[Insight-users] multi-resolution pyramid - origin problem

[Lars Friedrich Lars]

Hello Luis,

thank you for your kind explanation!

Although I'm pretty sure that the LandmarkInitializedMutualInformationRegistration InsightApplication considers the origins of the compared images, it might not be the best tool for comparing spatial image shifts in sub-millimeter space. The visual misplacements I presumed to see are possibly due to the effects of gaussian smoothing of sharp-edged object within the original image ...

Thank you again!

regards,

lars


-------- Original-Nachricht --------
> Datum: Mon, 19 Apr 2010 18:39:23 -0400
> Von: Luis Ibanez <luis.ibanez at kitware.com>
> An: Lars Friedrich Lars <lars-friedrich at gmx.net>
> CC: insight-users at itk.org
> Betreff: Re: [Insight-users] multi-resolution pyramid - origin problem

> Hi Lars,
> 
> Thanks for pointing this out.
> 
> The origins are actually computed correctly,
> 
> Although it is understandable that you may be surprised by the
> way the origin of the image changes at every resolution level.
> 
> Here is the rationale for recomputing the image origin:
> 
> We want to make sure that if the image contains an object,
> the center of mass of that object doesn't change its physical
> coordinates at every resolution level.
> 
> The sub-pixel refinement of the image origin must be done
> at every resolution level in order to ensure this stability
> property.  (It actually took a lot of hard work to get it to
> behave this way)
> 
> 
> The image will "look" shifted if you look at them with
> a viewer that doesn't take into account the coordinates
> of the image origin.
> 
> You may want to look at them with:
> 
> A) Paraview (www.paraview.org), and/or
> B) Slicer (www.slicer.org)
> 
> 
>    Regards,
> 
> 
>           Luis
> 
> 
> -------------------------------------------------------------------------------------
> On Fri, Apr 16, 2010 at 2:59 AM, Lars Friedrich Lars
> <lars-friedrich at gmx.net
> > wrote:
> 
> > Hello,
> >
> > when using itk::MultiResolutionPyramidImageFilter for shrinking a 2D
> image
> > over 3 levels (schedule 4 4 / 2 2 / 1 1) I recognized that the resultant
> > output origins of the images in level 0 (4 4) and level 1 (2 2) seem to
> be
> > 'incorrect'.
> >
> > original image (=level 2):
> > origin=+205,-205 dimension=410,410 spacing=1,1 direction=1 0 0 1
> >
> > --> pyramid output:
> > level 0:
> > ORIGIN=+206.5,-203.5 dimension=102,102 spacing=4,4 direction=1 0 0 1
> > level 1:
> > ORIGIN=+205.5,-204.5 dimension=205,205 spacing=2,2 direction=1 0 0 1
> >
> > To my mind the outputs at levels 0 and 1 seem to be incorrectly shifted
> by
> > 0.5*originalspacing.
> > --> desired output origins:
> > level 0:
> > ORIGIN=+206,-204
> > level 1:
> > ORIGIN=+205,-205
> >
> > When I compare the output images from level 0 and 1 to the original
> image,
> > I can see the shift visually. Manually changing the output to the
> desired
> > values 'fixes' the problem, and the overlay of the compared images shows
> a
> > better visual agreement.
> >
> > This is the code fragment from itkMultiResolutionPyramidImageFilter.txx
> > which is responsible for the shift:
> > //Now compute the new shifted origin for the updated levels;
> > const typename OutputImageType::PointType::VectorType outputOriginOffset
> >         =(inputDirection*(outputSpacing-inputSpacing))*0.5;
> >
> >
> > Is this a bug or a feature? Does the shift account for smoothing-effects
> of
> > the internally used discrete gaussian smoothing filter or
> shrinking-effects
> > of the shrink image filter?
> >
> > Thank you!
> >
> > regards,
> >
> > lars
> >
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Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01