ENH: add background value handling for stat estimation in pca

Modules/Applications/AppDimensionalityReduction/app/otbDimensionalityReduction.cxx

@@ -183,6 +183,10 @@ private:
     MandatoryOff("outmatrix");
     DisableParameter("outmatrix");
     
+    AddParameter(ParameterType_Float, "bv", "Background Value");
+    SetParameterDescription( "bv", "Background value to ignore in computation of the transformation matrix. Note that all pixels will still be processed when applying the transformation." );
+    MandatoryOff("bv");
+
     AddRAMParameter();
 
     // Doc example parameter settings
@@ -276,6 +280,11 @@ private:
             filter->SetUseVarianceForNormalization(false);
           }
 
+          if( HasValue( "bv" ) )
+          {
+            filter->SetStatisticsUserIgnoredValue(GetParameterFloat("bv"));
+          }
+          
           m_ForwardFilter->GetOutput()->UpdateOutputInformation();
 
           // Write eigenvalues

Modules/Filtering/DimensionalityReduction/include/otbPCAImageFilter.h

@@ -161,7 +161,17 @@ public:
     m_StdDevValues = vec;
     this->Modified();
   }
-
+  
+  void SetStatisticsUserIgnoredValue ( RealType value )
+  {
+    /** User ignored value for the normalizer */
+    m_Normalizer->GetCovarianceEstimator()->SetUserIgnoredValue(value);
+    m_Normalizer->GetCovarianceEstimator()->SetIgnoreUserDefinedValue(true);
+    /** User ignored value for the covariance estimator */
+    m_CovarianceEstimator->SetUserIgnoredValue(value);
+    m_CovarianceEstimator->SetIgnoreUserDefinedValue(true);
+  }
+  
 protected:
   PCAImageFilter();
   ~PCAImageFilter() override { }