Example script related to this discussion: https://discourse.slicer.org/t/trying-to-diagnose-an-issue-with-virtualgl-and-slicer/17685/14

vr-depthpeel-crop.py

#!/usr/bin/env python

comment = """
You can use this script with either Slicer's python wrapped VTK or with pip installed VTK like this:

~/Downloads/Slicer-4.13.0-2021-06-18-linux-amd64/bin/PythonSlicer vr-depth-crop.py

python vr-depth-crop.py

"""


import os
import urllib.request
import vtk


def main():
    fileName = get_program_parameters()

    colors = vtk.vtkNamedColors()

    # This is a simple volume rendering example that
    # uses a vtkFixedPointVolumeRayCastMapper

    # Create the standard renderer, render window
    # and interactor.
    ren1 = vtk.vtkRenderer()

    renWin = vtk.vtkRenderWindow()

    #
    # SP: for depth peeling per: https://vtk.org/Wiki/VTK/Depth_Peeling
    #
    renWin.SetAlphaBitPlanes(1)
    renWin.SetMultiSamples(0)
    ren1.SetUseDepthPeeling(1)
    ren1.SetMaximumNumberOfPeels(4)
    ren1.SetOcclusionRatio(0.1)
    ren1.SetUseDepthPeelingForVolumes(1) ;# also used in Slicer


    # add sphere to test depth peeling
    sphere = vtk.vtkSphereSource()
    sphere.SetCenter(200,0,100)
    sphere.SetRadius(100)
    sphereMapper = vtk.vtkPolyDataMapper()
    sphereMapper.SetInputConnection(sphere.GetOutputPort())
    sphereActor = vtk.vtkActor()
    sphereActor.SetMapper(sphereMapper)
    sphereActor.GetProperty().SetOpacity(0.5)
    ren1.AddActor(sphereActor)


    renWin.AddRenderer(ren1)

    iren = vtk.vtkRenderWindowInteractor()
    iren.SetRenderWindow(renWin)

    # Create the reader for the data.
    reader = vtk.vtkStructuredPointsReader()
    reader.SetFileName(fileName)

    # Create transfer mapping scalar value to opacity.
    opacityTransferFunction = vtk.vtkPiecewiseFunction()
    opacityTransferFunction.AddPoint(20, 0.0)
    opacityTransferFunction.AddPoint(255, 0.2)

    # Create transfer mapping scalar value to color.
    colorTransferFunction = vtk.vtkColorTransferFunction()
    colorTransferFunction.AddRGBPoint(0.0, 0.0, 0.0, 0.0)
    colorTransferFunction.AddRGBPoint(64.0, 1.0, 0.0, 0.0)
    colorTransferFunction.AddRGBPoint(128.0, 0.0, 0.0, 1.0)
    colorTransferFunction.AddRGBPoint(192.0, 0.0, 1.0, 0.0)
    colorTransferFunction.AddRGBPoint(255.0, 0.0, 0.2, 0.0)

    # The property describes how the data will look.
    volumeProperty = vtk.vtkVolumeProperty()
    volumeProperty.SetColor(colorTransferFunction)
    volumeProperty.SetScalarOpacity(opacityTransferFunction)
    volumeProperty.ShadeOn()
    volumeProperty.SetInterpolationTypeToLinear()

    # The mapper / ray cast function know how to render the data.
    volumeMapper = vtk.vtkFixedPointVolumeRayCastMapper()
    volumeMapper.SetInputConnection(reader.GetOutputPort())
    
    #
    # SP: add volume cropping for peeling example
    #
    volumeMapper.SetCroppingRegionPlanes(0,300,0,300,0, 300)
    volumeMapper.SetCropping(1)

    # The volume holds the mapper and the property and
    # can be used to position/orient the volume.
    volume = vtk.vtkVolume()
    volume.SetMapper(volumeMapper)
    volume.SetProperty(volumeProperty)

    ren1.AddVolume(volume)
    ren1.SetBackground(colors.GetColor3d('Wheat'))
    ren1.GetActiveCamera().Azimuth(45)
    ren1.GetActiveCamera().Elevation(30)
    ren1.ResetCameraClippingRange()
    ren1.ResetCamera()


    renWin.SetSize(600, 600)
    renWin.SetWindowName('DepthPeeling')
    renWin.Render()

    #
    # SP: for depth peeling per: https://vtk.org/Wiki/VTK/Depth_Peeling
    #
    print(f"Render used depth peeling: {ren1.GetLastRenderingUsedDepthPeeling()}")


    iren.Start()


#
# SP: switch to downloaded data for ease of use
#
def get_program_parameters():
    filePath = "/tmp/MRHead.vtk"
    if not os.path.exists(filePath):
      mrHeadURL = "https://s3.amazonaws.com/IsomicsPublic/SampleData/MRHead/MRHead.vtk"
      print(f"Downloading {mrHeadURL} to {filePath}")
      urllib.request.urlretrieve(mrHeadURL, filePath)
    else:
      print(f"Using {filePath}")

    return filePath


if __name__ == '__main__':
    print(f"VTK Version: {vtk.vtkVersion().GetVTKVersion()}")
    main()