# IsoSubsample

VTKExamples/Python/ImageProcessing/IsoSubsample

### Description¶

An artifact called aliasing occurs when sub-sampling and is often associated with stair-stepping edges. Sampling theory proves that discrete sampled signals with spacing S, completely describe continuous functions composed of frequencies less than S/2. When a signal is sub-sampled, its capacity to hold high frequency information is reduced. However, the high frequency energy does not disappear. It wraps around the frequency spectrum appearing as a low frequency alias artifact. The solution, which eliminates this artifact, is to low-pass filter before sub-sampling.

Low-pass smoothing reduces the high frequency range of an image that would cause aliasing. The same aliasing phenomena occurs when acquiring data. If a signal from an analog source contains high frequencies, saving the analog data in a discrete form requires sub-sampling that will introduce alias artifacts. For this reason, it is common practice to acquire data at high resolutions,then smooth and subsample to reduce the image to a manageable size.

This example demonstrates aliasing that occurs when a high-frequency signal is sub-sampled. High frequencies appear as low frequency artifacts. The left image is an isosurface of a skull after sub-sampling. The right image used a low-pass filter before sub-sampling to reduce aliasing.

Note

Other Languages

See (Cxx)

Question

### Code¶

IsoSubsample.py

#!/usr/bin/env python

"""
"""

import vtk

def main():
colors = vtk.vtkNamedColors()

colors.SetColor("ActorColor", [235, 235, 235, 255])

fileName = get_program_parameters()

# Smoothed pipeline.
smooth = vtk.vtkImageGaussianSmooth()
smooth.SetDimensionality(3)
smooth.SetStandardDeviations(1.75, 1.75, 0.0)

subsampleSmoothed = vtk.vtkImageShrink3D()
subsampleSmoothed.SetInputConnection(smooth.GetOutputPort())
subsampleSmoothed.SetShrinkFactors(4, 4, 1)

isoSmoothed = vtk.vtkImageMarchingCubes()
isoSmoothed.SetInputConnection(smooth.GetOutputPort())
isoSmoothed.SetValue(0, 1150)

isoSmoothedMapper = vtk.vtkPolyDataMapper()
isoSmoothedMapper.SetInputConnection(isoSmoothed.GetOutputPort())
isoSmoothedMapper.ScalarVisibilityOff()

isoSmoothedActor = vtk.vtkActor()
isoSmoothedActor.SetMapper(isoSmoothedMapper)
isoSmoothedActor.GetProperty().SetColor(colors.GetColor3d("ActorColor"))

# Unsmoothed pipeline.
# Sub sample the data.
subsample = vtk.vtkImageShrink3D()
subsample.SetShrinkFactors(4, 4, 1)

iso = vtk.vtkImageMarchingCubes()
iso.SetInputConnection(subsample.GetOutputPort())
iso.SetValue(0, 1150)

isoMapper = vtk.vtkPolyDataMapper()
isoMapper.SetInputConnection(iso.GetOutputPort())
isoMapper.ScalarVisibilityOff()

isoActor = vtk.vtkActor()
isoActor.SetMapper(isoMapper)
isoActor.GetProperty().SetColor(colors.GetColor3d("ActorColor"))

# The rendering Pipeline.

# Setup the render window, renderer, and interactor.
leftViewport = [0.0, 0.0, 0.5, 1.0]
rightViewport = [0.5, 0.0, 1.0, 1.0]

rendererLeft = vtk.vtkRenderer()
rendererLeft.SetViewport(leftViewport)

rendererRight = vtk.vtkRenderer()
rendererRight.SetViewport(rightViewport)

renderWindow = vtk.vtkRenderWindow()

renderWindowInteractor = vtk.vtkRenderWindowInteractor()
renderWindowInteractor.SetRenderWindow(renderWindow)

rendererLeft.GetActiveCamera().SetFocalPoint(0.0, 0.0, 0.0)
rendererLeft.GetActiveCamera().SetPosition(0.0, -1.0, 0.0)
rendererLeft.GetActiveCamera().SetViewUp(0.0, 0.0, -1.0)
rendererLeft.ResetCamera()
rendererLeft.GetActiveCamera().Azimuth(-20.0)
rendererLeft.GetActiveCamera().Elevation(20.0)
rendererLeft.ResetCameraClippingRange()

rendererLeft.SetBackground(colors.GetColor3d("SlateGray"))
rendererRight.SetBackground(colors.GetColor3d("LightSlateGray"))
rendererRight.SetActiveCamera(rendererLeft.GetActiveCamera())

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

renderWindowInteractor.Start()

def get_program_parameters():
import argparse
description = 'This figure demonstrates aliasing that occurs when a high-frequency signal is subsampled.'
epilogue = '''
High frequencies appear as low frequency artifacts.
The left image is an isosurface of a skull after subsampling.
The right image used a low-pass filter before subsampling to reduce aliasing.
'''
parser = argparse.ArgumentParser(description=description, epilog=epilogue,
formatter_class=argparse.RawDescriptionHelpFormatter)