[ITK Community] [Insight-users] inverse of affine transform using transform->GetInverseTransform();

[Matt McCormick]

Hi John,


> But am I interpreting the results from the inverse correctly here? It is a
> 2D transform.
>
>
>   TransformType::InverseTransformBasePointer inverse =
> transform->GetInverseTransform();
>   TransformType::ParametersType inverseParameters  =
> inverse->GetParameters();
>
>     float a11 = inverseParameters[0];
>     float a12 = inverseParameters[1];
>     float a21 = inverseParameters[2];
>     float a22 = inverseParameters[3];
>     float Tx =  inverseParameters[4]; //Is this correct?
>

Yes, this should be correct.

HTH,
Matt

PS.  Please reply-to-all so the message hits the lists.


>
>     float Ty =  inverseParameters[5];
>     Cx = transform->GetCenter()[0];
>     Cy = transform->GetCenter()[1];
>
> Thank you
> John
>
>
>
>
> On Wed, Feb 12, 2014 at 10:02 AM, Matt McCormick <
> matt.mccormick at kitware.com> wrote:
>
>> Hi John,
>>
>> Could a fully buildable version of the code demonstrating the issue be
>> posted, possibly through a Github Gist [1]?
>>
>> The center of the inverse should be the same, and it is important to set
>> it before calling GetInverse() as documented here: [2].
>>
>> Hope this helps,
>> Matt
>>
>> [1] https://gist.github.com/
>> [2]
>> http://www.itk.org/Doxygen/html/classitk_1_1MatrixOffsetTransformBase.html#a1e97fbe21d53aa805a9692ca9ab95fdb
>>
>>
>> On Wed, Feb 12, 2014 at 12:43 PM, John D <john.dz.eis at gmail.com> wrote:
>>
>>> Sorry. Forgot to include the code in the earlier post
>>>
>>> %The affine parameters come in the vAffineParameters vector to the
>>> function
>>>
>>> typedef itk::AffineTransform< double, 2 > TransformType;
>>>     TransformType::Pointer      transform     = TransformType::New();
>>>     TransformType::ParametersType
>>> affParameters(transform->GetNumberOfParameters());
>>>     for(int i=0;i<6;i++)
>>>         affParameters[i] = vAffineParameters[i];
>>>     transform->SetParameters(affParameters);
>>>
>>>    double center[2];
>>>     center[0] = vAffineParameters[6];
>>>     center[1] = vAffineParameters[7];
>>>     transform->SetCenter(center);
>>>
>>>     TransformType::InverseTransformBasePointer inverse =
>>> transform->GetInverseTransform();
>>>     TransformType::ParametersType inverseParameters  =
>>> inverse->GetParameters();
>>>
>>>     float a11 = inverseParameters[0];
>>>     float a12 = inverseParameters[1];
>>>     float a21 = inverseParameters[2];
>>>     float a22 = inverseParameters[3];
>>>     float Tx = inverseParameters[4]; //Tx and Ty seems to be not correct?
>>>     float Ty = inverseParameters[5]; //Tx and Ty seems to be not
>>> correct?
>>>     Cx = transform->GetCenter()[0]; //Is there a way to get the center
>>> from the inverse transform?
>>>     Cy = transform->GetCenter()[1];
>>>
>>>
>>>
>>> On Wed, Feb 12, 2014 at 9:40 AM, John D <john.dz.eis at gmail.com> wrote:
>>>
>>>> Hello users,
>>>>
>>>>   I have a problem where I need to compute the inverse of an affine
>>>> transform in 2D.
>>>>
>>>> I am seeing a problem in the results. Lets say my affine matrix has
>>>> just pure rotation at 45 degree.
>>>>
>>>> So my A matrix is [cost(45) -sin(45); sin(45 cos(45)]. There is no
>>>> translation and so [Tx Ty] = [0 0].  I am trying to rotate a square that is
>>>> 6000 on each side and my center of rotation is mid point of the square
>>>> [3000 3000]
>>>>
>>>> When I use transform->GetInverseTransform() to find the inverse
>>>> transform
>>>>
>>>> I am getting the correct coordinates for the A matrix. However the
>>>> returned translations in the inverse matrix are wrong. I would have
>>>> expected the Tx, Ty to be [0 0] in the inverse also since rotation is wrt
>>>> to the center. However I am getting [Tx, Ty] = [3000, -1243]
>>>>
>>>> What am I missing? Also when computing the inverse, will the center of
>>>> rotation get changed? How can I get the new center of rotation?
>>>>
>>>> Thanks,
>>>> John
>>>>
>>>>
>>>>
>>>
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>>
>
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