[Insight-users] Transformation in ModelToImageRegistration example

[Karthik Krishnan]

Looks like a bug to me too.

The example seems to swap index and physical points and won't work for 
images with spacing not equal to 1. (for instance if you say 
imageFilter->SetSpacing(..)) and it seems to swap the notion of Fixed 
and moving image.


Bradley Lowekamp wrote:

> Hello all,
>
> I am confused by the transformations going on in the example 
> ModelToImageRegistration.cxx with regards to the custom Metric. Here 
> are some highlights of the code:
>
> In SetMovingSpacialObject():
>       ...
>       typedef itk::ImageRegionConstIteratorWithIndex<TFixedImage> 
> myIteratorType;
>       myIteratorType 
> it(this->m_FixedImage,this->m_FixedImage->GetBufferedRegion());
>       itk::Point<double,2> point;
>       while(!it.IsAtEnd())
>         {
>         for(unsigned int i=0;i<Self::ObjectDimension;i++)
>           {
>           point[i]=it.GetIndex()[i];
>           }
>
>         if(this->m_MovingSpatialObject->IsInside(point,99999))
>           {
>           m_PointList.push_back(point);
>           }
>         ++it;
> ....
> In GetValue():
>        ...
>        PointListType::const_iterator it = m_PointList.begin();
>        ...
>        while(it != m_PointList.end())
>         {
>         PointType transformedPoint = 
> this->m_Transform->TransformPoint(*it);
>         
> this->m_FixedImage->TransformPhysicalPointToIndex(transformedPoint,index);
>         ...
>
> I have been reading and rereading the relevant sections in the ITK 
> software guide book, especially the "Direction of the Transform 
> Mapping". I believe I understand the usage and direction of the 
> transformations is the registration process. Perhaps this is a poor 
> example, which takes some short cuts. But this is my understanding of 
> this code:
>
> The created m_PointList contains a points in "image grid" space. 
> (Slightly ambiguous as to which since the two are already aligned).
>
> Then the m_Transform is applied. This transform maps points from 
> physical space of the fixed image to the physical space of the moving 
> model according to the definition of said transform. (We can overlook 
> the lack of image grid to physical space transformation since it is 
> the identity in both cases.) So "transformedPoint" ought to be in the 
> physical space of the moving model.
>
> Last the fixed image's physical to image grid index transform is 
> applied. So "transformedPoint" must be in fixed image's physical space.
>
> So this is a contradiction, and I am confused. What am I missing here?
>
> Sorry for the repost. I am hoping that this one doesn't get hijacked 
> too :)
>
> Thanks,
> Brad
>
>
>
> ========================================================
>
> Bradley Lowekamp  
>
> Management Systems Designers Contractor for
>
> Office of High Performance Computing and Communications
>
> National Library of Medicine 
>
> blowekamp at mail.nih.gov <mailto:blowekamp at mail.nih.gov>
>
>
>------------------------------------------------------------------------
>
>_______________________________________________
>Insight-users mailing list
>Insight-users at itk.org
>http://www.itk.org/mailman/listinfo/insight-users
>  
>