Get surface normal vector in C#?

Replies to This Discussion

Permalink Reply by Pirouz Nourian on December 18, 2013 at 11:36am

Hi, You first need to get the UV coordinates using this: http://4.rhino3d.com/5/rhinocommon/html/M_Rhino_Geometry_Surface_Cl...

Then you can use the property normal:

http://4.rhino3d.com/5/rhinocommon/html/AllMembers_T_Rhino_Geometry... 

I hope this helps,

Pirouz

Permalink Reply by Andreas Graumann on December 18, 2013 at 11:52am

Hello,

thanks for that. It appears to work for the most part. One problem i have is that it doesn't always line up with GH's EvalSrf component. Sometimes the EvalSrf gives {0,1,0}(which is correct) while my C# code results in {0,-1,0} (which is not).

Any ideas?

Permalink Reply by Andreas Graumann on December 18, 2013 at 1:02pm

Hm i realized some other things i dont understand. Here is what i would like to do: i have a bunch of planar trimmed surfaces (most have holes in them). I want to iterated over them and sum up their area, but only if they have a specific normal vector.

Here is what i have so far:

AreaMassProperties massProp = AreaMassProperties.Compute(surf);

Point3d cent = massProp.Centroid;

double u,v;
surf.ClosestPoint(cent, out u, out v);

SurfaceCurvature cur = surf.CurvatureAt(u, v);

Vector3d normal = cur.Normal;

double area = massProp.Area;

However, if i check the area in Rhino it gives a different result than AreaMassProperties.Area. And GH's EvalSrf component shows different(and correct) normal vectors than what i get with the code above.

Any hints appreciated,

Andy

Permalink Reply by Hannes Löschke on December 19, 2013 at 12:59am

The surface has a method NormalAt(u,v). Maybe, you get better results with that.

Permalink Reply by Mateusz Zwierzycki on December 19, 2013 at 1:11am

Also it's much easier to work with surfaces after reparametrizing - method .SetDomain

Permalink Reply by Andreas Graumann on December 19, 2013 at 8:50am

Mateusz: is there somewhere a description available, what SetDomain actually does? the docs are a bit short on that..

Permalink Reply by Mateusz Zwierzycki on December 19, 2013 at 8:56am

You can manually set the U and V domains of the surface. In GH we use reparametrize option to set it to 0 to 1 in both directions, however you can set it to any domain with code. It's just much more convinient to work with reparametrized surface, both in code and in grasshopper.