STYLE: Add constexpr to if (Dimension ...) statements

Made it clearer that the condition of these `if` statements can be evaluated at
compile-time. Cases found by the regular expression ` if \(\w*Dimension `.

Also addressed `-Wunused-but-set-variable` warnings from ITK.Linux
(Ubuntu 9.4.0-1ubuntu1~20.04.2) saying:

> itkSobelOperator.hxx: warning: variable 'center' set but not used
> itkJensenHavrdaCharvatTsallisPointSetMetricTest.cxx: warning: variable 'metricValues2D' set but not used
> itkJensenHavrdaCharvatTsallisPointSetMetricTest.cxx: warning: variable 'metricValues3D' set but not used

Modules/Core/Common/include/itkCrossHelper.h

@@ -51,13 +51,13 @@ class CrossHelper
   {
     VectorType oCross;
 
-    if (Dimension > 2)
+    if constexpr (Dimension > 2)
     {
       oCross[0] = iU[1] * iV[2] - iV[1] * iU[2];
       oCross[1] = iV[0] * iU[2] - iU[0] * iV[2];
       oCross[2] = iU[0] * iV[1] - iV[0] * iU[1];
 
-      if (Dimension > 3)
+      if constexpr (Dimension > 3)
       {
         for (unsigned int dim = 3; dim < Dimension; ++dim)
         {

Modules/Core/Common/include/itkIndex.h

@@ -521,7 +521,7 @@ operator<<(std::ostream & os, const Index<VDimension> & obj)
   {
     os << obj[i] << ", ";
   }
-  if (VDimension >= 1)
+  if constexpr (VDimension >= 1)
   {
     os << obj[VDimension - 1];
   }

Modules/Core/Common/include/itkOffset.h

@@ -464,7 +464,7 @@ operator<<(std::ostream & os, const Offset<VDimension> & ind)
   {
     os << ind[i] << ", ";
   }
-  if (VDimension >= 1)
+  if constexpr (VDimension >= 1)
   {
     os << ind[VDimension - 1];
   }

Modules/Core/Common/include/itkPoint.hxx

@@ -263,7 +263,7 @@ std::ostream &
 operator<<(std::ostream & os, const Point<T, TPointDimension> & vct)
 {
   os << '[';
-  if (TPointDimension == 1)
+  if constexpr (TPointDimension == 1)
   {
     os << vct[0];
   }

Modules/Core/Common/include/itkSize.h

@@ -435,7 +435,7 @@ operator<<(std::ostream & os, const Size<VDimension> & obj)
   {
     os << obj[i] << ", ";
   }
-  if (VDimension >= 1)
+  if constexpr (VDimension >= 1)
   {
     os << obj[VDimension - 1];
   }

Modules/Core/Common/include/itkSobelOperator.hxx

@@ -28,43 +28,46 @@ SobelOperator<TPixel, VDimension, TAllocator>::Fill(const CoefficientVector & co
 {
   this->InitializeToZero();
 
-  // Note that this code is only good for 2d and 3d operators.  Places the
-  // coefficients in the exact center of the neighborhood
-  const unsigned int center = this->GetCenterNeighborhoodIndex();
-
-  if (VDimension == 3)
+  if constexpr (VDimension == 2 || VDimension == 3)
   {
-    unsigned int coeff_index = 0;
-    for (int z = -1; z <= 1; ++z)
+    // Note that this code is only good for 2d and 3d operators.  Places the
+    // coefficients in the exact center of the neighborhood
+    const unsigned int center = this->GetCenterNeighborhoodIndex();
+
+    if constexpr (VDimension == 3)
     {
-      for (int y = -1; y <= 1; ++y)
+      unsigned int coeff_index = 0;
+      for (int z = -1; z <= 1; ++z)
       {
-        for (int x = -1; x <= 1; ++x)
+        for (int y = -1; y <= 1; ++y)
         {
-          const int pos = center + z * this->GetStride(2) + y * this->GetStride(1) + x * this->GetStride(0);
+          for (int x = -1; x <= 1; ++x)
+          {
+            const int pos = center + z * this->GetStride(2) + y * this->GetStride(1) + x * this->GetStride(0);
 
