Relationship between SegmentEditorWidget and SegmentEditorNode?

I am working on a set of custom extensions, and I have been able to muddle through to get several things working, but I am now running into problems and having trouble debugging. I think my problem may be related to not understanding the relationship between a segmentEditorWidget and a segmentEditorNode. For example, in my code I create a segmentEditorWidget and a segmentEditorNode, add them to the mrmlScene, and try to link them together:

segmentEditorWidget = slicer.qMRMLSegmentEditorWidget()
segmentEditorNode = slicer.vtkMRMLSegmentEditorNode()
segmentEditorWidget.setSegmentationNode(newSegmentationNode) # This doesn't seem to link the widget to the new segmentation node, it still seems to link to an older, pre-existing segmentation node

# Not sure if I should do this:
# or this:

segmentEditorNode.SetAndObserveSegmentationNode(newSegmentationNode) # this line does seem to successfully link to the new segmentation node

I’m not sure how to properly understand the relationship between the widget and the node. If I change something in one of them, when is that reflected in the other, and when is it not? For my purposes, I want to start from an existing segmentation, make a copy of it, and then make several modifications to the new, copied segmentation. Can I get by without using a widget at all, since I never need to interact with it graphically? If so, how can I do the equivalent of segmentEditorWidget.setActiveEffectByName() and effect = segmentEditorWidget.activeEffect()? I am aware of the documentation at and find it helpful sometimes, but it never contains anything like a readable explanation of the logical structure of things, just a few word notes (if I am lucky!).

For example, at, I see that there is a function SetActiveEffectName(), but it isn’t clear to me whether that function means I am assigning a name to an active effect or selecting a name. It allows me to use any string as an argument, even one which does not correspond to any of the set of possible active effects, which makes me worried that I am not selecting an effect but rather creating a name for an effect, but it could easily be that there just isn’t any error checking to make sure that the effect name I supply actually corresponds to an effect. I don’t see anything in the documentation which would help me figure out which of those is happening. I am a relatively novice python programmer and worse than novice C++ programmer, so when things get down to the C++ code, looking at the source code is only sometimes helpful and is entirely lacking in nuance for me.

Is there any reference which outlines the basic programming design for slicer and/or vtk? I mean something which might say something like “display nodes hold parameters related to the 2D and 3D display of segmentations. For a given segmentation node, it’s display node can be accessed through segmentationNode.GetDisplayNode()” or simple explanations like that. Thanks for any help you can provide.

slicer.qMRMLSegmentEditorWidget is a GUI widget. slicer.vtkMRMLSegmentEditorNode is a MRML node.

These slides should answer your questions.

It activates an effect.

There are many examples (short code snippets and full segmentation workflows) available in the script repository: Documentation/Nightly/ScriptRepository - Slicer Wiki

Feel free to ask if you cannot find the answer to any of your questions.


Thanks, those slides are very helpful and will help organize my thinking about how to properly structure and use modules.


Agreed. Why don’t we link them directly from here:

It was already added to nightly version, but now I’ve added it to 4.10, too.