[Insight-developers] Simplifying traits

[Joshua Cates]

I also agree that this is good idea.  We are running the risk of
alienating users by requiring so many levels of indirection for simple
data access.

Josh.
______________________________
 Josh Cates			
 School of Computer Science	
 University of Utah
 Email: cates@cs.utah.edu
 Phone: (801) 587-7697
 URL:   www.cs.utk.edu/~cates


On Thu, 29 Mar 2001, Miller, James V (CRD) wrote:

> I am still leaning towards eliminating scalar traits. I would really like to get back to standard
> pointer/iterator semantics.
> 
> foo = *it;
> *it = garf;
> 
> The effect of this would be:
> 
> 1. DataAccessors would have to return a reference.  This would mean that data accessors could only be
> used to map memory (i.e. access a scalar from a pixel that held a scalar and vector).  DataAccessors
> could not be used for casting or for calculating a function of a pixel.
> 
> 2. A given filter would assume its inputs were of the correct "type".  If a filter is expecting to
> work on scalars, dereferencing an iterator would have to return a scalar.  If a filter is expecting
> to work on vectors, derefencing an iterator would have to return a vector. If an algorithm needed
> both scalars and vectors, they would be passed in as separate inputs.
> 
> The reason for leaning in this direction is that the syntax for accessing a pixel within a filter is
> getting rather complicated. Furthermore, we are having reference and const issues in the current
> framework.  Last night I was working on a line of code that read
> 
> value = (double) ScalarTraits<PixelType>::GetScalar( it.Get() );
> 
> and the compiler was giving the error:
> 
> GetScalar() cannot convert parameter 1 from "float" to "float &"
> 
> Note that this error wasn't even a const issue! Just a reference issue.
> 
> Jim
> 
> 
> 
> 
> 
> -----Original Message-----
> From: Brad King [mailto:brad.king@kitware.com]
> Sent: Friday, March 23, 2001 4:37 PM
> To: Insight Developers
> Subject: [Insight-developers] Simplifying traits
> 
> 
> Hello, all:
> 
> Here are some ideas to consider before deciding to throw out the use of
> ScalarTraits an VectorTraits.  Our current approach is not the simplest
> approach to using traits.
> 
> We can make traits easier to use.  Consider the following filter:
> 
> template <class TInputImage, class TOutputImage>
> class MyFilter
> {
> public:
>   typedef TInputImage InputImageType;
>   typedef typename InputImageType::PixelType InputPixelType;
>   typedef ScalarTraits<InputPixelType> PixelScalarTraits;
>   typedef typename PixelScalarTraits::ScalarValueType
>           PixelScalarValueType;
>   
>   inline static PixelScalarValueType& GetPixelScalar(PixelType& p)
>     { return PixelScalarTraits::GetScalar(p); }
> 
>   inline static void SetPixelScalar(PixelType& p,
>                                     const PixelScalarValueType& v)
>     { PixelScalarTraits::SetScalar(p, v); }
> 
>   // ....
> };
> 
> Note that the function definitions can be put in ImageToImageFilter and be
> inherited into other filters.  Then, with minimal setup work, a filter
> implementation can access a Pixel's scalar value like this:
> 
> PixelScalarValueType v = GetPixelScalar(myPixel);
>   // do something to v.
> SetPixelScalar(myPixel, v);
> 
> This is in place of the current code:
> 
> typename ScalarTraits<PixelType>::ScalarValueType v =
>   ScalarTraits<PixelType>::GetScalar(myPixel);
>   // do something to v.
> ScalarTraits<PixelType>::SetScalar(myPixel, v);
> 
> Also, I have noticed that there are a bunch of extra ScalarTrait
> specializations for more than the basic types.  An example is
> "ScalarTraits< RGBPixel<float> >".  These should not be done, because if
> someone wants to use an RGBPixel with a template argument other than those
> defined in itkPixelTraits.h, they will have to figure out how to write
> their own trait specialization for it.  Currently, this can be solved (I
> think) by putting the proper typedefs (ValueType, etc) into RGBPixel and
> letting the primary template of ScalarTraits handle it from there.
> 
> The "real" way to do such a specialization of the ScalarTraits for
> RGBPixel would be to do a partial specialization:
> 
> template <typename TComponent>
> class ScalarTraits< RGBPixel<TComponent> > { /* ... */ };
> 
> Unfortunately, not all of our compilers support partial specialization, so
> we can't do this.
> 
> The other option is to not require scalar traits to be a speicalization of
> ScalarTraits at all.  If instead we make such traits a template argument,
> then any class can be defined and passed to the argument:
> 
> template <class TPixel, unsigned int VDimension,
>           class TPixelTraits = ScalarTraits<TPixel> >
> class Image
> {
> public:
>   typedef TPixel PixelType;
>   enum { Dimension = VDimension };
>   typedef TPixelTraits PixelTraits;
>   // ....
> };
> 
> Then, filters could pull the Pixel traits out of their image template
> parameters:
> 
> template <class TInputImage, class TOutputImage>
> class MyFilter
> {
> public:
>   typedef TInputImage InputImageType;
>   typedef typename InputImageType::PixelTraits InputPixelTraits;
>   // ... use traits as in previous MyFilter example.
> };
> 
> Making traits a template argument is currently the approach that the Mesh
> class uses.  It even encapsulates the "CellTraits" as a member of the
> "MeshTraits" template argument.  We could make the same relationship for
> the Image.  That is:
> 
> template <typename TPixel, unsigned int VDimension>
> class DefaultImageTraits
> {
> public:
>   typedef TPixel PixelType;
>   enum { Dimension = VDimension };
>   typedef DefaultPixelTraits<PixelType> PixelTraits;
>   // ... other useful traits?
> };
> 
> template <typename TPixel,
>           unsigned int VDimension,
>           class TImageTraits = DefaultImageTraits<TPixel, VDimension> >
> class Image
> {
> public:
>   typedef TImageTraits ImageTraits;
>   typedef typename ImageTraits::PixelType   PixelType;
>   typedef typename ImageTraits::PixelTraits PixelTraits;
> 
>   // ...
> };
> 
> Assume that we have encapsulated both the current scalar and vector traits
> into "DefaultPixelTraits".
> 
> Thoughts, anyone?
> -Brad
> 
> 
> 
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