[vtkusers] similarity between two images (compare two images)

[m26404030 at john.petra.ac.id]

hi all, i want to ask a question...
i have three 2D images, lets say image I, image II, and image III.
and suppose i use image I as reference image. i want to  
determine/compare the similarity between image II and image I ,   and  
between image III and image I.
(comparing based on their shape or pixel??)
i need this because i have to choose one image which have more  
similarity with image I..

is there a vtk class to do this?
or maybe someone know the algorithm to solve this problem..


thanx for the reply..^^
sorry if my english is bad..