[Insight-users] Centered Affine Transform

[Lucas Lorenzo]

Hi Luis,

I've been running some tests so as to understand a little bit better 
what's this transform doing and it is clear to me now that it is 
applying the translation first and then it is rotating the translated 
image with respect to the specified center of rotation.
My understanding is that it should perform these operations but in 
reverse order.
Please let me know if this is wrong.
Thanks,

Lucas

On Jul 16, 2004, at 2:14 PM, Lucas Lorenzo wrote:

> Hi Luis,
>
> thanks for your feedback.
>
> 1) Till now the way I was setting the parameters was using the 
> m_Transform->SetParameters(m_TransformParameters);
>
> 2) I have a 2D image so my understanding is that I'm going to have 8 
> parameters for this transformation where the first 4 parameters define 
> the rotation, shear, and scaling; the next two define the center of 
> rotation and the last two define the translation to be applied with 
> respect to the center of rotation (after the image has been rotated).
>
> Please, let me know where am I making a mistake.
>
> Thanks,
>
> Lucas
>
>
> On Jul 16, 2004, at 1:17 PM, Luis Ibanez wrote:
>
>>
>> Hi Lucas,
>>
>> It is likely that you are misinterpreting
>> the encoding of the parameters in this
>> transform.
>>
>> Please post the values of the parameters
>> that you are providing, and post also
>> what you are expecting to be the effect.
>>
>> Note that you have:
>>
>>     - Center of rotation
>>     - Translation
>>     - Offset
>>     - Rotation matrix
>>
>>
>> When you use this transform you should only
>> care about SetCenter(), SetTranlation() and
>> Rotation. Never touch the Offset directly.
>>
>>
>> A code snippet illustrating the calls that
>> you are making on the transform will also
>> be very useful....
>>
>>
>>  Thanks
>>
>>
>>    Luis
>>
>>
>> ----------------------
>> Lucas Lorenzo wrote:
>>
>>> 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
>>>> _______________________________________________
>>>> Insight-users mailing list
>>>> Insight-users at itk.org
>>>> http://www.itk.org/mailman/listinfo/insight-users
>>>>
>>> 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
>>> _______________________________________________
>>> Insight-users mailing list
>>> Insight-users at itk.org
>>> http://www.itk.org/mailman/listinfo/insight-users
>>
>>
>>
>>
> 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
>
>
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