[Insight-users] fix registration parameters

[Luis Ibanez]

Hi Carmen,

The easy way to understand the effect of the
parameters-scale is to look at what they do
at the level of the optimizer.

In the case of the gradient descent optimizers,
the parameters divide the value of the gradient
before it is applied for computing the next
step.

For example look at the code in:

   Insight/Code/Numerics/
     itkRegularStepGradientDescentBaseOptimizer.cxx

In the method AdvanceOneStep(), lines 181-187.


Therefore, if you want to disable a specific
parameter, you must set its scale to a very high
value.  In this way its gradient component will
be attenuated and the subsequent steps will not
move much in the dimension of this parameter.



Regards,


   Luis


-------------------------

Carmen Olmos wrote:
>     which value should I place for optimizerScales? I've tried with
>     scale=0 for the parameter to fix (I thought that it would make the
>     step to 0) but then the other paremeters are not optimized either.
> 
>      
> 
>     Thanks,
> 
>     Carmen
> 
>      
> 
>      
> 
>     -------
> 
>     Note that right now you can do the equivalent
>     of FixParameter by selecting appropriate values
>     for the optimizerScales. However that will not
>     result in any performance (computing time)
>     improvement since the optimizer will keep running
>     in a parametric space with the total number of
>     dimensions.
> 
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