[ITK] [ITK-users] Polar2Cartesian Behavior

[Somesh]

Thanks Matt. This is very helpful.

I had modeled my transform class on itkAzimuthElevationToCartesianTransform
which also inherited from Affine transform.
Should this class also override GetTransformCategory(). It doesn't look
like its doing it now.
https://itk.org/Doxygen/html/itkAzimuthElevationToCartesianTransform_8h_source.html

Thanks,
Somesh

On Sun, Sep 11, 2016 at 9:47 PM, Matt McCormick <matt.mccormick at kitware.com>
wrote:

> On Sun, Sep 11, 2016 at 9:19 PM, Somesh <someshsandbox at gmail.com> wrote:
> > Thanks Matt. In step 3, I generate a polar image and in Step 4 I want to
> get
> > the corresponding cartesian image. Since in Step 3, the maximum radius
> of my
> > polar image is R=20 , so I have the cartesian image range from -R to R
> for
> > both X and Y and I set the default pixel to 0. The pixels in the polar
> image
> > have value equal to R. So I expected a circular image similar to step 1.
> > However I get a distorted/cropped semi-circle.
>
> It looks like your Transform is inheriting from itk::AffineTransform,
> although it is not an AffineTransform. AffineTransform is a linear
> transform, and the ResampleImageFilter takes advantage of that
> property to optimize its computation:
>
>   https://github.com/InsightSoftwareConsortium/ITK/blob/
> 6ee10575445aa1c878797db4febf7d775a38f1f3/Modules/Filtering/
> ImageGrid/include/itkResampleImageFilter.hxx#L242-L249
>
> So, your class need to override ->GetTransformCategory() to return
> UnknownTransformCategory.
>
>
> > The use case of this program is the following. I have a cylindrical
> image. I
> > have to re-sample it at finer intervals in theta. Then I want to convert
> it
> > to cartesian co-ordinates. Given a polar to cartesian transform,
> shouldn't
> > the resample image filter be able to handle this ?
>
> Yes, but the SpecialCoordinatesImage allows you to keep your data in
> native form and it prevents abuse of "spacing", etc.
>
>
> > Regarding the CurvilinearArraySpecialCoordinatesImage class. How can I
> use
> > it to convert a cylindrical image to Cartesian image ? Since it is an
> > "Image" class and not a "Transform" class, I am a little unclear on how
> to
> > use it. Should I manually create the desired cartesian grid, find the
> > co--responding polar co-ordinate using
> > CurvilinearArraySpecialCoordinatesImage APIs and fill in the pixels.
> Also do
> > have have to manually handle the interpolation ? Or can I still use the
> > ResampleImageFileter to do these steps ?
>
> Yes, you can use ResampleImageFilter and this class to perform the
> desired operations. Here is an example:
>
>  https://github.com/KitwareMedical/ITKUltrasound/blob/
> 0cb92288333a9779da57511f779d0bde5ca35275/test/
> itkCurvilinearArraySpecialCoordinatesImageTest.cxx#L96-L110
>
> HTH,
> Matt
>
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