# Python - Dynamically change the opacity values of vtkPieceWiseFunction()

**URL:** https://discourse.vtk.org/t/python-dynamically-change-the-opacity-values-of-vtkpiecewisefunction/4698
**Category:** Development
**Created:** [November 20, 2020, 1:07pm UTC](https://discourse.vtk.org/t/python-dynamically-change-the-opacity-values-of-vtkpiecewisefunction/4698 "2020-11-20T13:07:34Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![Gibser](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/g/ecd19e/32.png) [@Gibser](https://discourse.vtk.org/u/Gibser)
#### Post date: [November 20, 2020, 1:07pm UTC](https://discourse.vtk.org/t/python-dynamically-change-the-opacity-values-of-vtkpiecewisefunction/4698/1 "2020-11-20T13:07:34Z")

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Hi everyone, this is my first post here. I used vtkplotter to show a 3d model starting from dicom scans with the following code:

```auto
import vtk
from vtkplotter import *
import numpy as np
from vtkmodules.vtkCommonDataModel import vtkPiecewiseFunction

from vtkmodules.vtkRenderingCore import vtkRenderer, vtkVolume, vtkVolumeProperty, vtkColorTransferFunction

volume = load('Letter:/path')

volumeColor = vtkColorTransferFunction()
r, g, b = getColor('white')
volumeColor.AddRGBPoint(-1024, r, g, b)
r, g, b = getColor('yellow')
volumeColor.AddRGBPoint(-1023, r, g, b)
r, g, b = getColor('blue')
volumeColor.AddRGBPoint(-1022, r, g, b)

volumeScalarOpacity = vtkPiecewiseFunction()
volumeScalarOpacity.AddPoint(-1024, 0.00)
volumeScalarOpacity.AddPoint(-1023, 0.5)
volumeScalarOpacity.AddPoint(-1022, 0.5)

volumeProperty = vtkVolumeProperty()
volumeProperty.SetColor(volumeColor)

volumeProperty.SetScalarOpacity(volumeScalarOpacity)
volumeProperty.SetInterpolationTypeToLinear()
volumeProperty.ShadeOn()
volumeProperty.SetAmbient(0.4)
volumeProperty.SetDiffuse(0.6)
volumeProperty.SetSpecular(0.2)

volume.SetProperty(volumeProperty)

show(volume)

```

How can I change the opacity values ​​dynamically with a slider?

Thanks in advance for your answers

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<div class="post-metadata">

### Author: ![marcomusy](https://discourse.vtk.org/user_avatar/discourse.vtk.org/marcomusy/32/95_2.png) [@marcomusy](https://discourse.vtk.org/u/marcomusy)
#### Post date: [November 22, 2020, 12:19pm UTC](https://discourse.vtk.org/t/python-dynamically-change-the-opacity-values-of-vtkpiecewisefunction/4698/2 "2020-11-22T12:19:39Z")

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You can try something like this:

```python
from vedo import load, datadir
from vedo.applications import RayCaster
embryo = load(datadir+"embryo.tif") # vtkVolume
embryo.mode(1).c('jet').alpha([0,0.1,0.9]) # change properties
# volumeProperty = embryo.GetProperty()
plt = RayCaster(embryo) # Plotter instance
plt.show(viewup="z", bg='black', bg2='blackboard', axes=7)

```

 ![Screenshot from 2020-11-22 13-06-52](https://discourse.vtk.org/uploads/default/original/2X/c/c5147d5e09bd2bafa824b3c74841c16f93d504c3.png)

or customize it the way you like using [this](https://github.com/marcomusy/vedo/blob/867dc3b70045c8efcc7f7d70e901306bbebe722a/vedo/applications.py#L277). If you encounter any problems please submit an issue [here](https://github.com/marcomusy/vedo/issues).

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<div class="post-metadata">

### Author: ![Gibser](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/g/ecd19e/32.png) [@Gibser](https://discourse.vtk.org/u/Gibser)
#### Post date: [November 26, 2020, 11:45am UTC](https://discourse.vtk.org/t/python-dynamically-change-the-opacity-values-of-vtkpiecewisefunction/4698/3 "2020-11-26T11:45:05Z")

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I’ll try, thank you very much!