[vtkusers] calculating the mean value of x-coordinate

[xujf]

Hi,
  this is part of my program:

import sys,os 
from paths import *
from vtkMINCReader import *

reader=vtkMINCReader()
reader.SetFileName("E:/picture/sbrain.mnc")

filter=vtkImageMarchingCubes()
filter.SetInput(reader.GetOutput())
filter.SetValue(0,1200)

I use vtkImageMarchingCubes to get the isosurface of a volume,then I want to calculate the mean value of x-coordinates of all points in the isosurface.
I know I should add up all the values and divide by the total number! but how can I get the total number and every point\'s x-coordinate value.

Thanks in advance!
xujf
 
what should I do next?