AW: [Insight-users] Fw: Mean Squares Metric Computation does not match

[Stein Daniel]

Hi Emma,

 

A mean square error of 958 over a scale of 0-255 says that, in the mean,
the difference is about 31. You will receive this solution if you just
take the root, you still have the squares.

 

Regards,

 

Daniel

 

---

 

Dipl.-Inform. Med. Daniel Stein
German Cancer Research Center
Div. Medical and Biological Informatics (E130)
Im Neuenheimer Feld 280
D-69120 Heidelberg, Germany
Phone:  (+49) 6221/42-3547
Fax:    (+49) 6221/42-2345
http://www.dkfz-heidelberg.de/de/mbi/
Homepage: http://www.dkfz-Heidelberg.de/de/mbi/people/Daniel_Stein.html

 

 

________________________________

Von: insight-users-bounces+d.stein=dkfz-heidelberg.de at itk.org
[mailto:insight-users-bounces+d.stein=dkfz-heidelberg.de at itk.org] Im
Auftrag von Emma Ryan
Gesendet: Donnerstag, 16. August 2007 15:20
An: insight-users at itk.org
Betreff: [Insight-users] Fw: Mean Squares Metric Computation does not
match

 

Hi,

   I posted this a few days ago and did not get response. I was
wondering if anyone tried the same experiments on their data ?

Essentially, if you have any results of final metric values from
synthetic tests using registration/optimizations algorithms and by using
direct matrix computations of known transforms,
please let me know. 

Thank you,
Emma

----- Forwarded Message ----
From: Emma Ryan <eryanvtk at yahoo.com>
To: insight-users at itk.org
Sent: Tuesday, August 14, 2007 2:01:58 PM
Subject: Mean Squares Metric Computation does not match

Hi,

   I use versor-based 3D rigid registration and gradient descent -based
affine registration algos to register two 3D volumes.

The metric is mean squares and interpolator type is linear.

To perform a cross-check on the final metric received after
registration, I -recompute the metric values as follows :

1.  Grayscale images
MetricType::Pointer         metricOrig           = MetricType::New();

TransformType::Pointer  transformOrig = TransformType::New();    

metricOrig->SetInterpolator(interpolator);
metricOrig->SetTransform(transformOrig);
metricOrig->SetFixedImage(fixedImageOrig);
metricOrig->SetMovingImage(movingImageOrig); 
metricOrig->SetFixedImageRegion(fixedImageOrig->GetBufferedRegion());
MetricType::TransformParametersType finalParam =
registration->GetLastTransformParameters();
metricOrig->Initialize();    
std::cout<<"MetricOnOriginalImage =
"<<metricOrig->GetValue(finalParam)<<std::endl;
        

For the above code, I get metric values like 50 and 90 (for two
different datasets )whereas the bestValue = optimizer->GetValue()
returned by the optimizer (after registration) is 0.70 and 1.70
respectively.
How does one explain this ? Especially when the Mean Square Error is the
MEAN and not the total error ?



2. If the volumes sent to the registrator were binary, and the resultant
transform is to be applied to binary moving image.

 MetricType::Pointer         metricBin           = MetricType::New();


 metricBin->SetInterpolator(interpolator);
 metricBin->SetTransform(transformOrig);
 metricBin->SetFixedImage(fixedBinImage);
 metricBin->SetFixedImageRegion(fixedImage->GetBufferedRegion());
 metricBin->SetMovingImage(movingBinImage); 
 metricBin->Initialize();
std::cout<<"MetricOnBinImage =
"<<metricBin->GetValue(finalParam)<<std::endl;



Then I get values of metric = 958 and 220 (for two different datasets)
when the optimizer returns a value of 32 and 90 respectively. 

So my questions are :

a)  How does one explain a MEAN square error of 958 over a scale of
0-255 ?
b) For the binary images, when I compute the mean square error using
other softwares, I get a value of 5. Whereas ITK optimizer returns the
final metric value at 32. Both softwares use linear interpolator. I dont
think I should get such huge differences even if I were to uses nearest
neighbor .
c) In an earlier itk posting,
 
http://public.kitware.com/pipermail/insight-users/2005-July/014045.html,
Lydia mentions that this would be due to roundoff errors, but it does
not explain large differences.

Any clues ?

Emma

 

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