[Insight-users] Re: NMI registration

[Luis Ibanez]

Hi Vineet,

Well, if you start your optimization at the
optimal value, it is to expect that the
optimizer will take a tour around that value
and eventually come back.

In that process, you will necesarily see
smaller values of the metric.

If the optimizer is not comming back to the
original position, that means that your
step length is too large.


Please try smaller values for

  optimizer->SetMaximumStepLength( 0.5 );

Note that the angular parameters are measured
in radians. A rotation of 0.5 radians is almost
30 degrees which is too large for the step of
an optimization method in the angle space.


BTW, you did not mention the values you are
using for the scaling parameters. Those are
also critical here.... Look at how much the
translation parameters are changing with
respect to the angular parameters. That will
give you an idea of whether you need to change
your parameter scaling or not.


   Regards,


      Luis


-----------------------
Vineet Kulkarni wrote:

> Hi Luis,
> I was running a registration program with the same image as moving and fixed 
> to test the module.
> I am using
> -Normalized Mutual Info as the metric
> -Regular Step Gradient Descent Optimizer 
> -Euler 3d Transform
> 
>     optimizer->SetMaximumStepLength( 0.5 );
>     optimizer->SetMinimumStepLength( 0.001 );
>     optimizer->SetNumberOfIterations( 100 );
> 
> The transform is set to identity initially. 
> The problem is that the metric value starts at 2.0 as it should but then it 
> keeps on decreasing it. This happens with both MaximizeOn and MaximizeOff.
> Could you please suggest whats going wrong?
> Thanks,
> Regards,
> Vineet Kulkarni
> 
>