-          this->operator[](pos) = static_cast<TPixel>(coeff[coeff_index]);
+            this->operator[](pos) = static_cast<TPixel>(coeff[coeff_index]);
 
-          ++coeff_index;
+            ++coeff_index;
+          }
         }
       }
     }
-  }
-  else if (VDimension == 2)
-  {
-    unsigned int coeff_index = 0;
-    for (int y = -1; y <= 1; ++y)
+    else // So now VDimension == 2
     {
-      for (int x = -1; x <= 1; ++x)
+      unsigned int coeff_index = 0;
+      for (int y = -1; y <= 1; ++y)
       {
-        const int pos = center + y * this->GetStride(1) + x * this->GetStride(0);
-        // Note, The following line copies the double precision
-        // coefficients of SobelOperator to the pixel type
-        // of the neighborhood operator which may not support
-        // negative numbers, or floating point numbers.
-        this->operator[](pos) = static_cast<TPixel>(coeff[coeff_index]);
-
-        ++coeff_index;
+        for (int x = -1; x <= 1; ++x)
+        {
+          const int pos = center + y * this->GetStride(1) + x * this->GetStride(0);
+          // Note, The following line copies the double precision
+          // coefficients of SobelOperator to the pixel type
+          // of the neighborhood operator which may not support
+          // negative numbers, or floating point numbers.
+          this->operator[](pos) = static_cast<TPixel>(coeff[coeff_index]);
+
+          ++coeff_index;
+        }
       }
     }
   }

Modules/Core/Common/include/itkVector.hxx

@@ -167,7 +167,7 @@ std::ostream &
 operator<<(std::ostream & os, const Vector<T, TVectorDimension> & vct)
 {
   os << '[';
-  if (TVectorDimension == 1)
+  if constexpr (TVectorDimension == 1)
   {
     os << vct[0];
   }

Modules/Core/Common/test/itkNeighborhoodAlgorithmTest.cxx

@@ -133,7 +133,7 @@ NeighborhoodAlgorithmTest()
   region.SetIndex(ind);
 
   size.Fill(5);
-  if (VDimension > 1)
+  if constexpr (VDimension > 1)
   {
     size[VDimension - 1] = 1;
   }
@@ -184,7 +184,7 @@ NeighborhoodAlgorithmTest()
     return false;
   }
 
-  if (VDimension == 2)
+  if constexpr (VDimension == 2)
   {
     ind[0] = 0;
     ind[1] = 249;

Modules/Core/ImageAdaptors/test/itkVectorImageTest.cxx

@@ -864,7 +864,7 @@ itkVectorImageTest(int, char * argv[])
         failed = true;
       }
 
-      if (Dimension == 3)
+      if constexpr (Dimension == 3)
       {
 
         //

Modules/Core/SpatialObjects/include/itkMetaBlobConverter.hxx

@@ -114,7 +114,7 @@ MetaBlobConverter<VDimension>::SpatialObjectToMetaObject(const SpatialObjectType
     Blob->GetPoints().push_back(pnt);
   }
 
-  if (VDimension == 2)
+  if constexpr (VDimension == 2)
   {
     Blob->PointDim("x y red green blue alpha");
   }

Modules/Core/SpatialObjects/include/itkMetaContourConverter.hxx

@@ -172,11 +172,11 @@ MetaContourConverter<VDimension>::SpatialObjectToMetaObject(const SpatialObjectT
     contourMO->GetControlPoints().push_back(pnt);
   }
 
