[Insight-users] Re: VersorRigid3DTransform

[Luis Ibanez]

Hi Radhika,

A) The memory problem that you are encountering is
    due to the fact that the "ParameterType" used by
    ITK transform for representing the array of
    parameters is indeed a vnl_vector<>.

    This is a variable size vector that by default has
    size=0. You must replace the declaration:

TransformType::ParametersType finalParameters;

    with something like:

TransformType::ParametersType finalParameters(
                      transform->GetNumberOfParameters() );

    or something like:

TransformType::ParametersType finalParameters =
                      transform->GetParameters();

     In both cases the vector-size of finalParameters
     will take the correct value.



B)  The value that you provided for the parameter[0]
     doesn't correspond to 10 degrees.  A Versors is
     a unit quaternion. The magnitude of the three
     versor components is equal to sin(angle/2).

     You probably computed

        parameter[0] = 0.174532 = sin( 10 )

     while you should do

        parameter[0] = 0.087155 = sin( 10 / 2 )


C) Yes there is a better way of initializing the transform.
    Please look at the methods of the Versor class:
http://www.itk.org/Insight/Doxygen/html/classitk_1_1Versor.html

    You can simply do:

       TransformType::VersorType  versor;
       versor.SetRotationAroundX( 10.0 * PI / 180.0 );
       transform->SetRotation( versor );

    or you could do

       TransformType::AxisType axis;
       axis[0] = 1.0;
       axis[1] = 0.0;
       axis[2] = 0.0;

       transform->SetRotation( axis, 10.0 * PI / 180.0 );



Please let us know if you have further questions,


    Thanks


      Luis


-------------------------------
Radhika Sivaramakrishna wrote:

> Hi Luis,
> I was trying to use the VersorRigid3DTransform to artificially transform a
> given image by 10 degrees about the center of the image only in the
> X-direction (ie no rotation in Y and Z) and no translation either. I am
> doing this to get a better understanding of this transform because I will
> need to use it to register a large number of unknown cases. Unfortunately, I
> am running into some problems.
> I modified the example in ImageRegistration8.cxx to do this.
> 
> Here is what I did:
> 
> I read in the fixed and moving image (in my case both are from the same
> file). Since no registration is required this time, I only defined a
> Transform of type VersorRigid3DTransform.
> 
> I then set the finalparameters of my transform directly in the following
> way:
> 
> TransformType::ParametersType finalParameters;
> 
> 
> 
>   finalParameters[0] = 0.174532;
>   finalParameters[1] = 0;
>   finalParameters[2] = 0;
>   finalParameters[3] = 128.0;
>   finalParameters[4] = 128.0;
>   finalParameters[5] = 82;
>   finalParameters[6] = 0;
>   finalParameters[7] = 0;
>   finalParameters[8] = 0;
> 
> since I want a 10 degree rotation in X, my image is of size 256x256x164.
> 
> I then did the usual resampling and casting to create my final image.
> However a memory error is reported when I try to set the finalParameters
> directly. Can you tell me what the problem is?
> 
> Is there a better way of artificially creating a known transformation?
> 
> Also, when I get to the actual image registration stage, what is the best
> optimizer for this transform? Is it the VersorTransformOptimizer?
> 
> Thanks
> Radhika
> 
> 
>