vtkDecimatePolylineFilter not reducing the number of points

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1

Using vtk for python, when using vtkDecimatePolylineFilter, the output has the same number of points (and lines) as the input.
Here below a minimal example supposed decimate from 30 to 10 points (target reduction of 0.667). But there is still 30 points in the output.

Am I using this filter wrong?
Thanks

import vtk
import numpy as np

# Create example data with 30 points and 30 lines

n = 30
n_down = 10

points = vtk.vtkPoints()
for i in range(int(n/2)):
    t = (4*i-n)/n
    points.InsertNextPoint(t, np.cos(t), 0) 
for i in range(int(n/2)):
    t = -(4*i-n)/n
    points.InsertNextPoint(t, -np.cos(t), 0) 

lines = vtk.vtkCellArray()
for i in range(n-1):
    line = vtk.vtkLine()
    line.GetPointIds().SetId(0, i)
    line.GetPointIds().SetId(1, i + 1)
    lines.InsertNextCell(line)
line.GetPointIds().SetId(0, i + 1)
line.GetPointIds().SetId(1, 0)
lines.InsertNextCell(line)

poly = vtk.vtkPolyData()
poly.SetPoints(points)
poly.SetLines(lines)

# Decimation
down_factor = (n - n_down) / n
decimator = vtk.vtkDecimatePolylineFilter()
decimator.SetInputData(poly)
decimator.SetTargetReduction(down_factor)
decimator.Update()
poly_down = decimator.GetOutput()

# Check number of points and lines in the downsampled poly
print(f"Original Number of Points: {poly.GetNumberOfPoints()}") 
print(f"Original Number of Lines: {poly.GetNumberOfLines()}")     
print(f"Downsampled Number of Points: {poly_down.GetNumberOfPoints()}") 
print(f"Downsampled Number of Lines: {poly_down.GetNumberOfLines()}")  

Original Number of Points: 30
Original Number of Lines: 30
Downsampled Number of Points: 30
Downsampled Number of Lines: 30
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2

I suspect that the filter is reducing the size of the polyline cells, and leaving the points untouched. Whether this is a feature, bug, or laziness on the part of the developer (probably me :slight_smile: ) is hard to say, certainly a mode could be added to discard unused points. In the mean time, if you use vtkCleanPolyData or vtkStaticCleanPolyData with RemoveUnusedPoints enabled, you should be able to remove the unused points.

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