[vtkusers] about the center of the image

Wed Nov 28 11:26:07 EST 2001

Alex,

I suspect its because half the pixels are 0-127 and the other half are 128-255
so the exact center is at 127.5, and this becomes the offset at -127.5. Only if
the number of pixels is odd would the image center be directly on a pixel
center...john

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From: "Alex zhang" <zjzhang at ee.cuhk.edu.hk>@public.kitware.com on 11/28/2001
11:48 PM ZE8

       "Alex zhang" <zjzhang at ee.cuhk.edu.hk>           To:   <vtkusers at public.kitware.com>        
                         @public.kitware.com           Cc:    (bcc: John Anast-JM/PGI)            
                                               Subject:      [vtkusers] about the center of the   
                                                    image                                         
 Sent by:  vtkusers-admin at public.kitware.com                                                      
                         11/28/2001 10:48 AM                                                      

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