BUG: ZScore computations invalid for small values

Images with small values have standard deviations
that could be smaller than 1.0, but be perfectly valid.

For the case when the standard deviation is zero,
set all the z-score values to zero.

Author: hjmjohnson

include/itkNonLocalPatchBasedImageFilter.hxx

@@ -112,16 +112,21 @@ NonLocalPatchBasedImageFilter<TInputImage, TOutputImage>::VectorizeImagePatch(co
 
   if (normalize)
   {
+    // Convert the current image patch to a z-score.
     RealType mean = 0.0;
     RealType standardDeviation = 0.0;
     this->GetMeanAndStandardDeviationOfVectorizedImagePatch(patchVector, mean, standardDeviation);
-
-    standardDeviation = std::max(standardDeviation, NumericTraits<RealType>::OneValue());
-
-    typename InputImagePixelVectorType::iterator it;
-    for (it = patchVector.begin(); it != patchVector.end(); ++it)
+    if (standardDeviation < NumericTraits<RealType>::epsilon())
+    {
+      for (auto & it : patchVector)
+      {
+        it = 0.;
+      }
+    }
+    const auto inv_standardDeviation = 1. / standardDeviation;
+    for (auto & it : patchVector)
     {
-      *it = (*it - mean) / standardDeviation;
+      it = (it - mean) * inv_standardDeviation;
     }
   }
   return patchVector;
@@ -138,13 +143,13 @@ NonLocalPatchBasedImageFilter<TInputImage, TOutputImage>::GetMeanAndStandardDevi
   RealType sumOfSquares = 0.0;
   RealType count = 0.0;
 
-  typename InputImagePixelVectorType::const_iterator it;
-  for (it = patchVector.begin(); it != patchVector.end(); ++it)
+  for (const auto it : patchVector)
   {
-    if (std::isfinite(*it))
+    if (std::isfinite(it)) // Silently skip non-finite values used to indicate
+                           // out-of-bounds.
     {
-      sum += *it;
-      sumOfSquares += itk::Math::sqr(*it);
+      sum += it;
+      sumOfSquares += itk::Math::sqr(it);
       count += itk::NumericTraits<RealType>::OneValue();
     }
   }
@@ -204,13 +209,11 @@ NonLocalPatchBasedImageFilter<TInputImage, TOutputImage>::ComputeNeighborhoodPat
   {
     return NumericTraits<RealType>::max();
   }
-
   if (this->m_SimilarityMetric == SimilarityMetricEnum::PEARSON_CORRELATION)
   {
-    RealType varianceX = sumOfSquaresX - itk::Math::sqr(sumX) / N;
-    varianceX = std::max(varianceX, static_cast<RealType>(1.0e-6));
-
-    RealType measure = itk::Math::sqr(sumXY) / varianceX;
+    const RealType varianceX = sumOfSquaresX - itk::Math::sqr(sumX) / N;
+    const RealType measure =
+      (varianceX > std::numeric_limits<RealType>::epsilon()) ? itk::Math::sqr(sumXY) / varianceX : 0.;
     if (sumXY > 0)
     {
       return -measure;