbsSurface” probably is the same as a surface.
Other than that:

[FEEDBACK] In Geometry Pipeline looks like missing the surface type option. How to soft reference or link all "surface" from a Rhino layer?

The nomenclature is not consistent I’m afraid. In Rhino: Surface = single face with or without trimming data Polysurface = two or more joined faces with or without trimming data. In Rhino SDK: Surface = single face without trimming data Brep = one or more faces with or without trimming data In Grasshopper: Surface = single face with or without trimming data Brep = one or more joined faces with or without trimming data. Basically, what Grasshopper calls a “surface” is just an SDK Brep …

HTH,
-wim

[Michael_Pryor (ekim)]

NurbsSurface is same as Surface except NurbsSurface let’s you access more properties, constructors, and methods. Surfaces are a protected class and are the root of NurbsSurface. Same goes for Curve > NurbsCurve. You can see it here in the inheritance tree: https://developer.rhino3d.com/api/RhinoCommon/html/T_Rhino_Geometry_NurbsSurface.htm

[AndrewAhn (Andrew Jinho AHN)]

Never knew the existence of such class… Thank you.