VTK/Examples/Python/SumVTKImages

import vtk
#ImageSets=List of Image sets
#Weights  = List of wieghts e.g [1,1,1]
def SumVTKImages(ImageSets,Weights):
    NumOfImages=len(ImageSets)
    SumFilter=vtk.vtkImageWeightedSum()
    for x in range(0,NumOfImages,1):
        SumFilter.AddInput(ImageSets[x])
        SumFilter.SetWeight(x,Weights[x])
    SumFilter.Update()
    return SumFilter.GetOutput()# Summed Image as vtkImageData