[Insight-developers] What are the bounds of an itkImageRegion?

[Luis Ibanez]

Hi Steve,


Image Regions are defined in the digital space of the image grid,
not in the continous space of physical coordinates.


The region defined by

         start = 1
         size  = 2

includes only the two pixels of indices :

         a)   index = 1
         b)   index = 2

not the continous range of real coordinates

        [1.0, 2.99)


The point at 2.99 is not part of that region.

In fact, this ties up to the discussion on how we define the
center of a pixel, and how we compute rounding.

The currect code in itkImageRegion is:


   /** Test if an index is inside */
   template <typename TCoordRepType>
   bool
   IsInside(const ContinuousIndex<
              TCoordRepType,VImageDimension> &index) const
     {
     for(unsigned int i=0; i<ImageDimension; i++)
       {
       if( index[i] < static_cast<TCoordRepType>( m_Index[i] ) )
         {
         return false;
         }
       // bound is the last valid pixel location
       const TCoordRepType bound = static_cast<TCoordRepType>(
                  m_Index[i] + static_cast<long>(m_Size[i]) - 1);

       if( index[i] > bound )
         {
         return false;
         }
       }
     return true;
     }


We have to review our definition of Voronoi regions
associated to the image grid, before we can tell
if this is a bug or not.


We plan to discuss this next week at the NAMIC meeting...


    Regards,


        Luis


-------------------------
Steve M. Robbins wrote:
> Hi,
> 
> While pondering issue 6558 "Physical coordinates of a pixel - Severe
> inconsistency and bug in ImageBase", I started writing some
> tests for itkImageRegion.
> 
> I came to the conclusion that either I misunderstand this class or
> there's a more fundamental bug in there.
> 
> The class doc says:
> 
>  * \brief An image region represents a structured region of data.
>  *
>  * ImageRegion is an class that represents some structured portion or
>  * piece of an Image. The ImageRegion is represented with an index and
>  * a size in each of the n-dimensions of the image. (The index is the
>  * corner of the image, the size is the lengths of the image in each of
>  * the topological directions.)
> 
> Consider a 1-dimensional region with start=1, size=2.  I interpreted
> the above to mean that the region is the half-open interval [1,3) on
> the real line.  Or maybe it's the closed interval [1,3] ?
>
> In either case, I expected 2.99 to be included in the region,
> but it isn't as the following test-case shows (written
> using Boost.Test; c.f. my earlier post from this evening).
> 
> 
> BOOST_AUTO_TEST_CASE( testContinuousIndexPointIsInsideEdgeConvention )
> {
>     Fixture1 line( 1, 2 );
>     Fixture1::CIndexType p0, p1, p2, p3;
> 
>     p0[0] = 0.99;
>     p1[0] = 1.01;
>     p2[0] = 2.99;
>     p3[0] = 3.01;
> 
>     BOOST_CHECK( ! line.mRegion.IsInside( p0 ) );
>     BOOST_CHECK(   line.mRegion.IsInside( p1 ) );
>     BOOST_CHECK(   line.mRegion.IsInside( p2 ) );
>     BOOST_CHECK( ! line.mRegion.IsInside( p3 ) );
> }
> 
> 
> See below for the Fixture code.  This test fails on the p2 line -- it
> is treated as being outside the region.
> 
> By the way, is there any concensus to fix this?  I believe the code
> should be modified to follow the ITK Software Guide; i.e. to
> use pixel-centre convention, in which case the test needs to
> be modified to:
> 
>     p0[0] = 0.49;
>     p1[0] = 0.51;
>     p2[0] = 2.49;
>     p3[0] = 2.51;
> 
> 
> Regards,
> -Steve
> 
> P.S.  The Fixture code follows.
> 
> template< unsigned int VImageDimension >
> struct Fixture
> {
>     typedef itk::ImageRegion<VImageDimension>            RegionType;
>     typedef typename RegionType::IndexType               IndexType;
>     typedef typename RegionType::SizeType                SizeType;
>     typedef itk::ContinuousIndex<float,VImageDimension>  CIndexType;
> 
>     RegionType mRegion;
> };
> 
> struct Fixture1 : public Fixture<1>
> {
>     Fixture1( int start0,
> 	      int size0 )
>     {
> 	IndexType start = {{ start0 }};
> 	SizeType  size  = {{ size0 }};
> 	mRegion = RegionType( start, size );
>     }
> };
> 
> 
> 
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> 
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