Hi @dale

Is there Api for Curve side(Similar this grasshopper component){for open curve}

In c#?

I don t think so.

you will need a reference plane / normal / z-vector to judge this

if you implement your own function:

P the point to evaluate

S the closest point on the curve

t the corresponding parameter on the curve

you can use

https://developer.rhino3d.com/api/RhinoCommon/html/Overload_Rhino_Geometry_Curve_ClosestPoint.htm

to get the t

get S and tangent

now calculate a curve normal in respect of the plane / z-Vector

(project to cplane, Cross Product)

compare it to the PS vector

get the angle between the PS vector and the curve normal

angle < 90 degree => left

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