-  if (VDimension == 2)
+  if constexpr (VDimension == 2)
   {
     contourMO->ControlPointDim("id x y xp yp v1 v2 r g b a");
   }
-  else if (VDimension == 3)
+  else if constexpr (VDimension == 3)
   {
     contourMO->ControlPointDim("id x y z xp yp zp v1 v2 v3 r gn be a");
   }
@@ -201,11 +201,11 @@ MetaContourConverter<VDimension>::SpatialObjectToMetaObject(const SpatialObjectT
     contourMO->GetInterpolatedPoints().push_back(pnt);
   }
 
-  if (VDimension == 2)
+  if constexpr (VDimension == 2)
   {
     contourMO->InterpolatedPointDim("id x y r g b a");
   }
-  else if (VDimension == 3)
+  else if constexpr (VDimension == 3)
   {
     contourMO->InterpolatedPointDim("id x y z r g b a");
   }

Modules/Core/SpatialObjects/include/itkMetaDTITubeConverter.hxx

@@ -278,7 +278,7 @@ MetaDTITubeConverter<VDimension>::SpatialObjectToMetaObject(const SpatialObjectT
     {
       pnt->AddField("v1x", it->GetNormal1InObjectSpace()[0]);
       pnt->AddField("v1y", it->GetNormal1InObjectSpace()[1]);
-      if (VDimension == 3)
+      if constexpr (VDimension == 3)
       {
         pnt->AddField("v1z", it->GetNormal1InObjectSpace()[2]);
       }
@@ -288,7 +288,7 @@ MetaDTITubeConverter<VDimension>::SpatialObjectToMetaObject(const SpatialObjectT
     {
       pnt->AddField("v2x", it->GetNormal2InObjectSpace()[0]);
       pnt->AddField("v2y", it->GetNormal2InObjectSpace()[1]);
-      if (VDimension == 3)
+      if constexpr (VDimension == 3)
       {
         pnt->AddField("v2z", it->GetNormal2InObjectSpace()[2]);
       }
@@ -298,7 +298,7 @@ MetaDTITubeConverter<VDimension>::SpatialObjectToMetaObject(const SpatialObjectT
     {
       pnt->AddField("tx", it->GetTangentInObjectSpace()[0]);
       pnt->AddField("ty", it->GetTangentInObjectSpace()[1]);
-      if (VDimension == 3)
+      if constexpr (VDimension == 3)
       {
         pnt->AddField("tz", it->GetTangentInObjectSpace()[2]);
       }

Modules/Core/SpatialObjects/include/itkMetaLandmarkConverter.hxx

@@ -112,7 +112,7 @@ MetaLandmarkConverter<VDimension>::SpatialObjectToMetaObject(const SpatialObject
     landmarkMO->GetPoints().push_back(pnt);
   }
 
-  if (VDimension == 2)
+  if constexpr (VDimension == 2)
   {
     landmarkMO->PointDim("x y red green blue alpha");
   }

Modules/Core/SpatialObjects/include/itkMetaLineConverter.hxx

@@ -134,11 +134,11 @@ MetaLineConverter<VDimension>::SpatialObjectToMetaObject(const SpatialObjectType
     lineMO->GetPoints().push_back(pnt);
   }
 
-  if (VDimension == 2)
+  if constexpr (VDimension == 2)
   {
     lineMO->PointDim("x y v1x v1y v2x v2y red green blue alpha");
   }
-  else if (VDimension == 3)
+  else if constexpr (VDimension == 3)
   {
     lineMO->PointDim("x y z v1x v1y v1z v2x v2y v2z red green blue alpha");
   }

Modules/Core/SpatialObjects/include/itkMetaSurfaceConverter.hxx

@@ -123,11 +123,11 @@ MetaSurfaceConverter<VDimension>::SpatialObjectToMetaObject(const SpatialObjectT
     surfaceMO->GetPoints().push_back(pnt);
   }
 
-  if (VDimension == 2)
+  if constexpr (VDimension == 2)
   {
     surfaceMO->PointDim("x y v1 v2 red green blue alpha");
   }
-  else if (VDimension == 3)
+  else if constexpr (VDimension == 3)
   {
     surfaceMO->PointDim("x y z v1 v2 v3 red green blue alpha");
   }

