[Insight-users] Re: itkVTKtoITKtoVTK modification

[Mathieu Lamard]

Thank Luis,

Your mail is very clear and now it works fine !

Thanks again

Mathieu


Luis Ibanez wrote:
> 
> Hi Mathieu,
> 
> 
> I just reproduced the error that you reported.
> 
> Here is what is happening :
> 
> 1) ITK uses exceptions for managing error conditions.
> Your modifications are conceptually fine but they are
> generating a pixel type conflict between VTK and ITK,
> this results in an exception being thrown.
> 
> Iin general you may want to put ITK code inside
> try/catch blocks. Specially  the code that updates filters.
> 
> Typically this will look like:
> 
> try
> {
>    myFilter->Update()
> }
> catch( itk::ExceptionObject & e )
> {
>    std::cerr << "Exception catched " << e << std::endl;
> }
> 
> 
> (Note that the exception is catched by reference)
> 
> 
> 
> 2) After adding this try/catch block,
> here is the message that you can see for the thrown exception:
> 
> 
>  Exception catched !!
>  itk::ExceptionObject (0x84e60c0)
>  Location: "Unknown"
>  File: /home/ibanez/test/Insight/Code/BasicFilters/itkVTKImageImport.txx
>  Line: 203
>  Description: itk::ERROR: VTKImageImport(0x84d1368):
>  Input scalar type is unsigned short but should be float
> 
> 
> The last line specifies that there is a conflict of types at
> the input of the VTKImageImport...
> 
> VTK is templated over types internally while ITK templates are
> exposed to the user.  VTK makes decisions on types at run time
> by using internal switch  statements on the filters.  ITK asks the
> user to make those decisions at compile time.
> 
> When you replaced the Noise source image by a PNGReader you
> implicitly replaced the type of the input image. PNG images are
> typically (not always...) read as "unsigned short".  Because VTK
> made this decision at run time, your compilation passed Ok.
> 
> When you execute the pipeline, VTK reads the file and place the
> data in an "unsigned short" image. Then it tries to pass it to the
> Exporter but the exporter is templated over type and is already
> expecting a "float" image.  Not knowing what to do, it throws an
> exception with the hope that the user will catch it in a try/catch
> block and solve the problem somehow.
> 
> 
> The solution is to change the type of the image pixel on line 127:
> 
>  typedef itk::Image<float, 2> ImageType;
>  typedef itk::VTKImageImport<ImageType> ImageImportType;
> 
> 
> replacing "float" by "unsigned short" will prevent the exception
> from happening.
> 
> but....
> 
> at compile time you will find that the itk::CurvatureFlowImageFilter
> actually expect the input image to be of pixel type float or double.
> This fact will produce a large number of warnings related to
> assignemes from 'double' to 'unsigned short'. They look like:
> 
> itkCurvatureFlowFunction.txx:101: warning: assignment
>   to `short unsigned int' from `double'
> 
> So now you will have to introduce a CastImageFilter in between
> the vtkImageLuminance filter and the VTKImageImport filter.
> This Cast filter should convert the input image from "unsigned
> short" pixels to "float" pixels.
> 
> Note that Cast is pretty naive in the sense that no effort is made
> to adjust the intensity range of the images. It will just do a
> pixel-wise cast on the image. If you want to adjust the intensity
> range the itkShiftScaleImageFilter could do that for you.
> 
> Once you introduce this Cast filter the pipeline should
> execute ok.
> 
> A modified version of itkVTKtoITKtoVTK.cxx has been
> checked  in. If you update  your cvs checkout you will find
> 
>        itkPNGVTKtoITKtoVTK.cxx
> 
> Which is based on the code that you posted to the list.
> 
> You will also see a modified CMakeList.txt (that defines an
> executable for  this example)
> 
> This executable is requiring two PNG files right now, so you
> will have to change the filenames in lines : 121 and 223  in
> order to set any of your PNG files.
> 
> =============
> 
> A Comment on Casting:
> 
> The use of CastFilters is a trade-off between speed and memory.
> When you use a CastFilter you are paying for an extra copy of
> the image on memory but the process should be quite fast. 
> If memory is a concern, you may use ImageAdaptors instead. 
> http://www.itk.org/Doxygen/html/classitk_1_1ImageAdaptor.html
> 
> ImageAdaptors allow to make an image of type TA look like
> an image of type TB.
> 
> This is done by an internal conversion on the iterators that access
> the pixels on the image. So each time that you read a pixel, the
> iterator will do the cast for you.   Using ImageAdaptors allow to
> avoid the extra memory consumption of the copied image but
> will be slower at computing time.
> 
> ImageAdaptors can also be used to compute pixel-wise functions
> on images, see for example:
> 
> http://www.itk.org/Doxygen/html/classitk_1_1SinImageAdaptor.html
> 
> 
> 
> Now...
> most ITK filters are templated over the input image type and the
> output image type. So, most of them will do casting for you.
> 
> Whether this Casting is exactly what you wanted to do or not....
> well, this is another question     :-)
> 
> 
> 
> 
> Please let us know if you encounter any other problem
> with this code
> 
> 
> Thanks
> 
> 
>   Luis
> 
> 
> 
> 
> 
>