ENH: Add kernel helper methods to itk::DiscreteGaussianImageFilter

Exposes methods in `itk::DiscreteGaussianImageFilter` public interface
for getting details on the separable Gaussian kernel to be used in
smoothing. This is useful for handling cases such as image padding and
overlap for convolution.

Modules/Filtering/Smoothing/include/itkDiscreteGaussianImageFilter.h

@@ -193,19 +193,24 @@ class ITK_TEMPLATE_EXPORT DiscreteGaussianImageFilter : public ImageToImageFilte
   }
 
   /** Set the standard deviation of the Gaussian used for smoothing.
-   * Sigma is measured in the units of image spacing. You may use the method
-   * SetSigma to set the same value across each axis or use the method
-   * SetSigmaArray if you need different values along each axis. */
+   * Sigma is measured in the units of image spacing. */
   void
-  SetSigmaArray(const ArrayType & sigmas)
+  SetSigma(const ArrayType & sigma)
   {
     ArrayType variance;
     for (unsigned int i = 0; i < ImageDimension; ++i)
     {
-      variance[i] = sigmas[i] * sigmas[i];
+      variance[i] = sigma[i] * sigma[i];
     }
     this->SetVariance(variance);
   }
+  /** SetSigmaArray is preserved for interface
+   *  backwards compatibility. */
+  void
+  SetSigmaArray(const ArrayType & sigmas)
+  {
+    this->SetSigma(sigmas);
+  }
   void
   SetSigma(double sigma)
   {
@@ -256,6 +261,19 @@ class ITK_TEMPLATE_EXPORT DiscreteGaussianImageFilter : public ImageToImageFilte
     this->SetMaximumError(dv);
   }
 
+  /** Get the radius of the generated directional kernel */
+  unsigned int
+  GetKernelRadius(const unsigned int dimension) const;
+
+  /** Get the radius of the separable kernel in each direction */
+  ArrayType
+  GetKernelRadius() const;
+
+  /** Get the size of the separable kernel in each direction.
+   *  size[i] = radius[i] * 2 + 1 */
+  ArrayType
+  GetKernelSize() const;
+
   /** Set/Get whether or not the filter will use the spacing of the input
    * image in its calculations. Use On to take the image spacing information
    * into account and to specify the Gaussian variance in real world units;
@@ -343,10 +361,6 @@ class ITK_TEMPLATE_EXPORT DiscreteGaussianImageFilter : public ImageToImageFilte
   void
   GenerateKernel(const unsigned int dimension, KernelType & oper) const;
 
-  /** Get the radius of the generated directional kernel */
-  unsigned int
-  GetKernelRadius(const unsigned int dimension) const;
-
   /** Get the variance, optionally adjusted for pixel spacing */
   ArrayType
   GetKernelVarianceArray() const;

Modules/Filtering/Smoothing/include/itkDiscreteGaussianImageFilter.hxx

@@ -51,6 +51,31 @@ DiscreteGaussianImageFilter<TInputImage, TOutputImage>::GetKernelRadius(const un
   return oper.GetRadius(dimension);
 }
 
+template <typename TInputImage, typename TOutputImage>
+auto
+DiscreteGaussianImageFilter<TInputImage, TOutputImage>::GetKernelRadius() const -> ArrayType
+{
+  ArrayType kernelRadius;
+  for (unsigned int dim = 0; dim < ImageDimension; ++dim)
+  {
+    kernelRadius[dim] = this->GetKernelRadius(dim);
+  }
+  return kernelRadius;
+}
+
+
+template <typename TInputImage, typename TOutputImage>
+auto
+DiscreteGaussianImageFilter<TInputImage, TOutputImage>::GetKernelSize() const -> ArrayType
+{
+  ArrayType kernelSize;
+  for (unsigned int dim = 0; dim < ImageDimension; ++dim)
+  {
+    kernelSize[dim] = this->GetKernelRadius(dim) * 2 + 1;
+  }
+  return kernelSize;
+}
+
 template <typename TInputImage, typename TOutputImage>
 typename DiscreteGaussianImageFilter<TInputImage, TOutputImage>::ArrayType
 DiscreteGaussianImageFilter<TInputImage, TOutputImage>::GetKernelVarianceArray() const
@@ -59,7 +84,7 @@ DiscreteGaussianImageFilter<TInputImage, TOutputImage>::GetKernelVarianceArray()
   {
     if (this->GetInput() == nullptr)
     {
-      itkExceptionMacro("UseImageSpacing is ON but no input image was provided");
+      itkExceptionMacro("Could not get kernel variance! UseImageSpacing is ON but no input image was provided");
     }
 
     ArrayType adjustedVariance;

Modules/Filtering/Smoothing/test/itkDiscreteGaussianImageFilterTest.cxx

@@ -70,6 +70,9 @@ itkDiscreteGaussianImageFilterTest(int argc, char * argv[])
   filter->SetSigma(sigmaValue);
   ITK_TEST_SET_GET_VALUE(sigmaValue, filter->GetSigma());
 
+  filter->SetSigma(sigma);
+  ITK_TEST_SET_GET_VALUE(sigma, filter->GetSigmaArray());
+
   filter->SetSigmaArray(sigma);
   ITK_TEST_SET_GET_VALUE(sigma, filter->GetSigmaArray());
 
@@ -101,6 +104,25 @@ itkDiscreteGaussianImageFilterTest(int argc, char * argv[])
   filter->SetFilterDimensionality(Dimension);
   ITK_TEST_SET_GET_VALUE(Dimension, filter->GetFilterDimensionality());
 
+  // Verify kernel radius matches expectations for test parameters
+  filter->SetUseImageSpacingOff();
+  constexpr unsigned int EXPECTED_RADIUS = 3;
+  auto                   radius = filter->GetKernelRadius();
+  auto                   kernelSize = filter->GetKernelSize();
+  for (unsigned int idx = 0; idx < Dimension; ++idx)
+  {
+    ITK_TEST_EXPECT_EQUAL(radius[idx], EXPECTED_RADIUS);
+    ITK_TEST_EXPECT_EQUAL(filter->GetKernelRadius(idx), EXPECTED_RADIUS);
+    ITK_TEST_EXPECT_EQUAL(kernelSize[idx], EXPECTED_RADIUS * 2 + 1);
+  }
+
+  // Verify filter throws exception when trying to get kernel information
+  // if UseImageSpacing is ON and an input image is not set
+  filter->SetUseImageSpacingOn();
+  ITK_TRY_EXPECT_EXCEPTION(filter->GetKernelRadius());
+  ITK_TRY_EXPECT_EXCEPTION(filter->GetKernelRadius(0));
+  ITK_TRY_EXPECT_EXCEPTION(filter->GetKernelSize());
+
   auto useImageSpacing = static_cast<bool>(std::stoi(argv[1]));
 #if !defined(ITK_FUTURE_LEGACY_REMOVE)
   if (useImageSpacing)