[Insight-developers] CastImageFilter

[Luis Ibanez]

Jim,

Maybe I'm missing something...please correct me if I'm wrong.

The Functor type is defined at compile time using the third parameter
of the Unary/Binary/Ternary filters.  This same Functor type is expected
by  the Set/Get Functor methods:

 - SetFunctor( Functor & );
 - Functor & GetFunctor();

These two methods allow to manage Functors that have ivars and
facilitate to users to set this Ivars.   For example, Thresholding can
be done very easily with a functor with line 46 =

    return   A >  m_Threshold ? 0 : 1;

here, the user may want to set the m_Threshold value at will.
But the Functor itself is always of type "ThresholdingFunctor".

We cannot pluggin a "RGBtoFloatRedFunctor" here because the
type will not fit the SetFunctor() method signature.  It is not like
passing a pointer to a base class for calling a virtual function.

It is a bit missleading because it has almost the same API as an
approach of type:

  SetFunctor( FunctorBaseClass * );
  where new functor classes oveload a virtual function "Evaluate()".

The advantage of the current templated approach is that the function
is fully resolved at compile time and the code of line 46 end up
being inlined (at least when optimization is on ).

The polymorphic approach, on the other hand, will cost a virtual call
per pixel which is probably too much for a simple operation like
casting.


    Luis



=====================================

Miller, James V (CRD) wrote:

>Luis,
>
>Why doesn't SetFunctor() work?
>
>I think this should work for the same reason that you can set the comparitor used in
>std::priority_queue
>
>Jim
>
>
>-----Original Message-----
>From: Luis Ibanez [mailto:luis.ibanez@kitware.com]
>Sent: Friday, March 22, 2002 9:54 AM
>To: Miller, James V (CRD)
>Cc: 'Joshua Cates'; Insight-Developers
>Subject: Re: [Insight-developers] CastImageFilter
>
>
>
>Josh, Jim,
>
>
>Miller, James V (CRD) wrote:
>
>>I think you can use the CastImageFilter as it is:
>>
>>1) As long as a cast operator is defined for converting RGB to itk::Vector, it
>>should work out of the box.
>>
>
>That's right, the basic castings are all covered by this filter.
>That is: 
>all the C/C++ normal castings and, as Jim already said, all those involving
>classes for which the cast operator to the output class is defined.
>
>>2) You can call SetFunctor() on the CastImageFilter (defined in UnaryFunctorImageFilter) to set the
>>functor to anyone you would like.
>>
>There is no "SetFunctor()" in this case.
>The Functor has to be defined as a type at compile time.
>It is not set at run time.
>
>>There is no real problem adding the template parameter. Just not sure whether 
>>it is really needed to accomplish what you want.
>>
>
>Adding another template parameter to CastFilter will make it equal to 
>the UnaryFilter.  
>
>If some fancy casting is needed the easiest way to obtain it is to copy 
>the file itkCastingFilter.h
>into  "myCastingFilter" and redefine the "Functor" that is already 
>inside. That will require to
>modify only one line of code: Line 46 where the current conversion is 
>defined as:
>
>   return  static_cast< TOutput >( A );
>
>Let's say casting from RGB to float by extracting only the Red component 
>will be done
>with:
>
>    return static_cast< float >( A[0] );         or
>    return static_cast< float >( A.GetRed() );
>
>
>The whole file is 87 lines  so it is almost to the point were in can be 
>managed by a macro
>that creates the whole filter is we provide : (1) Filter name  (2) code 
>for line 46
>
>
>Luis
>
>
>
>