And example explaining the difference between PreMultiply and PostMultiply methods of vtkTransform objects

PrePostMultiply.py

# Example to show difference between PreMultiply and PostMultiply

# Create coordinate models using the CreateModels module

createModelsLogic = slicer.modules.createmodels.logic()
preModelNode = createModelsLogic.CreateCoordinate(20,2)
preModelNode.SetName('PreModel')
postModelNode = createModelsLogic.CreateCoordinate(20,2)
postModelNode.SetName('PostModel')
originModelNode = createModelsLogic.CreateCoordinate(20,2)
originModelNode.SetName('OriginModel')

# Change the color of models

preModelNode.GetDisplayNode().SetColor(1,0,0)
postModelNode.GetDisplayNode().SetColor(0,1,0)

# Create transform nodes
# Google vtkMRMLLinearTransformNode for documentation

preModelToRas = slicer.vtkMRMLLinearTransformNode()
preModelToRas.SetName('PreModelToRas')
slicer.mrmlScene.AddNode(preModelToRas)

postModelToRas = slicer.vtkMRMLLinearTransformNode()
postModelToRas.SetName('PostModelToRas')
slicer.mrmlScene.AddNode(postModelToRas)

# Set transform of the transform node
# Google vtkTransform for documentation

preModelToRasTransform = vtk.vtkTransform()
preModelToRasTransform.PreMultiply() // This is default, but it makes explicit
preModelToRasTransform.Translate(0, 100, 0)
preModelToRasTransform.RotateX(30)
preModelToRasTransform.Update()
preModelToRas.SetAndObserveTransformToParent(preModelToRasTransform)

postModelToRasTransform = vtk.vtkTransform()
postModelToRasTransform.PostMultiply()
postModelToRasTransform.Translate(0, 100, 0)
postModelToRasTransform.RotateX(30)
postModelToRasTransform.Update()
postModelToRas.SetAndObserveTransformToParent(postModelToRasTransform)

# Transform the models with transform nodes

preModelNode.SetAndObserveTransformNodeID(preModelToRas.GetID())
postModelNode.SetAndObserveTransformNodeID(postModelToRas.GetID())