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

[Steve M. Robbins]

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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