BUG: Add ImageAdaptor::ComputeOffset, to fix ImageRegionIterator support

Fixed issue #5870,
"ImageAdaptor should have its own ComputeOffset, ImageRegionIterator support
broken"

`ImageRegionConstIterator<AdapterType>::Increment()` depends on
`AdapterType::ComputeOffset`.

Author: N-Dekker

Modules/Core/ImageAdaptors/include/itkImageAdaptor.h

@@ -234,6 +234,13 @@ class ITK_TEMPLATE_EXPORT ImageAdaptor : public ImageBase<TImage::ImageDimension
   IndexType
   ComputeIndex(OffsetValueType offset) const;
 
+  /** Computes the offset from the beginning of the buffer. */
+  OffsetValueType
+  ComputeOffset(const IndexType & index) const
+  {
+    return m_Image->ComputeOffset(index);
+  }
+
   /** PixelContainer type alias support Used to construct a container for
    * the pixel data. */
   using PixelContainer = typename TImage::PixelContainer;

Modules/Core/ImageAdaptors/test/itkImageAdaptorGTest.cxx

@@ -17,11 +17,12 @@
  *=========================================================================*/
 
 // First include the header file to be tested:
-#include "itkImageBase.h"
+#include "itkImageAdaptor.h"
 
 #include "itkCovariantVector.h"
 #include "itkImage.h"
-#include "itkImageAdaptor.h"
+#include "itkImageRegionConstIteratorWithIndex.h"
+#include "itkImageRegionIterator.h"
 #include "itkVector.h"
 
 #include <gtest/gtest.h>
@@ -185,3 +186,70 @@ TEST(ImageAdapter, TransformMemberFunctionsReturnSameAsForImage)
   const auto image3D = itk::Image<unsigned char, 3>::New();
   Expect_Transform_member_functions_return_the_same_for_an_ImageAdapter_as_for_its_image(*image3D);
 }
+
+
+// Tests that ComputeOffset(index) returns the same value for an adaptor as for its internal image.
+TEST(ImageAdaptor, ComputeOffset)
+{
+  constexpr unsigned int dimension{ 2 };
+  using PixelType = int;
+  using ImageType = itk::Image<PixelType, dimension>;
+
+  const auto image = ImageType::New();
+  const auto adaptor = itk::ImageAdaptor<ImageType, DummyPixelAccessor<PixelType>>::New();
+
+  // Use a very small image region, to speed up the unit test.
+  const itk::ImageRegion imageRegion{ itk::Index<dimension>::Filled(1), itk::Size<dimension>::Filled(2) };
+
+  adaptor->SetImage(image);
+  adaptor->SetRegions(imageRegion);
+  adaptor->AllocateInitialized();
+
+  itk::ImageRegionConstIteratorWithIndex<ImageType> it(image, imageRegion);
+  for (it.GoToBegin(); !it.IsAtEnd(); ++it)
+  {
+    EXPECT_EQ(adaptor->ComputeOffset(it.GetIndex()), image->ComputeOffset(it.GetIndex()));
+  }
+}
+
+
+// Tests that ImageAdaptor supports iteration by means of ImageRegionIterator.
+TEST(ImageAdaptor, SupportsRegionIterator)
+{
+  constexpr unsigned int dimension{ 2 };
+  using PixelType = int;
+  using ImageType = itk::Image<PixelType, dimension>;
+
+  const auto image = ImageType::New();
+  const auto adaptor = itk::ImageAdaptor<ImageType, DummyPixelAccessor<PixelType>>::New();
+
+  adaptor->SetImage(image);
+  adaptor->SetRegions(itk::Size<dimension>::Filled(4));
+  adaptor->AllocateInitialized();
+
+  constexpr auto         regionIndex = itk::Index<dimension>::Filled(1);
+  constexpr auto         regionSize = itk::Size<dimension>::Filled(2);
+  const itk::ImageRegion region{ regionIndex, regionSize };
+
+  // Set the pixel values by means of the adaptor, for the specified region.
+  PixelType           pixelValue{};
+  constexpr PixelType maxPixelValue{ regionSize.CalculateProductOfElements() };
+
+  using AdaptorType = itk::ImageAdaptor<ImageType, DummyPixelAccessor<PixelType>>;
+  for (itk::ImageRegionIterator<AdaptorType> iterator(adaptor, region); !iterator.IsAtEnd(); ++iterator)
+  {
+    ++pixelValue;
+    ASSERT_LE(pixelValue, maxPixelValue);
+    iterator.Set(pixelValue);
+    EXPECT_EQ(iterator.Get(), pixelValue);
+  }
+
+  // Now check if the intenal image has got the expected pixel values, in the specified region.
+  PixelType expectedPixelValue{};
+
+  for (itk::ImageRegionConstIterator<ImageType> iterator(image, region); !iterator.IsAtEnd(); ++iterator)
+  {
+    ++expectedPixelValue;
+    EXPECT_EQ(iterator.Get(), expectedPixelValue);
+  }
+}