[Insight-users] Centered Affine Transform
Lucas Lorenzo
lucas at cvrti.utah.edu
Fri Jul 16 19:11:51 EDT 2004
Hi Luis,
I'm selecting the spatial coordinates of the center of the object I'm
interested in.
Lucas
On Jul 16, 2004, at 5:06 PM, Luis Ibanez wrote:
> Hi Lucas,
>
>
> What center of rotation did you selected ?
>
> Luis
>
>
> ---------------------
> Lucas Lorenzo wrote:
>
>> 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
>
>
>
>
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
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