Modules/Core/SpatialObjects/include/itkSurfaceSpatialObject.hxx

@@ -194,7 +194,7 @@ SurfaceSpatialObject<TDimension, TSurfacePointType>::ComputeNormals()
       PointType v2 = this->m_Points[identifier[1]].GetPositionInObjectSpace();
       PointType v3 = this->m_Points[identifier[2]].GetPositionInObjectSpace();
 
-      if (TDimension == 3)
+      if constexpr (TDimension == 3)
       {
         double coa = -(v1[1] * (v2[2] - v3[2]) + v2[1] * (v3[2] - v1[2]) + v3[1] * (v1[2] - v2[2]));
         double cob = -(v1[2] * (v2[0] - v3[0]) + v2[2] * (v3[0] - v1[0]) + v3[2] * (v1[0] - v2[0]));

Modules/Core/SpatialObjects/include/itkTubeSpatialObject.hxx

@@ -427,7 +427,7 @@ TubeSpatialObject<TDimension, TTubePointType>::ComputeTangentsAndNormals()
 
     // Compute the normal
     CovariantVectorType n1;
-    if (TDimension == 2)
+    if constexpr (TDimension == 2)
     {
       // The normal to the tangent in 2D is the orthogonal direction to the tangent.
       n1[0] = t[1];
@@ -447,7 +447,7 @@ TubeSpatialObject<TDimension, TTubePointType>::ComputeTangentsAndNormals()
       ((TubePointType *)(this->GetPoint(it1)))->SetNormal1InObjectSpace(n1);
       prevN1 = n1;
     }
-    else if (TDimension == 3)
+    else if constexpr (TDimension == 3)
     {
       // The normal to the tangent in 3D is the cross product of adjacent tangent directions.
       n1[0] = t[1] * t2[2] - t[2] * t2[1];

Modules/Filtering/BiasCorrection/include/itkMRIBiasFieldCorrectionFilter.hxx

@@ -192,7 +192,7 @@ MRIBiasFieldCorrectionFilter<TInputImage, TOutputImage, TMaskImage>::MRIBiasFiel
 {
   m_NormalVariateGenerator->Initialize(time(nullptr));
 
-  if (ImageDimension == 3)
+  if constexpr (ImageDimension == 3)
   {
     m_UsingInterSliceIntensityCorrection = true;
     m_SlicingDirection = 2;

Modules/Filtering/DisplacementField/include/itkTimeVaryingVelocityFieldIntegrationImageFilter.hxx

@@ -35,7 +35,7 @@ TimeVaryingVelocityFieldIntegrationImageFilter<TTimeVaryingVelocityField,
   this->m_NumberOfTimePoints = 0;
   this->SetNumberOfRequiredInputs(1);
 
-  if (InputImageDimension - 1 != OutputImageDimension)
+  if constexpr (InputImageDimension - 1 != OutputImageDimension)
   {
     itkExceptionMacro("The time-varying velocity field (input) should have "
                       << "dimensionality of 1 greater than the deformation field (output). ");

Modules/Filtering/FastMarching/include/itkFastMarchingImageFilterBase.hxx

@@ -570,13 +570,13 @@ FastMarchingImageFilterBase<TInput, TOutput>::InitializeOutput(OutputImageType *
   // Initialize indices if this->m_TopologyCheck is activated
   if (this->m_TopologyCheck != Superclass::TopologyCheckEnum::Nothing)
   {
-    if (ImageDimension == 2)
+    if constexpr (ImageDimension == 2)
     {
       InitializeIndices2D();
     }
     else
     {
-      if (ImageDimension == 3)
+      if constexpr (ImageDimension == 3)
       {
         InitializeIndices3D();
       }
@@ -596,7 +596,7 @@ bool
 FastMarchingImageFilterBase<TInput, TOutput>::DoesVoxelChangeViolateWellComposedness(const NodeType & idx) const
 {
   bool isChangeWellComposed = false;
-  if (ImageDimension == 2)
+  if constexpr (ImageDimension == 2)
   {
     isChangeWellComposed = this->IsChangeWellComposed2D(idx);
   }

