[vtkusers] about the center of the image

[Alex zhang]

Dear all:
       In many examples we have these code:
vtkImageReader reader
  reader ReleaseDataFlagOff
  reader SetDataByteOrderToLittleEndian
  reader SetDataExtent 0 255 0 255 1 93
  reader SetDataOrigin -127.5 -127.5 -47
  reader SetFilePrefix "$VTK_DATA/fullHead/headsq"
  reader SetDataMask 0x7fff
  reader Update
we use the setDataOrigin to make the image center to be the the image center, 
but I don't know why the data origin is -127.5 but not -128,  there are totally 
256 pixels and I think if the center is 0, then there will be both 128 pixel in the 
positive side and in negtive coordinate system, why -127.5?

regards,
zhang zhijun