[Insight-users] Can a metric be negative in ITK?

[Rick Frank]

I believe that is the case. You are trying to minimize a metric, which
is going to be where it's derivative at its global minimum is 0,
Not necessarily the value of the metric.

Also, one must be aware that typically MI has a very "jaggy" metric
trace, which will create local minima. One needs an optimizer that can
Jump out of the local minima, or use a pyramid of resolutions,  and/or,
you can jitter pixels (as per Joachim Hornegger's "Korea"
talk/presentation) to try to smooth the metric. I have not tried the
latter but he claims it will smooth the resulting trace making
optimization less likely to stick in local minima.


You can find his lectures here

http://www5.informatik.uni-erlangen.de/en/our-team/hornegger-joachim/lec
tures/

Rick
-----Original Message-----

Date: Sun, 04 Mar 2012 16:56:54 +0100
From: Christopher Kappe <cpkappe at gmx.de>
Subject: Re: [Insight-users] Can a metric be negative in ITK?
To: insight-users at itk.org
Message-ID: <1377188.YjJcHt4Ey7 at chrisbook.site>
Content-Type: text/plain; charset="iso-8859-1"

So it's absolutely normal that this metric returns negative values and
it's 
okay when they get even "more negative" during the registration process.
(?)

Paul Muetze

Am Freitag 02 M?rz 2012, 12:04:01 schrieb Rick Frank:
> "High" negative  is what you want from this metric, as I understand
it.
> 
> Rick
>
> 
> Hello everybody, 
> 
> I'm new to this list. 
> I'm trying to do registration with two 3D images. I basically used the
> ImageRegistration8.cxx example (using itk 4.0 btw) but with
> MattesMutualInformationMetric instead of MeanSquaresMetric. 
> And, well, I got output like this from the observer: 
> 
> 0   -0.00130253  
> 1   -0.000181399 
> 2   -0.00019049   
> 3   -5.82618e-06
> 4   -6.76423e-06 
> 5   -1.81517e-05  
> 6   -1.78196e-05   
> 7   -7.79246e-06  
> 8   -4.0121e-06
> 9   -3.87818e-06 
> 10   -1.20782e-05   
> 11   -2.655e-06   
> 12   -1.13154e-05  
> 13   -7.53967e-06  
> 14   -8.0584e-06  
> 15   -3.52788e-06 
> 16   -2.35729e-06
> 17   -5.10754e-06
> 18   -6.61478e-05
> 19   -0.000101335
> ...
> 49   -0.000806308 
> Optimizer stop condition: VersorRigid3DTransformOptimizer: Maximum
> number of iterations (50) exceeded.
>           Probe Tag    Starts    Stops             Time (s)
>         Registration           1            1           4509.42
> 
> The thing is, it's not even converging (at least not towards 0).
> And is it okay to get a negative metric as result in the first place?
> 
> Best
> Paul Muetze


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