VTK: render concave polygon with vtkDelaunay2D

Question

I want to render a concave polygon with the use of a vtkDelaunay2D. I've read that it should work with a vtkDelaunay2D, but it doesn't. Why? Here is the my code:

import vtk

points = vtk.vtkPoints()

ls = [
    [5, 5], [-5, 5], [-8, 0], [-5, -5], [5, -5], [2, 0]
]
for x, y in ls:
    points.InsertNextPoint(x, y, 0)

aPolyData = vtk.vtkPolyData()
aPolyData.SetPoints(points)

delaunay = vtk.vtkDelaunay2D()
delaunay.SetInputData(aPolyData)

meshMapper = vtk.vtkPolyDataMapper()
meshMapper.SetInputConnection(delaunay.GetOutputPort())

colors = vtk.vtkNamedColors()

meshActor = vtk.vtkActor()
meshActor.SetMapper(meshMapper)
meshActor.GetProperty().EdgeVisibilityOn()
meshActor.GetProperty().SetEdgeColor(colors.GetColor3d("Peacock"))
meshActor.GetProperty().SetInterpolationToFlat()

renderer = vtk.vtkRenderer()
renderWindow = vtk.vtkRenderWindow()
renderWindow.AddRenderer(renderer)
renderWindowInteractor = vtk.vtkRenderWindowInteractor()
renderWindowInteractor.SetRenderWindow(renderWindow)

renderer.AddActor(meshActor)
renderer.SetBackground(colors.GetColor3d("Mint"))

renderWindow.SetSize(640, 480)
renderWindow.Render()
renderWindowInteractor.Start()

The result looks like this:

The triangle on the right side shouldn't be there.

So why does this happen? And what do I have to do so this doesn't happen?

Answer 1

From the documentation (specially the Warning part), output is expected to be convex.

As workaround for your example, decreasing the Offset value of the filter leads to the result you want (for instance delaunay.SetOffset(0.1) ). But as it is not intended by the filter, it may have unexpected results.

Answer 2

You may add a counterclockwise polydata cotaining the line that defines the boundary using delaunay.SetSourceData(polyline) , then you get the desired result:

but this only works if you assume you know the boundary