Convex Hull in VB component

Hi, is there any way to use the convex hull function inside a VB component?

Replies to This Discussion

Permalink Reply by David Rutten on September 26, 2011 at 5:16am

Which convex hull function?

David Rutten

david@mcneel.com

Poprad, Slovakia

Permalink Reply by Liviu Teodorescu on September 26, 2011 at 5:46am

I'm generating a list of points inside a VB component and I want to use convex hull on them but inside the same component.

Permalink Reply by David Rutten on September 26, 2011 at 10:36am

Oh sorry, you meant the 2D convex hull that ships with Grasshopper! I was confused and thought you wanted 3D convex hulls.

Have a look at the Grasshopper.Kernel.Geometry.ConvexHull namespace. There's a Solver class in there that has static methods for solving hulls.

David Rutten

david@mcneel.com

Poprad, Slovakia

Permalink Reply by Liviu Teodorescu on September 26, 2011 at 12:15pm

Yes, 2d points. I couldn't find what you said in the Grasshopper SDK. Any thoughts on how to actually write this. I have a list of points say

PList as new list (of point3d) and I want the hull for them (the Z unit is 0 for all points)

Permalink Reply by David Rutten on September 26, 2011 at 12:45pm

Dim nodes As New Grasshopper.Kernel.Geometry.Node2List(pts)

A = Grasshopper.Kernel.Geometry.ConvexHull.Solver.ComputeHull(nodes)

If pts is a List of Point3d, then the above will work.

David Rutten

david@mcneel.com

Poprad, Slovakia

Permalink Reply by Liviu Teodorescu on September 26, 2011 at 12:55pm

It worked, thanks a lot ! I was writing the convex hull algorithm myself, but this saves me some time.

This isn't in the SDK is it? The grasshopper.kernel.geometry.etc ..

Anyway, thank you again.

Cheers

Permalink Reply by David Rutten on September 26, 2011 at 1:35pm

If it wasn't in the SDK you couldn't use it. But the Grasshopper.Kernel.Geometry namespace is omitted from the SDK helpfile.

In fact a lot of stuff is omitted from the helpfile, either because I felt it was pointless to anyone but me or because I never took the time to write decent comments. In this case because of the latter.

David Rutten

david@mcneel.com

Poprad, Slovakia

Permalink Reply by Liviu Teodorescu on September 26, 2011 at 2:58pm

Sorry one more question.

Say I get the convex hull, now I need the area centroid or center of gravity for the hull.

I am really sorry for asking these questions but I need this to drive my algorithm.

I was thinking of making a planar surface out of the polyline hull and then just evaluating the surface at surface parameter (0.5, 0.5).

Thanks in advance.

Permalink Reply by David Rutten on September 27, 2011 at 1:41am

You can use Rhino.Geometry.AreaMassProperties to find the centroid of your hull. But first you'll need to convert it from a Polyline into a class that derives from the abstract Curve class:

Dim nodes As New Geometry.Node2List(pts)

Dim hull As Polyline = ConvexHull.Solver.ComputeHull(nodes)

Dim am As AreaMassProperties

am = AreaMassProperties.Compute(new PolylineCurve(hull))

Dim pt As Point3d = am.Centroid

David Rutten

david@mcneel.com

Poprad, Slovakia

Permalink Reply by Liviu Teodorescu on September 30, 2011 at 6:06am

Hey, I've tried your code and it works, but when I put it in a function

I get "object reference not set to an instance of an object". The function returns the hull center. Here's the code

Function get_HullCenter(ByVal _PPList As list (Of PtPointer)) As point3d

    Dim i As Integer
    Dim newList As New list(Of Point3d)

    For i = 0 To _PPList.count - 1

      newList.add(_PPList(i).getPoint)

    Next

    If newList.count = 1 Then                               'test if the list has one or two points and return hullcenter

      Return newList(0)

    Else

      If newList.count = 2 Then

        Dim v1 As New vector3d(newList(0))
        Dim v2 As New vector3d(newList(1))
        Dim _pt As New point3d((v1.X + v2.X) / 2, (v1.Y + v2.Y) / 2, (v1.Z + v2.Z) / 2)

        Return _pt

      End If

    End If

    Dim nodes As New Grasshopper.Kernel.Geometry.Node2List(newList)                               'get hullcenter for 3 or more points
    Dim _crv As polyline = Grasshopper.Kernel.Geometry.ConvexHull.Solver.ComputeHull(nodes)

    Dim am As AreaMassProperties
    am = AreaMassProperties.Compute(New PolylineCurve(_crv))

    Dim pt As point3d = am.Centroid

    Return pt

End Function