[Insight-developers] transform internal types

[Emmanuel Christophe]

Hi,

On a related note, in the itkTransform, the TScalarType is used to
define the InputPointType.

Correct me if I'm wrong, but I think there is a confusion between the
type of the data we are transforming (which could be double,
variableLengthVector, std::complex) and the precision of the
coordinates (which is unlikely to be something else than float or
double).
Not sure if the issue is in the Transform or in the way
itkResampleImageFilter instanciates the transform (with
TInterpolatorPrecisionType as a template).

Emmanuel


On Tue, May 4, 2010 at 22:43,  <M.Staring at lumc.nl> wrote:
> Hi all,
>
> I have a question about the internal floating types that are used in the
> itk::TransformBase and the itk::Transform classes.
>
> Currently, the TransformBase contains the typedef:
>
>  typedef  double  ParametersValueType;
>
> and the Transform:
>
>  typedef  Array2D< double >  JacobianType;
>
> So, the ParametersType and the JacobianType are hard-coded to be
> doubles.
>
> For my application I like them to be floats.
>
> It seems intuitive to use the first template parameter of the Transform
> class for this. This template parameter namely is a PrecisionType /
> ScalarType. This PrecisionType then defines the type of for example the
> input and output points, the rotation matrix for the affine transform,
> and the B-spline weights function type of the B-spline transform. So, in
> the end it determines the precision of the output, regardless of the
> ParametersType being doubles.
>
> For example:
> - The SetParameters() function in the MatrixOffsetTransformBase copies
> the parameters to m_Matrix, which is defined as
>  typedef Matrix<TScalarType, OutputSpaceDimension, InputSpaceDimension>
> MatrixType;
> so, defined as a float matrix if I choose TScalarType = float. The added
> precision of the parameters being double is immediately thrown away.
>
> Of course, using a double type can sometimes add some precision for
> intermediate computations. However, it would be nice to be able to use
> floats if for a specific application float precision is all what is
> needed. But mainly, I would like to pass the parameters to the GPU, and
> not all GPUs support double precision.
>
>
>
> Is there a reason that ParametersValueType and JacobianType are
> hard-coded to be doubles ?
>
> -------------------------
>
> A solution to fix this would be to move the ParametersValueType typedef
> from the TransformBase to the Transform and change it to:
>
>  typedef  TScalarType  ParametersValueType;
>
> And also change
>  typedef  Array2D< TScalarType >  JacobianType;
>
> But I guess that would break backwards compatibility. A solution to that
> could be the use of a cmake variable and some #ifdefs.
>
> Does anyone see objections to these changes, or can you recommend a
> better fix?
>
> Regards,
>
> Marius Staring
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