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

[Matt McCormick]

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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