ENH: Throw exception if "NumberOfResolutions" parameter has value zero

Distinguished clearly between zero and one. In this case, the value zero must be a mistake.

Improved consistency between fixed and moving pyramid.

Discussed at our weekly internal elastix sprint meeting.

Components/Registrations/MultiMetricMultiResolutionRegistration/elxMultiMetricMultiResolutionRegistration.hxx

@@ -49,6 +49,11 @@ MultiMetricMultiResolutionRegistration<TElastix>::BeforeRegistration()
   /** Set the number of resolutions.*/
   unsigned int numberOfResolutions = 3;
   this->m_Configuration->ReadParameter(numberOfResolutions, "NumberOfResolutions", 0);
+  if (numberOfResolutions == 0)
+  {
+    itkExceptionMacro("The NumberOfResolutions parameter must have a non-zero value!");
+  }
+
   this->SetNumberOfLevels(numberOfResolutions);
 
   /** Set the FixedImageRegions to the buffered regions. */

Components/Registrations/MultiResolutionRegistration/elxMultiResolutionRegistration.hxx

@@ -54,6 +54,11 @@ MultiResolutionRegistration<TElastix>::BeforeRegistration()
   /** Set the number of resolutions. */
   unsigned int numberOfResolutions = 3;
   this->m_Configuration->ReadParameter(numberOfResolutions, "NumberOfResolutions", 0);
+  if (numberOfResolutions == 0)
+  {
+    itkExceptionMacro("The NumberOfResolutions parameter must have a non-zero value!");
+  }
+
   this->SetNumberOfLevels(numberOfResolutions);
 
   /** Set the FixedImageRegion. */

Components/Registrations/MultiResolutionRegistrationWithFeatures/elxMultiResolutionRegistrationWithFeatures.hxx

@@ -38,6 +38,10 @@ MultiResolutionRegistrationWithFeatures<TElastix>::BeforeRegistration()
   /** Set the number of resolutions. */
   unsigned int numberOfResolutions = 3;
   this->m_Configuration->ReadParameter(numberOfResolutions, "NumberOfResolutions", 0);
+  if (numberOfResolutions == 0)
+  {
+    itkExceptionMacro("The NumberOfResolutions parameter must have a non-zero value!");
+  }
   this->SetNumberOfLevels(numberOfResolutions);
 
   /** Set the FixedImageRegions to the buffered regions. */

Components/elxGenericPyramidHelper.h

@@ -65,7 +65,7 @@ class GenericPyramidHelper
     configuration.ReadParameter(numberOfResolutions, "NumberOfResolutions", 0, true);
     if (numberOfResolutions == 0)
     {
-      numberOfResolutions = 1;
+      itkGenericExceptionMacro("The NumberOfResolutions parameter must have a non-zero value!");
     }
 
     /** Create a default schedule. Set the numberOfLevels first. */

Core/ComponentBaseClasses/elxFixedImagePyramidBase.hxx

@@ -103,7 +103,7 @@ FixedImagePyramidBase<TElastix>::SetFixedSchedule()
   configuration.ReadParameter(numberOfResolutions, "NumberOfResolutions", 0, true);
   if (numberOfResolutions == 0)
   {
-    numberOfResolutions = 1;
+    itkExceptionMacro("The NumberOfResolutions parameter must have a non-zero value!");
   }
 
   /** Create a default schedule. Set the numberOfLevels first. */

Core/ComponentBaseClasses/elxMovingImagePyramidBase.hxx

@@ -104,9 +104,8 @@ MovingImagePyramidBase<TElastix>::SetMovingSchedule()
   configuration.ReadParameter(numberOfResolutions, "NumberOfResolutions", 0, true);
   if (numberOfResolutions == 0)
   {
-    log::error("ERROR: NumberOfResolutions not specified!");
+    itkExceptionMacro("The NumberOfResolutions parameter must have a non-zero value!");
   }
-  /** \todo quit program? Actually this check should be in the ::BeforeAll() method. */
 
   /** Create a default schedule. Set the numberOfLevels first. */
   this->GetAsITKBaseType()->SetNumberOfLevels(numberOfResolutions);

Core/Main/GTesting/itkElastixRegistrationMethodGTest.cxx

@@ -203,6 +203,47 @@ GTEST_TEST(itkElastixRegistrationMethod, IsDefaultInitialized)
 }
 
 
+// Tests that the value zero is rejected for the "NumberOfResolutions" parameter.
+GTEST_TEST(itkElastixRegistrationMethod, RejectZeroValueForNumberOfResolution)
+{
+  constexpr auto ImageDimension = 2U;
+  using PixelType = float;
+  using ImageType = itk::Image<PixelType, ImageDimension>;
+  const itk::Size<ImageDimension> imageSize{ { 5, 6 } };
+
+  elx::DefaultConstruct<elx::ParameterObject>                     parameterObject{};
+  elx::DefaultConstruct<ElastixRegistrationMethodType<ImageType>> registration{};
+
+  registration.SetFixedImage(CreateImage<PixelType>(imageSize));
+  registration.SetMovingImage(CreateImage<PixelType>(imageSize));
+  registration.SetParameterObject(&parameterObject);
+
+  elx::ParameterObject::ParameterMapType parameterMap{ // Parameters in alphabetic order:
+                                                       { "ImageSampler", { "Full" } },
+                                                       { "MaximumNumberOfIterations", { "2" } },
+                                                       { "Metric", { "AdvancedNormalizedCorrelation" } },
+                                                       { "Optimizer", { "AdaptiveStochasticGradientDescent" } },
+                                                       { "Transform", { "TranslationTransform" } }
+  };
+
+  parameterObject.SetParameterMap(parameterMap);
+
+  // OK: "NumberOfResolutions" is unspecified, so use its default value.
+  EXPECT_NO_THROW(registration.Update());
+
+  for (const unsigned int numberOfResolutions : { 1, 2 })
+  {
+    // OK: Use a value greater than zero.
+    parameterMap["NumberOfResolutions"] = { std::to_string(numberOfResolutions) };
+    EXPECT_NO_THROW(registration.Update());
+  }
+
+  // Expected to be rejected: the value zero.
+  parameterMap["NumberOfResolutions"] = { "0" };
+  EXPECT_THROW(registration.Update(), itk::ExceptionObject);
+}
+
+
 // Tests registering two small (5x6) binary images, which are translated with respect to each other.
 GTEST_TEST(itkElastixRegistrationMethod, Translation)
 {