PERF: Refactor RelabelComponentImageFilter

Parallelized the computation of the label counting with parallel
reduction of the map. Parallelized applying the output production
step. Modernizes the use of STL data structures and
loops. Stream-lined cases. Save iterators used for searching to
exploit coherence in images.

Updated some type to avoid casts. The filter clearly maps input pixels
to output pixels, so those type are used. And the number of objects
type has been changed to SizeValueType.

Modules/Segmentation/ConnectedComponents/include/itkRelabelComponentImageFilter.h

@@ -21,6 +21,7 @@
 #include "itkInPlaceImageFilter.h"
 #include "itkImage.h"
 #include <vector>
+#include <mutex>
 
 namespace itk
 {
@@ -131,14 +132,14 @@ class ITK_TEMPLATE_EXPORT RelabelComponentImageFilter : public InPlaceImageFilte
   itkNewMacro(Self);
 
   /** Type used as identifier for the different component labels. */
-  using LabelType = IdentifierType;
+  using LabelType = InputPixelType;
 
   /** Type used to count number of pixels in objects. */
   using ObjectSizeType = SizeValueType;
 
   /** Get the number of objects in the image. This information is only
    * valid after the filter has executed. */
-  itkGetConstMacro(NumberOfObjects, LabelType);
+  itkGetConstMacro(NumberOfObjects, SizeValueType);
 
   using ObjectSizeInPixelsContainerType = std::vector<ObjectSizeType>;
   using ObjectSizeInPhysicalUnitsContainerType = std::vector<float>;
@@ -148,12 +149,12 @@ class ITK_TEMPLATE_EXPORT RelabelComponentImageFilter : public InPlaceImageFilte
    * the filter has executed. If the caller has not specified a
    * minimum object size, OriginalNumberOfObjects is the same as
    * NumberOfObjects. */
-  itkGetConstMacro(OriginalNumberOfObjects, LabelType);
+  itkGetConstMacro(OriginalNumberOfObjects, SizeValueType);
 
   /** Get/Set the number of objects enumerated and described when the
    * filter is printed. */
-  itkSetMacro(NumberOfObjectsToPrint, LabelType);
-  itkGetConstReferenceMacro(NumberOfObjectsToPrint, LabelType);
+  itkSetMacro(NumberOfObjectsToPrint, SizeValueType);
+  itkGetConstReferenceMacro(NumberOfObjectsToPrint, SizeValueType);
 
   /** Set the minimum size in pixels for an object. All objects
    * smaller than this size will be discarded and will not appear
@@ -254,6 +255,9 @@ class ITK_TEMPLATE_EXPORT RelabelComponentImageFilter : public InPlaceImageFilte
   void
   GenerateData() override;
 
+  void
+  ParallelComputeLabels(const RegionType & inputRegion);
+
   /** RelabelComponentImageFilter needs the entire input. Therefore
    * it must provide an implementation GenerateInputRequestedRegion().
    * \sa ProcessObject::GenerateInputRequestedRegion(). */
@@ -266,46 +270,28 @@ class ITK_TEMPLATE_EXPORT RelabelComponentImageFilter : public InPlaceImageFilte
 
   struct RelabelComponentObjectType
   {
-    LabelType      m_ObjectNumber;
     ObjectSizeType m_SizeInPixels;
-    float          m_SizeInPhysicalUnits;
-  };
 
-  // put the function objects here for sorting in descending order
-  class RelabelComponentSizeInPixelsComparator
-  {
-  public:
-    bool
-    operator()(const RelabelComponentObjectType & a, const RelabelComponentObjectType & b)
+    RelabelComponentObjectType &
+    operator+=(const RelabelComponentObjectType & other)
     {
-      if (a.m_SizeInPixels > b.m_SizeInPixels)
-      {
-        return true;
-      }
-      else if (a.m_SizeInPixels < b.m_SizeInPixels)
-      {
-        return false;
-      }
-      // size in pixels and physical units are the same, sort based on
-      // original object number
-      else if (a.m_ObjectNumber < b.m_ObjectNumber)
-      {
-        return true;
-      }
-      else
-      {
-        return false;
-      }
+      this->m_SizeInPixels += other.m_SizeInPixels;
+      return *this;
     }
   };
 
 private:
-  LabelType      m_NumberOfObjects{ 0 };
-  LabelType      m_NumberOfObjectsToPrint{ 10 };
-  LabelType      m_OriginalNumberOfObjects{ 0 };
+  SizeValueType  m_NumberOfObjects{ 0 };
+  SizeValueType  m_NumberOfObjectsToPrint{ 10 };
+  SizeValueType  m_OriginalNumberOfObjects{ 0 };
   ObjectSizeType m_MinimumObjectSize{ 0 };
   bool           m_SortByObjectSize{ true };
 
+  std::mutex m_Mutex;
+
+  using MapType = std::map<LabelType, RelabelComponentObjectType>;
+  MapType m_SizeMap;
+
   ObjectSizeInPixelsContainerType        m_SizeOfObjectsInPixels;
   ObjectSizeInPhysicalUnitsContainerType m_SizeOfObjectsInPhysicalUnits;
 };