ENH: Fix hardening of warping transforms on labelmap volumes

Labelmap volumes must be resampled using nearest neighbor interpolation.

Libs/MRML/Core/vtkMRMLLabelMapVolumeNode.cxx

@@ -72,3 +72,11 @@ void vtkMRMLLabelMapVolumeNode::CreateDefaultDisplayNodes()
   dispNode->SetDefaultColorMap();
   this->SetAndObserveDisplayNodeID(dispNode->GetID());
 }
+
+//----------------------------------------------------------------------------
+int vtkMRMLLabelMapVolumeNode::GetResamplingInterpolationMode()
+{
+  // Labelmap volumes must be resampled using nearest neighbor method to avoid
+  // introducing new label values at boundaries.
+  return VTK_NEAREST_INTERPOLATION;
+}

Libs/MRML/Core/vtkMRMLLabelMapVolumeNode.h

@@ -58,6 +58,8 @@ class VTK_MRML_EXPORT vtkMRMLLabelMapVolumeNode : public vtkMRMLScalarVolumeNode
   ~vtkMRMLLabelMapVolumeNode() override;
   vtkMRMLLabelMapVolumeNode(const vtkMRMLLabelMapVolumeNode&);
   void operator=(const vtkMRMLLabelMapVolumeNode&);
+
+  int GetResamplingInterpolationMode() override;
 };
 
 #endif

Libs/MRML/Core/vtkMRMLVolumeNode.cxx

@@ -1050,7 +1050,19 @@ void vtkMRMLVolumeNode::ApplyNonLinearTransform(vtkAbstractTransform* transform)
     }
 
   reslice->SetInputConnection(this->ImageDataConnection);
-  reslice->SetInterpolationModeToLinear();
+
+  // GetResamplingInterpolationMode does not use VTK_RESLICE... constants because it is an implementation
+  // detail that currently vtkImageReslice is used for resampling.
+  int resamplingMode = this->GetResamplingInterpolationMode();
+  switch (resamplingMode)
+    {
+    case VTK_NEAREST_INTERPOLATION: reslice->SetInterpolationModeToNearestNeighbor(); break;
+    case VTK_LINEAR_INTERPOLATION: reslice->SetInterpolationModeToLinear(); break;
+    case VTK_CUBIC_INTERPOLATION: reslice->SetInterpolationModeToCubic(); break;
+    default:
+      vtkErrorMacro("ApplyNonLinearTransform: invalid interpolation mode: " << this->GetResamplingInterpolationMode());
+    }
+
   double backgroundColor[4] = { 0, 0, 0, 0 };
   for (int i = 0; i < 4; i++)
     {
@@ -1106,6 +1118,13 @@ void vtkMRMLVolumeNode::ApplyNonLinearTransform(vtkAbstractTransform* transform)
   this->EndModify(wasModified);
 }
 
+//---------------------------------------------------------------------------
+int vtkMRMLVolumeNode::GetResamplingInterpolationMode()
+{
+  // By default linear interpolation is used for all volumes.
+  return VTK_LINEAR_INTERPOLATION;
+}
+
 //---------------------------------------------------------------------------
 void vtkMRMLVolumeNode::ShiftImageDataExtentToZeroStart()
 {

Libs/MRML/Core/vtkMRMLVolumeNode.h

@@ -285,6 +285,10 @@ class VTK_MRML_EXPORT vtkMRMLVolumeNode : public vtkMRMLDisplayableNode
   /// If useParentTransform is false then parent transform is ignored.
   void GetCenterPositionRAS(double* centerPositionRAS, bool useParentTransform=true);
 
+  /// Returns the interpolation algorithm that should be used for resampling the volume.
+  /// The value is one of VTK_NEAREST_INTERPOLATION, VTK_LINEAR_INTERPOLATION, or VTK_CUBIC_INTERPOLATION.
+  virtual int GetResamplingInterpolationMode();
+
   /// these are unit length direction cosines
   double IJKToRASDirections[3][3];