ENH: Added GetCenterPoint and SetCenterPoint to EllipseSpatialObject

Wraps access to ellipseSpatialObject->GetObjectToWorldTransform()->m_Offset.

These new member functions allow simplifying the implementation, as well as
use cases of itk::HoughTransform2DCirclesImageFilter::GetCircles(). With this
commit, the implementation, test, and example of the HoughTransform2DCircles
filter are adapted to make use of them.

Note that this change silently breaks old ITK user code (ITK < 5.0) calling
HoughTransform2DCirclesImageFilter::GetCircles().

Triggered by comments from Tim Evain at ITK Discourse, starting at
https://discourse.itk.org/t/hough-transform-2d-circles-image-filter-getcircles-patch/350/46

Change-Id: I49a9f0504c0fc85aeefddd813fcabca309bd09e8

Examples/Segmentation/HoughTransform2DCirclesImageFilter.cxx

@@ -215,7 +215,7 @@ int main( int argc, char *argv[] )
   while( itCircles != circles.end() )
     {
     std::cout << "Center: ";
-    std::cout << (*itCircles)->GetObjectToParentTransform()->GetOffset()
+    std::cout << (*itCircles)->GetCenterPoint()
               << std::endl;
     std::cout << "Radius: " << (*itCircles)->GetRadius()[0] << std::endl;
     // Software Guide : EndCodeSnippet
@@ -231,15 +231,14 @@ int main( int argc, char *argv[] )
          angle <= itk::Math::twopi;
          angle += itk::Math::pi/60.0 )
       {
-      using TransformType = HoughTransformFilterType::CircleType::TransformType;
-      using OffsetType = TransformType::OutputVectorType;
-      const OffsetType offset =
-        (*itCircles)->GetObjectToParentTransform()->GetOffset();
+      const HoughTransformFilterType::CircleType::PointType centerPoint =
+        (*itCircles)->GetCenterPoint();
+      using IndexValueType = ImageType::IndexType::IndexValueType;
       localIndex[0] =
-         itk::Math::Round<long int>(offset[0]
+         itk::Math::Round<IndexValueType>(centerPoint[0]
                     + (*itCircles)->GetRadius()[0]*std::cos(angle));
       localIndex[1] =
-         itk::Math::Round<long int>(offset[1]
+         itk::Math::Round<IndexValueType>(centerPoint[1]
                     + (*itCircles)->GetRadius()[0]*std::sin(angle));
       OutputImageType::RegionType outputRegion =
                                   localOutputImage->GetLargestPossibleRegion();

Modules/Core/SpatialObjects/include/itkEllipseSpatialObject.h

@@ -68,6 +68,12 @@ class ITK_TEMPLATE_EXPORT EllipseSpatialObject:
   /** Get radii via an array of radius values */
   itkGetConstReferenceMacro(Radius, ArrayType);
 
+  /* Returns the center point (in world coordinates) of the ellipse */
+  PointType GetCenterPoint()  const;
+
+  /* Set the center point (in world coordinates) of the ellipse */
+  void SetCenterPoint(const PointType& point);
+
   /** Returns a degree of membership to the object.
    *  That's useful for fuzzy objects. */
   bool ValueAt(const PointType & point, double & value,
@@ -109,6 +115,7 @@ class ITK_TEMPLATE_EXPORT EllipseSpatialObject:
   /** Print the object informations in a stream. */
   void PrintSelf(std::ostream & os, Indent indent) const override;
 };
+
 } // end namespace itk
 
 #ifndef ITK_MANUAL_INSTANTIATION

Modules/Core/SpatialObjects/include/itkEllipseSpatialObject.hxx

@@ -51,6 +51,27 @@ EllipseSpatialObject< TDimension >
     }
 }
 
+template< unsigned int TDimension >
+auto
+EllipseSpatialObject<TDimension>::GetCenterPoint() const
+-> PointType
+{
+  PointType originPoint;
+  originPoint.Fill(0);
+  // GetObjectToWorldTransform() never returns nullptr, no need to check.
+  return originPoint + this->GetObjectToWorldTransform()->GetOffset();
+}
+
+template< unsigned int TDimension >
+void
+EllipseSpatialObject<TDimension>::SetCenterPoint(const PointType& point)
+{
+  PointType originPoint;
+  originPoint.Fill(0);
+  // GetModifiableObjectToWorldTransform() never returns nullptr, no need to check.
+  this->GetModifiableObjectToWorldTransform()->SetOffset(point - originPoint);
+}
+
 /** Test whether a point is inside or outside the object
  *  For computational speed purposes, it is faster if the method does not
  *  check the name of the class and the current depth */

Modules/Filtering/ImageFeature/include/itkHoughTransform2DCirclesImageFilter.hxx

@@ -248,10 +248,10 @@ HoughTransform2DCirclesImageFilter< TInputPixelType, TOutputPixelType, TRadiusPi
       Circle->SetId(static_cast<int>( circles ));
       Circle->SetRadius( m_RadiusImage->GetPixel( indexOfMaximum ) );
 
-      CircleType::VectorType center;
+      CircleType::PointType center;
       center[0] = indexOfMaximum[0];
       center[1] = indexOfMaximum[1];
-      Circle->GetObjectToParentTransform()->SetOffset(center);
+      Circle->SetCenterPoint(center);
       Circle->ComputeBoundingBox();
 
       m_CirclesList.push_back(Circle);

Modules/Filtering/ImageFeature/test/itkHoughTransform2DCirclesImageTest.cxx

@@ -198,21 +198,14 @@ namespace
       return false;
     }
 
-    const CircleType::TransformType* const transform1 = circle1->GetObjectToParentTransform();
-    const CircleType::TransformType* const transform2 = circle2->GetObjectToParentTransform();
-
-    if ( (transform1 == nullptr) || (transform2 == nullptr) )
-    {
-      std::cout << "A GetObjectToParentTransform() call appears to be incorrect!" << std::endl;
-      return false;
-    }
-
     bool success = true;
 
-    const itk::Vector<double, 2>& center1 = transform1->GetOffset();
-    const itk::Vector<double, 2>& center2 = transform2->GetOffset();
+    using PointType = CircleType::PointType;
+
+    const PointType& centerPoint1 = circle1->GetCenterPoint();
+    const PointType& centerPoint2 = circle2->GetCenterPoint();
 
-    if (center1 != center2)
+    if (centerPoint1 != centerPoint2)
     {
       // The choice of the radius image type should not affect the center estimation.
       std::cout << "center1 and center2 should be equal, even while the radius image types differ!"