BUG: incorrect neighbor determination in scan-line derived filters

Thanks to @DVigneault for finding the bug and creating a MWE to demonstrate it

Modules/Filtering/ImageLabel/include/itkScanlineFilterCommon.h

@@ -236,12 +236,21 @@ class ScanlineFilterCommon
   {
     // This checks whether the line encodings are really neighbors. The first
     // dimension gets ignored because the encodings are along that axis.
+    SizeValueType diffSum = 0;
     for (unsigned i = 1; i < OutputImageDimension; i++)
     {
-      if (Math::abs(A[i] - B[i]) > 1)
+      SizeValueType diff = Math::abs(A[i] - B[i]);
+      if (diff > 1)
       {
         return false;
       }
+      diffSum += diff;
+    }
+
+    assert(diffSum > 0); // we are checking potential neighbors, index difference must exist
+    if (!this->m_FullyConnected)
+    {
+      return (diffSum <= 1); // indices can differ only along one dimension
     }
     return true;
   }

Modules/Segmentation/ConnectedComponents/test/CMakeLists.txt

@@ -83,5 +83,6 @@ itk_add_test(NAME itkMaskConnectedComponentImageFilterTest
 
 set(ITKConnectedComponentsGTests
         itkRelabelComponentImageFilterGTest.cxx
+        itkConnectedComponentImageFilterGTest.cxx
         )
 CreateGoogleTestDriver(ITKConnectedComponents "${ITKConnectedComponents-Test_LIBRARIES}" "${ITKConnectedComponentsGTests}")

Modules/Segmentation/ConnectedComponents/test/itkConnectedComponentImageFilterGTest.cxx

@@ -0,0 +1,86 @@
+/*=========================================================================
+ *
+ *  Copyright Insight Software Consortium
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0.txt
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *=========================================================================*/
+
+
+#include "itkPrintHelper.h"
+using itk::print_helper::operator<<;
+
+#include "itkGTest.h"
+#include "itkImage.h"
+#include "itkConnectedComponentImageFilter.h"
+
+#include <bitset>
+
+namespace
+{
+typename itk::Image<unsigned char, 3>::Pointer
+CreateTestImageA(void)
+{
+  std::bitset<8> bits(105); // 3D Checkerboard: 01101001
+
+  using namespace itk::GTest::TypedefsAndConstructors::Dimension3;
+
+  using PixelType = unsigned char;
+  using ImageType = itk::Image<PixelType, Dimension>;
+
+  auto image = ImageType::New();
+  image->SetRegions(ImageType::RegionType(MakeSize(2u, 2u, 2u)));
+  image->Allocate(true);
+
+  for (size_t i = 0; i < 8; ++i)
+  {
+    std::bitset<3> idx(i);
+    image->SetPixel({ { idx[0], idx[1], idx[2] } }, bits[i]);
+  }
+
+  return image;
+}
+} // namespace
+
+
+TEST(ConnectedComponentImageFilter, checkerboard_3D)
+{
+  auto image = CreateTestImageA();
+  using ImageType = decltype(image)::ObjectType;
+
+  auto connected = itk::ConnectedComponentImageFilter<ImageType, ImageType>::New();
+  connected->SetInput(image);
+  connected->FullyConnectedOff();
+  connected->Update();
+
+  itk::ImageRegionConstIterator<ImageType> it(connected->GetOutput(),
+                                              connected->GetOutput()->GetLargestPossibleRegion());
+
+  EXPECT_EQ(it.Get(), 1);
+  ++it;
+  EXPECT_EQ(it.Get(), 0);
+  ++it;
+  EXPECT_EQ(it.Get(), 0);
+  ++it;
+  EXPECT_EQ(it.Get(), 2);
+  ++it;
+  EXPECT_EQ(it.Get(), 0);
+  ++it;
+  EXPECT_EQ(it.Get(), 3);
+  ++it;
+  EXPECT_EQ(it.Get(), 4);
+  ++it;
+  EXPECT_EQ(it.Get(), 0);
+  ++it;
+  EXPECT_TRUE(it.IsAtEnd());
+}