Summary of vtkThreshold (updated from time to time)

What is vtkThreshold?

A filter that extracts only elements that contain values within the specified range.

** class reference ** https://vtk.org/doc/nightly/html/classvtkThreshold.html

Specifically, the example in the following paraview is easy to understand

Check the function with paraview

For the calculation example, I used pitzDaily from the tutorial of ʻOpenFOAM`.

Before applying Threshold

After reading the calculation result, press this button

After applying Threshold

Result of applying a specified range of 5 to 15 for pressure p

After applying the TInvert option

Take the complement

Operation check with python

environment

python 3.7 vtk 8.1.2

Confirmation method

import Vtk
print(vtk.vtkVersion.GetVTKSourceVersion())
>> vtk version 8.1.2

Base code

I use vtkOpenFOAMReader to read the result of OpenFOAM About vtkOpenFOAMReader, I will also summarize it in here.

import vtk
#Read the result of OpenFOAM
filename = "case1.foam"
reader = vtk.vtkOpenFOAMReader()
reader.SetFileName(filename)
reader.CreateCellToPointOn()
reader.DecomposePolyhedraOn()
reader.EnableAllCellArrays()
reader.Update()
#Apply the latest Time result
n_step = reader.GetTimeValues().GetNumberOfValues()
latest_time = reader.GetTimeValues().GetValue(n_step-1)
reader.UpdateTimeStep(latest_time)
reader.Update()

filter_threshold = vtk.vtkThreshold()
filter_threshold.SetInputConnection(reader.GetOutputPort())

###################
#Add settings here#
###################
filter_threshold.Update()


filter = vtk.vtkGeometryFilter()
filter.SetInputConnection(filter_threshold.GetOutputPort())
filter.Update()

mapper = vtk.vtkCompositePolyDataMapper2()
mapper.SetInputConnection(filter.GetOutputPort()) #Set filter in mapper
mapper.SetScalarModeToUseCellFieldData() #Set for scalar data

# renderer
renderer = vtk.vtkRenderer()
renderer.AddActor(actor)            #Set actor in renderer

##Background color setting
renderer.GradientBackgroundOn()      #Set gradient background
renderer.SetBackground2(0.2,0.4,0.6) #Top color
renderer.SetBackground(1,1,1)        #Bottom color

#Window
renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(renderer)         #Set renderer in Window
iren = vtk.vtkRenderWindowInteractor();
iren.SetRenderWindow(renWin);
renWin.SetSize(850, 850)
renWin.Render()
iren.Start();

Basic

ThresholdBetween()

Limitation by specifying upper and lower limits

#Set the threshold between 5 and 15
filter_threshold.ThresholdBetween(5,15)

#enum FieldAssociations
#  {
#    FIELD_ASSOCIATION_POINTS,
#    FIELD_ASSOCIATION_CELLS,
#    FIELD_ASSOCIATION_NONE,
#    FIELD_ASSOCIATION_POINTS_THEN_CELLS,
#    FIELD_ASSOCIATION_VERTICES,
#    FIELD_ASSOCIATION_EDGES,
#    FIELD_ASSOCIATION_ROWS,
#    NUMBER_OF_ASSOCIATIONS
#  };
FIELD_ASSOCIATION_POINTS = 0
FIELD_ASSOCIATION_CELLS = 1
# "p"Is filtering against pressure
filter_threshold.SetInputArrayToProcess(0,0,0,FIELD_ASSOCIATION_CELLS ,"p")
filter_threshold.Update()

Consistent with the results in paraview above

If you don't know how to use the method, the help function may help.

help(filter_threshold.SetInputArrayToProcess)

>> 

Help on built-in function SetInputArrayToProcess:

SetInputArrayToProcess(...) method of vtkFiltersCorePython.vtkThreshold instance
    V.SetInputArrayToProcess(int, int, int, int, string)
    C++: virtual void SetInputArrayToProcess(int idx, int port,
        int connection, int fieldAssociation, const char *name)
    V.SetInputArrayToProcess(int, int, int, int, int)
    C++: virtual void SetInputArrayToProcess(int idx, int port,
        int connection, int fieldAssociation, int fieldAttributeType)
    V.SetInputArrayToProcess(int, vtkInformation)
    C++: virtual void SetInputArrayToProcess(int idx,
        vtkInformation *info)
    V.SetInputArrayToProcess(int, int, int, string, string)
    C++: virtual void SetInputArrayToProcess(int idx, int port,
        int connection, const char *fieldAssociation,
        const char *attributeTypeorName)
    
    Set the input data arrays that this algorithm will process.
    Specifically the idx array that this algorithm will process
    (starting from 0) is the array on port, connection with the
    specified association and name or attribute type (such as
    SCALARS). The fieldAssociation refers to which field in the data
    object the array is stored. See vtkDataObject::FieldAssociations
    for detail.

ThresholdByUpper()

Limit by specifying the upper limit Only areas larger than the specified value are displayed

#filter_threshold.ThresholdBetween(5,15)
filter_threshold.ThresholdByUpper(10)

Nothing is displayed because the target area does not exist

ThresholdByLower()

Limit by specifying the lower limit Only areas smaller than the specified value are displayed

#filter_threshold.ThresholdBetween(5,15)
filter_threshold.ThresholdByLower(5)

As a result, it matched the result of Invert in the above paraview.

Invert relationship

Apparently the Invert relationship is already implemented in c ++, but not in the python version.

filter_threshold.SetInvert(True)

AttributeError  
---> 31 filter_threshold.SetInvert(True)
AttributeError: 'vtkFiltersCorePython.vtkThreshold' object has no attribute 'SetInvert'

Other methods

GetUpperThreshold Get the upper limit of the threshold

filter_threshold.ThresholdBetween(-100,100)
filter_threshold.GetUpperThreshold()
>> 100

GetLowerThreshold Get the lower limit of the threshold

filter_threshold.ThresholdBetween(-100,100)
filter_threshold.GetLowerThreshold()
>> -100

Set/GetAttributeMode

filter_threshold.SetAttributeModeToDefault()
filter_threshold.GetAttributeMode()
>>0

filter_threshold.SetAttributeModeToUsePointData()
filter_threshold.GetAttributeMode()
>>1
filter_threshold.GetAttributeModeAsString()
>>'UsePointData'

filter_threshold.SetAttributeModeToUseCellData()
filter_threshold.GetAttributeMode()
>>2
filter_threshold.GetAttributeModeAsString()
>>'UseCellData'

filter_threshold.SetAttributeMode(2)
filter_threshold.GetAttributeModeAsString()
>>'UseCellData'

I will update it when I feel like it