[Insight-users] Centered Affine Transform

[Lucas Lorenzo]

Hi Gavin,

thanks a lot for your answer.

I'm setting the parameters using

	m_Transform->SetParameters(m_TransformParameters);

So, afterwards I'm adding

	m_Transform->ComputeOffset();

But still the results are not as expected (after rotation the object is  
not been translated to the desired location). Is there anything wrong  
in what I'm doing ?
Thanks,

Lucas

On Jul 15, 2004, at 10:07 PM, Gavin Baker wrote:

>
> Hi Lucas,
>
> On Thu, Jul 15, 2004 at 06:01:26PM -0600, Lucas Lorenzo wrote:
>
>> I'm trying to use a Centered Affine Transform. So first I'm taking a
>> look at the following example:
>>
>> Examples/Registration/ImageRegistration9.cxx.
>>
>> I'm a little bit confused on how to use the ComputeOffset() method
>> because I don't see where it is been used in this example and when
>> reading the online documentation my understanding is that it is
>> mandatory to use this particular method.
>>
>> Could anyone please help me to get a better understanding on this  
>> issue
>> ?
>
> When using the centered affine transform, there is an implicit  
> translation
> due to the center.  The offset takes into account the affect of any
> rotations you specify on this center, to give a new translation.  The
> ComputeOffset() method recalcuates this offset (which need only be done
> once), and needs to be updated before applying the transform.
>
> This was discussed a while ago:
>
>    
> http://public.kitware.com/pipermail/insight-users/2003-October/ 
> 005230.html
>    
> http://public.kitware.com/pipermail/insight-users/2003-October/ 
> 005272.html
>
>> From the comments at the top of ImageRegistration9.cxx (from the  
>> Software
> Guide) show:
>
>   //                      Note that the final total offset of the  
> transform
>   //  is to be computed as a combination of the shift due rotation  
> plus the
>   //  explicit translation set on the transform.
>   //
>
> General usage would be:
>
>   xform->Translate( delta );
>   xform->Rotate( theta );
>   // ...
>   xform->ComputeOffset();
>
>   resample->Update();
>
>
> ComputeOffset() is actually called explicitly in all the
> CenteredAffineTransform methods that affect the transform, but  
> (obviously)
> not any AffineTransform methods.  It isn't called automagically (after  
> each
> change) since it only needs to be done once, although it probably  
> wouldn't
> hurt if it were.
>
>   :: Gavin
>
> -- 
> Gavin Baker                                      Complex Systems Group
> http://www.cs.mu.oz.au/~gavinb             The University of Melbourne
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>
Lucas Lorenzo

University of Utah
Nora Eccles Harrison CardioVascular Research and Training Institute
Fellows Room
95 South 2000 East
Salt Lake City, UT 84112-5000

e-mail:  lucas at cvrti.utah.edu
telephone: 801-587-9536