[Insight-users] how to calculate point using eigen vector

[Luis Ibanez]

Hi Kana,

Please see my comments below
interlaced with your text.


   Thanks


         Luis


----------------------------------------------------------
On Thu, Mar 11, 2010 at 7:13 AM, Arunachalam Kana <Kana.Arunachalam at fh->
> It is clear that the eigen values and vectors are correctly arranged as per
> the increasing order of magnitude, when the parameter is “true”. My
> observation was that the last value is still returned in negative. Due to
> curiosity i dug inside the code J and found what is the possible reason.
> This is in the itkSymmetricEigenAnalysis line 692, code below:
>
>
>
> else if( m_OrderEigenValues == OrderByMagnitude )
>
>     {
>
>     // Order by magnitude
>
>     for (i = 0; i < m_Order-1; ++i)
>
>       {
>
>       k = i;
>
>       p = d[i];
>
>
>
> I think instead of “m_Order” , “m_Order – 1” is used.
>
>
>
> As this is the first time i am working with eigen values, i am not sure
> whether the magnitude of the eigen will be used for any other calculation. I
> would like to know whether this is acceptable or there has to be a change in
> code.
>
>
------------------------------------------------------------


Hi Kana,

It is great that you are looking at the internals of the code.
That's the best way of harvesting the power of Open Source.


The Relevant section of code is:

   692	    for (i = 0; i < m_Order-1; ++i)
   693	      {
   694	      k = i;
   695	      p = d[i];
   696	
   697	      for (j = i+1; j < m_Order; ++j)
   698	        {
   699	        if (vnl_math_abs(d[j]) >= vnl_math_abs(p))
   700	          {
   701	          continue;
   702	          }
   703	        k = j;
   704	        p = d[j];
   705	        }
   706	
   707	      if (k == i)
   708	        {
   709	        continue;
   710	        }
   711	      d[k] = vnl_math_abs(d[i]);
   712	      d[i] = vnl_math_abs(p);
   713	
   714	      for (j = 0; j < m_Order; ++j)
   715	        {
   716	        p = z[j + i * m_Dimension];
   717	        z[j + i * m_Dimension] = z[j + k * m_Dimension];
   718	        z[j + k * m_Dimension] = p;
   719	        }
   720	      }
   721	    }


The combination of the two for-loops

   692	    for (i = 0; i < m_Order-1; ++i)
   697	      for (j = i+1; j < m_Order; ++j)

is a common method for sorting a small array.


---

Have you found that the current ordering is incorrect ?


In such case, we should file a bug report at
http://public.kitware.com/Bug/


Please let us know,


     Thanks


          Luis