# Change vtkStencil used for the display of volumes

**URL:** <https://discourse.slicer.org/t/change-vtkstencil-used-for-the-display-of-volumes/21292>\
**Category:** Development\
**Created:** [December 31, 2021, 2:54pm UTC](https://discourse.slicer.org/t/change-vtkstencil-used-for-the-display-of-volumes/21292 "2021-12-31T14:54:50Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![riep](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/riep/32/9933_2.png) [@riep](https://discourse.slicer.org/u/riep)\
**Post date:** [December 31, 2021, 2:54pm UTC](https://discourse.slicer.org/t/change-vtkstencil-used-for-the-display-of-volumes/21292/1 "2021-12-31T14:54:50Z")

</div>

Dear everyone,  
I am working on a python script that overwrite the Background Image Stencil Data used in the display volume node.  
It is natively defined in [Slicer/vtkMRMLSliceLayerLogic.cxx at 0673b1c05404b21d259cfaf85a5f89d9cf1f3771 · Slicer/Slicer · GitHub](https://github.com/Slicer/Slicer/blob/0673b1c05404b21d259cfaf85a5f89d9cf1f3771/Libs/MRML/Logic/vtkMRMLSliceLayerLogic.cxx#L797)  
I have basically commented all occurrences of SetBackgroundImageStencilDataConnection in the slice layer logic.

Instead, I have initialized my own stencil from a vtkMRMLAnnotationROINode (roiNode)

```auto
 # Create a box implicit function that will be used as a stencil
    roiBox = vtk.vtkBox()
    roiCenter = [0, 0, 0]
    roiNode.GetXYZ(roiCenter)
    roiRadius = [0, 0, 0]
    roiNode.GetRadiusXYZ(roiRadius)
    roiBox.SetBounds(roiCenter[0] - roiRadius[0], roiCenter[0] + roiRadius[0], roiCenter[1] - roiRadius[1], roiCenter[1] + roiRadius[1], roiCenter[2] - roiRadius[2], roiCenter[2] + roiRadius[2])

    rasToBox = vtk.vtkMatrix4x4()
    if roiNode.GetTransformNodeID() != None:
      roiBoxTransformNode = slicer.mrmlScene.GetNodeByID(roiNode.GetTransformNodeID())
      boxToRas = vtk.vtkMatrix4x4()
      roiBoxTransformNode.GetMatrixTransformToWorld(boxToRas)
      rasToBox.DeepCopy(boxToRas)
      rasToBox.Invert()

    ijkToRas = vtk.vtkMatrix4x4()
    volumeNode.GetIJKToRASMatrix(ijkToRas)

    ijkToBox = vtk.vtkMatrix4x4()
    vtk.vtkMatrix4x4.Multiply4x4(rasToBox, ijkToRas, ijkToBox)
    ijkToBoxTransform = vtk.vtkTransform()
    ijkToBoxTransform.SetMatrix(ijkToBox)
    roiBox.SetTransform(ijkToBoxTransform)

    # Use the stencil to fill the volume
    imageData = volumeNode.GetImageData()

    # Convert the implicit function to a stencil
    functionToStencil = vtk.vtkImplicitFunctionToImageStencil()
    functionToStencil.SetInput(roiBox)
    functionToStencil.SetOutputOrigin(imageData.GetOrigin())
    functionToStencil.SetOutputSpacing(imageData.GetSpacing())
    functionToStencil.SetOutputWholeExtent(imageData.GetExtent())
    functionToStencil.Update()

```

Then, I bound the output to the volume

```auto
volumeNode.GetScalarVolumeDisplayNode().SetBackgroundImageStencilDataConnection(functionToStencil.GetOutputPort())

```

This has no effects and I cannot figure out why.  
Does anyone have an idea why?

Thank you in advance,  
Pierre  
Pierre