[Insight-developers] GaussianDerivativeImageFunction does not transform points from physical space to pixel space in Evaluate(..)

[Andinet Enquobahrie]

Hello Dan,

Thank you for pointing out the bug and for your detailed information. 
Not only Evaluate(PointType ) method, but also 
EvaluateAtContinuousIndex(..) method was doing incorrect transformation. 
I entered a bug (# 2891) report on this and just committed a fix to 
cvs.  Please, update your cvs version and give it a try. Let us know if 
you face any other problems using these functions.

-Andinet

> Hi all,
>
> For my application I need to calculate the image derivative at a 
> number of physical locations. I have found two suitable 
> ImageFunctions: CentralDifferenceImageFunction and 
> GaussianDerivativeImageFunction. The GaussianDerivativeImageFunction 
> is more suitable to my application because it uses a Gaussian kernel 
> to smooth noise artifacts.
>
> Unfortunately, there seems to be a discrepancy with the 
> GaussianDerivativeImageFunction::Evaluate(const PointType& point) 
> function. The Evaluate(const PointType& point) function should 
> evaluate the function at the given physical point (ie. taking into 
> account the image spacing and origin). For example see 
> itkCentralDifferenceImageFunction.h  which - via 
> itk::ImageFunction::ConvertPointToNearestIndex(..) - uses 
> itk::Image::TransformPhysicalPointToContinuousIndex(..) to transform 
> the physical point to a ContinuousIndex in pixel space before evaluation.
>
> It appears that GaussianDerivativeImageFunction *does not* transform 
> the point from physical space to pixel space before evaluation. If you 
> look at the Evaluate(..) function in the 
> itkGaussianDerivativeImageFunction.txx file, the physical point is 
> cast to an index (the rounding differences are used as the offset to 
> recompute the kernel) and then itk::Image::GetPixel(const IndexType 
> &index) is invoked - no transformation from physical space to pixel 
> space occurred!
>
> Have I missed something, or is this an omission?
>
> Thanks for your help.
>
> Dan
> -- 
> Dan Mueller (d.mueller[at]qut.edu.au)
> School of Engineering Systems (ES)
> Faculty of Built Environment and Engineering (BEE)
> Queensland University of Technology (QUT)
> CRICOS No: 00213J
>
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