[Insight-users] itkMatrix error: discards qualifiers??

[Luis Ibanez]

Hi Michiel,

You don't need to get that direction matrix as a reference.

If you get it as a copy, things should go smoothly and
the compiler should be happy.


Instead of:

   const ImageType::DirectionType & direction = image->GetDirection();

you need to do:

   ImageType::DirectionType   direction = image->GetDirection();

then you modify "direction", to apply your rotation,
and when you are done,
you set it back to the image by calling:

             image->SetDirection(   direction  );



    Regards,


         Luis

-------------------------------------------------------------------------------
On Tue, Mar 30, 2010 at 6:14 AM, michiel mentink
<michael.mentink at st-hughs.ox.ac.uk> wrote:
> Dear Luis,
>
> thank you for your reply, now I understand what is meant!
>
> Direction is from an image:
>
>   const ImageType::DirectionType & direction = image->GetDirection();
>
> I'm trying to rotate the image by just changing the direction matrix of the
> image.
>
> Unfortunately, when I just leave out 'const' from that line, the compiler
> complains
> that it's not the same type as expected.
>
> Maybe I should just create a new matrix, copy the value of
> image->GetDirection()
> into it, apply the rotation, and use image->SetDirection()?
>
> cheers, Michael
>
>
> On Mon, Mar 29, 2010 at 11:04 PM, Luis Ibanez <luis.ibanez at kitware.com>
> wrote:
>>
>> Hi Michiel,
>>
>>
>> It is excellent that you are diving into the code
>> looking for deeper understanding !
>>
>>
>> The message that you get indicates that you are
>> attempting to change the value of a variable that
>> has been declared "const" previously.
>>
>> The "const" declaration is a promise made to the
>> compiler, stating that you will not change the value
>> of that variable.
>>
>> When the compiler catch you breaking that promise,
>> it get angry at you and gives you the error message:
>>
>> "error: passing ‘const   X to  Z discards qualifiers"
>>
>>
>> Where the "qualifier" refers to the "const" property
>> of that variable.
>>
>>
>> The error message indicates that you are calling
>> the operator=() method on a const matrix (as left
>> argument of the = ).
>>
>>
>> You did so, with an expression such as:
>>
>>
>>  const MatrixType  myConstMatrixThatWillNeverChange;
>>
>>  myConstMatrixThatWillNeverChange = anotherMatrix;
>>
>>
>> This may also happens implicitly if the matrix
>> happens to be a member variable of a class,
>> and you attempt to do such assignment inside
>> a "const" method of the class.
>>
>>
>> If you share with use the exact declaration of your
>> "direction" matrix we could tell you more about how
>> to get around the problem.
>>
>>
>>       Regards,
>>
>>
>>             Luis
>>
>>
>>
>> --------------------------------------------------------------------------
>> On Mon, Mar 29, 2010 at 12:01 PM, michiel mentink
>> <michael.mentink at st-hughs.ox.ac.uk> wrote:
>> >
>> > In an autodidactic effort, I'm trying to rotate an image by 'hand', by
>> > multiplying the 'direction' matrix of an image by a rotation matrix.
>> >
>> > To do that, I'm copying bits out of itkEuler3DTransform.txx
>> > In that file, a RotationX matrix is defined:
>> >
>> > Matrix<TScalarType,3,3> RotationX
>> >
>> > However, when I try doing that, I'm getting the following error:
>> >
>> > error: passing ‘const itk::Matrix<double, 3u, 3u>’ as ‘this’ argument of
>> > ‘const itk::Matrix<T, NRows, NColumns>& itk::Matrix<T, NRows,
>> > NColumns>::operator=(const itk::Matrix<T, NRows, NColumns>&) [with T =
>> > double, unsigned int NRows = 3u, unsigned int NColumns = 3u]’ discards
>> > qualifiers
>> >
>> > 1) What does this mean?
>> > 2) How do I get my beloved Matrix?
>> >
>> > #include <itkMatrix.h>
>> > #include <vnl/vnl_matrix.h>
>> >
>> >   typedef itk::Matrix<double,3,3> Matrix;
>> >
>> >   Matrix RotationX;
>> >  // ImageType::DirectionType RotationX;
>> >
>> > //  vnl_matrix_fixed<double,3,3> RotationX;
>> >
>> >   RotationX[0][0] = 1; RotationX[0][1] =   0; RotationX[0][2] = 0;
>> >   RotationX[1][0] = 0; RotationX[1][1] =  cx; RotationX[1][2] = sx;
>> >   RotationX[2][0] = 0; RotationX[2][1] = -sx; RotationX[2][2] = cx;
>> >
>> >   direction = direction*RotationX;
>> >
>> > cheers,
>> >
>> > Michael
>> >
>> >
>> >
>> >
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