Convex hull of set of points lying on a 3D plane

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1

Hi, How do I find the boundary of point cloud lying on a 3D plane. This plane is not parallel to XY or YZ or ZX planes. I am looking for the polygon on the plane that surrounds the point cloud. vtkConvexHull2D is supressing the z-coordinates.

2

We don’t have a filter that easily projects points to a plane, but that is what’s required before you can use vtkConvexHull2D.

The closest thing I can think of would be to use vtkPCAStatistics to compute the first two eigenvectors of your point cloud and use these as the x and y axes of the projected cloud.

3

Did you try: Filters/Points/vtkProjectPointsToPlane?

1 Like
4

Derp. I have not seen that one before.

5

Thanks Will. That worked.
After I compute the ConvexHull2D, I project it back to the plane.

I see problem with ConvexHull2D in some cases when the point cloud is on vatical plane which is parallel to YZ or ZX planes.

6

I’m not sure what the issue is, is it the vtkProjectPointsToPlane filter? Can you share a snippet of code?

7

Will, Here is the code. Basically I have points on a plane and I need t find the boundary polygon.

def ProjectPointsToPlane(polyData, origin = (0,0,0), normal = (0,0,1)):

p2plaFilter = vtk.vtkProjectPointsToPlane()
p2plaFilter.SetInputData(polyData)
p2plaFilter.SetProjectionTypeToSpecifiedPlane()
p2plaFilter.SetOrigin(origin)
p2plaFilter.SetNormal(normal)
p2plaFilter.Update()
return p2plaFilter.GetOutput()

def GetConvexHull2D(pointCloudPolyData):

#PolyData holds the point cloud in a plane
points = pointCloudPolyData.GetPoints()
numPoints = points.GetNumberOfPoints()

#Plane normal and origin
pt0 = points.GetPoint(0)
pt1 = points.GetPoint(int(numPoints/2))
pt2 = points.GetPoint(numPoints-1)

planeNormal = [0,0,1]
planeOrigin = (pt1[0], pt1[1], pt1[2])
vtk.vtkTriangle.ComputeNormal(pt0, pt1, pt2, planeNormal)

#Project the points to XY plane
projectedPolyData = ProjectPointsToPlane(pointCloudPolyData, (0,0,0), (0, 0, 1))

hull = vtk.vtkConvexHull2D()
hull.SetHullShape(1) #0: BoundingRectangle or 1: ConvexHull.
hull.SetInputData(projectedPolyData)
hull.Update()

#Project back the hull to original plane
hullPolyData = ProjectPointsToPlane(hull.GetOutput(), planeOrigin, (0, 0, 1))
return hullPolyData

def CreateHullFromJOBFile(renderer, jobFileName):

mapper = ReadJOBFiles([jobFileName])
if type(mapper) is str: 
    print(mapper)
    return

mapper.SetInputData(GetConvexHull2D(mapper.GetInput()))

actor = vtk.vtkActor()
actor.SetMapper(mapper)
actor.GetProperty().SetColor([1,0,0])
actor.GetProperty().SetRepresentationToWireframe()

renderer.AddActor(actor)