[Insight-users] Mean Curvature Calculation in Level Set
Luis Ibanez
luis . ibanez at kitware . com
Thu, 20 Nov 2003 21:44:22 -0500
Hi Sah,
The ITK macro itkNotUsed() is converted to an
empty string in most platforms.
A method declaration/implementation like
myMethod( int & itkNotUsed( x ) )
is equivalent to
myMethod( int & )
this second expression is valid C++.
---
ComputeMeanCurvature() is a virtual function.
This means that it is declared in a base class
and it is intended to be overloaded in derived
classes. When a method in the base class
(itkLevelSet) invokes "ComputeMeanCurvature()"
the execution will be delegated to the
"ComputeMeanCurvature()" as defined in the derived
class.
The elegance of the LevelSetFunction implementation
that Josh did in ITK is that the generic computations
are done at the top level in the base class, while
the details of the specific type of partial differential
equation to use are defined in the derived classes.
In this way the common functionality is factorized
in a central location at the level of the base class
and the code doesn't have to be repeated for every new
level set approach. If you look at the derived classes
of SegmentationLevelSet:
http://www . itk . org/Insight/Doxygen/html/classitk_1_1SegmentationLevelSetFunction . html
it becomes more evident how poweful this implementation is.
Note for example that all this 8 different levelSet methods
are sharing the basic math and only changing the way in
which the actual components of the equation are computed.
You may want to look at a good C++ textbook and
check on the description of polymorphism, virtual
functions, and abstract functions. That will give you
a better background for following the implementation
in ITK.
Please let us know if you have further questions.
Regards,
Luis
-------------------
Sah Rayman wrote:
> Hi Lius,
>
> Sorry I am still confused.
>
> First, I also don't use "neighborhood" or "offset" in
> my implementation.
>
> If we take a look at itkLevelSetFunction.txx, Line
> 158-162:
> template <class TImageType>
> typename LevelSetFunction<TImageType>::ScalarValueType
> LevelSetFunction<TImageType>::ComputeMeanCurvature(
> const NeighborhoodType &itkNotUsed(neighborhood),
> const FloatOffsetType &itkNotUsed(offset),
> GlobalDataStruct *gd)
> { ...
> }
>
> Does it euqavalent to the following? (just replace
> itkNotUsed(AnyThing) to EMPTY)
> template <class TImageType>
> typename LevelSetFunction<TImageType>::ScalarValueType
> LevelSetFunction<TImageType>::ComputeMeanCurvature(
> const NeighborhoodType &,
> const FloatOffsetType &, GlobalDataStruct *gd)
> { ...
> }
>
> Is the above a leagal C++ function implementation?
>
> Also, in the same file, function
> ComputeCurvatureTerm() calls
> ComputeMeanCurvature(neighborhood, offset, gd), then
> where is it implemented?
>
> I never used those "advanced" C++ feature before.
> Sorry if it's a naive qustions.
>
> Thanks.
> Sah
>
>
> --- Luis Ibanez <luis . ibanez at kitware . com> wrote:
>
>>Hi Sah,
>>
>>The itkNotUse() macro was used to make some
>>compilers
>>happy and avoid the warnings for "non used variable"
>>in the cases where the arguments were not used in a
>>method. This typically happens in virtual functions
>>when in order to respect the API, a number of
>>arguments
>>are present, but not used inside the method.
>>
>>If your implementation of the method is actually
>>using
>>the argument of the function, you must then remove
>>the
>>itkNotUse() macro, otherwise the argument is not
>>declared
>>at all.
>>
>>The reason why there are not errors in
>>itkLevelSetFunction
>>is because the argument "neighbor" is not used in
>>the
>>ComputeMeanCurvature, so the signature could be as
>>well:
>>
>> ComputeMeanCurvature( const NeighborhoodType & )
>>
>>
>>
>>Regards,
>>
>>
>>
>> Luis
>>
>>
>>
>
> --------------------------------------------------------
>
>>Sah Rayman wrote:
>>
>>>A related question: when I implement
>>>ComputeMeanCurvature() in class
>>
>>MyLevelSetFunction.txx
>>
>>>If I write as
>>>ComputeMeanCurvature(const NeighborhoodType
>>>&itkNotUse(neighborhood), ...)
>>>as it is in itkLevelSetFunction.txx, I will get a
>>
>>lot
>>
>>>syntax error (VC 6.0), and I have to write as
>>>ComputeMeanCurvature(const NeighborhoodType
>>>&neighborhood, ...)
>>>
>>>I checked the definition of itkNotUse(x), it is a
>>>macro to nothing, (#define itkNotUse(x) )
>>>
>>>Then how come in itkLevelSetFunction.txx there is
>>
>>no
>>
>>>syntax error?
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>--- Sah Rayman <sahrayman at yahoo . com> wrote:
>>>
>>>
>>>>I am now 90% sure that this is a bug of
>>>>itkLevelSetFunction.txx as of ITK 1.4.
>>>>
>>>>I rewrote my ComputeMeanCurvature() in my derived
>>>>LevelSetFunction class, and the segmentation
>>>
>>result
>>
>>>>makes more sense.
>>>>
>>>>My fix:
>>>>move line "curvature_term -= gd->m_dx[i] *
>>>>gd->m_dx[j]
>>>>* gd->m_dxy[i][j];"
>>>>to the inner of "if( j!=i) ".
>>>>
>>>>Author of the file, if you're interested in more
>>>>details, please write to me.
>>>>
>>>>
>>>>
>>>>--- Sah Rayman <sahrayman at yahoo . com> wrote:
>>>>
>>>>
>>>>>I am looking at function:
>>>>>
>>>>>template <class TImageType>
>>>>>typename
>>>>>LevelSetFunction<TImageType>::ScalarValueType
>>>>>
>>>>
>>>LevelSetFunction<TImageType>::ComputeMeanCurvature()
>>>
>>>>>in itkLevelSetFunction.txx, line 158 (ITK 1.4)
>>>>>
>>>>>When I compared that to Sethian's "Level Set
>>>>
>>>>Methods
>>>>
>>>>
>>>>>and Fast Marching Methods", 2ndEd, pp. 70, Eq.
>>>>
>>>>6.36,
>>>>
>>>>
>>>>>I
>>>>>found that the C++ implementation contain some
>>>>
>>>>extra
>>>>
>>>>
>>>>>terms in nominator:
>>>>>
>>>>>-dx[0]*dx[0]*dxy[0][0]-dx[1]*dx[1]*dxy[1][1]-...
>>>>>
>>>>>Can someone tell me what are these additional
>>>>
>>>>terms
>>>>
>>>>
>>>>>for? Is the result still mean curvature? Any
>>>>>reference
>>>>>for that?
>>>>>
>>>>>
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