Modules/Filtering/GPUAnisotropicSmoothing/include/itkGPUScalarAnisotropicDiffusionFunction.hxx

@@ -36,7 +36,7 @@ GPUScalarAnisotropicDiffusionFunction<TImage>::GPUScalarAnisotropicDiffusionFunc
   // load GPU kernel
   std::ostringstream defines;
 
-  if (ImageDimension > 3 || ImageDimension < 1)
+  if constexpr (ImageDimension > 3 || ImageDimension < 1)
   {
     itkExceptionMacro("GPUScalarAnisotropicDiffusionFunction supports 1/2/3D image.");
   }

Modules/Filtering/GPUSmoothing/test/itkGPUMeanImageFilterTest.cxx

@@ -67,7 +67,7 @@ runGPUMeanImageFilterTest(const std::string & inFile, const std::string & outFil
   typename InputImageType::SizeType indexRadius;
   indexRadius[0] = 2; // radius along x
   indexRadius[1] = 2; // radius along y
-  if (VImageDimension > 2)
+  if constexpr (VImageDimension > 2)
   {
     indexRadius[2] = 2; // radius along z
   }

Modules/Filtering/GPUThresholding/test/itkGPUImageFilterTest.cxx

@@ -71,7 +71,7 @@ runGPUImageFilterTest(const std::string & inFile, const std::string & outFile)
   typename InputImageType::SizeType indexRadius;
   indexRadius[0] = 2; // radius along x
   indexRadius[1] = 2; // radius along y
-  if (VImageDimension > 2)
+  if constexpr (VImageDimension > 2)
   {
     indexRadius[2] = 2; // radius along z
   }

Modules/Filtering/ImageFeature/include/itkDiscreteGaussianDerivativeImageFilter.hxx

@@ -176,7 +176,7 @@ DiscreteGaussianDerivativeImageFilter<TInputImage, TOutputImage>::GenerateData()
   }
 
   // Create a chain of filters
-  if (ImageDimension == 1)
+  if constexpr (ImageDimension == 1)
   {
     // Use just a single filter
     SingleFilterPointer singleFilter = SingleFilterType::New();
@@ -212,7 +212,7 @@ DiscreteGaussianDerivativeImageFilter<TInputImage, TOutputImage>::GenerateData()
 
     // Middle filters convolves from real to real
     std::vector<IntermediateFilterPointer> intermediateFilters;
-    if (ImageDimension > 2)
+    if constexpr (ImageDimension > 2)
     {
       const unsigned int max_dim = ImageDimension - 1;
       for (unsigned int i = 1; i != max_dim; ++i)
@@ -239,7 +239,7 @@ DiscreteGaussianDerivativeImageFilter<TInputImage, TOutputImage>::GenerateData()
     LastFilterPointer lastFilter = LastFilterType::New();
     lastFilter->SetOperator(oper[ImageDimension - 1]);
     lastFilter->ReleaseDataFlagOn();
-    if (ImageDimension > 2)
+    if constexpr (ImageDimension > 2)
     {
       const unsigned int temp_dim = ImageDimension - 3;
       lastFilter->SetInput(intermediateFilters[temp_dim]->GetOutput());

Modules/Filtering/ImageFeature/include/itkLaplacianRecursiveGaussianImageFilter.hxx

@@ -217,7 +217,7 @@ LaplacianRecursiveGaussianImageFilter<TInputImage, TOutputImage>::GenerateData()
 
   // Because the output of last filter in the mini-pipeline is not
   // pipelined the data must be manually released
-  if (ImageDimension > 1)
+  if constexpr (ImageDimension > 1)
   {
     m_SmoothingFilters[ImageDimension - 2]->GetOutput()->ReleaseData();
   }