VTK: Invert IJK indices to match ResInsight convention

VTK coordinates are negated (z positive downward) to match ResInsight
internal convention, which flips the IJK directions relative to what the
structured grid algorithm expects. Add invertIJK flag to RigFemPartGrid
and RigFemPart so the inversion is applied during struct grid construction.

ApplicationLibCode/GeoMech/GeoMechDataModel/RigFemPart.cpp

@@ -30,6 +30,7 @@ RigFemPart::RigFemPart()
     : m_elementPartId( -1 )
     , m_characteristicElementSize( std::numeric_limits<float>::infinity() )
     , m_enabled( true )
+    , m_invertIJKInStructGrid( false )
 {
 }
 
@@ -227,12 +228,20 @@ const RigFemPartGrid* RigFemPart::getOrCreateStructGrid() const
     if ( m_structGrid.isNull() )
     {
         m_structGrid = new RigFemPartGrid();
-        m_structGrid->setFemPart( this );
+        m_structGrid->setFemPart( this, m_invertIJKInStructGrid );
     }
 
     return m_structGrid.p();
 }
 
+//--------------------------------------------------------------------------------------------------
+///
+//--------------------------------------------------------------------------------------------------
+void RigFemPart::setInvertIJKInStructGrid( bool invert )
+{
+    m_invertIJKInStructGrid = invert;
+}
+
 //--------------------------------------------------------------------------------------------------
 ///
 //--------------------------------------------------------------------------------------------------

ApplicationLibCode/GeoMech/GeoMechDataModel/RigFemPart.h

@@ -95,6 +95,7 @@ public:
     cvf::Vec3f faceNormal( int elementIndex, int faceIndex ) const;
 
     const RigFemPartGrid*   getOrCreateStructGrid() const;
+    void                    setInvertIJKInStructGrid( bool invert );
     const std::vector<int>& elementIdxToId() const;
 
     void        setName( std::string name );
@@ -111,6 +112,7 @@ private:
     int         m_elementPartId;
     std::string m_name;
     bool        m_enabled;
+    bool        m_invertIJKInStructGrid;
 
     std::vector<int>            m_elementId;
     std::vector<RigElementType> m_elementTypes;

ApplicationLibCode/GeoMech/GeoMechDataModel/RigFemPartGrid.cpp

@@ -43,16 +43,16 @@ RigFemPartGrid::~RigFemPartGrid()
 //--------------------------------------------------------------------------------------------------
 ///
 //--------------------------------------------------------------------------------------------------
-void RigFemPartGrid::setFemPart( const RigFemPart* femPart )
+void RigFemPartGrid::setFemPart( const RigFemPart* femPart, bool invertIJK )
 {
     m_femPart = femPart;
-    generateStructGridData();
+    generateStructGridData( invertIJK );
 }
 
 //--------------------------------------------------------------------------------------------------
 ///
 //--------------------------------------------------------------------------------------------------
-void RigFemPartGrid::generateStructGridData()
+void RigFemPartGrid::generateStructGridData( bool invertIJK )
 {
     //[X] 1. Calculate neighbors for each element
     //[X]    record the ones with 3 or fewer neighbors as possible grid corners
@@ -179,6 +179,19 @@ void RigFemPartGrid::generateStructGridData()
 
     m_elmIdxPrIJK.resize( m_elementIJKCounts[0], m_elementIJKCounts[1], m_elementIJKCounts[2] );
 
+    if ( invertIJK )
+    {
+        size_t iCount = m_elementIJKCounts[0];
+        size_t jCount = m_elementIJKCounts[1];
+        size_t kCount = m_elementIJKCounts[2];
+        for ( auto& ijk : m_ijkPrElement )
+        {
+            if ( ijk[0] >= 0 ) ijk[0] = static_cast<int>( iCount ) - 1 - ijk[0];
+            if ( ijk[1] >= 0 ) ijk[1] = static_cast<int>( jCount ) - 1 - ijk[1];
+            if ( ijk[2] >= 0 ) ijk[2] = static_cast<int>( kCount ) - 1 - ijk[2];
+        }
+    }
+
     for ( int elmIdx = 0; elmIdx < m_femPart->elementCount(); ++elmIdx )
     {
         size_t i, j, k;

ApplicationLibCode/GeoMech/GeoMechDataModel/RigFemPartGrid.h

@@ -31,7 +31,7 @@ public:
     RigFemPartGrid();
     ~RigFemPartGrid() override;
 
-    void setFemPart( const RigFemPart* femPart );
+    void setFemPart( const RigFemPart* femPart, bool invertIJK = false );
 
     size_t cellCountI() const override;
     size_t cellCountJ() const override;
@@ -47,7 +47,7 @@ public:
     cvf::Vec3d                              cellCentroid( size_t cellIndex ) const override;
 
 private:
-    void generateStructGridData();
+    void generateStructGridData( bool invertIJK );
     int  findElmIdxForIJK000();
     int  perpendicularFaceInDirection( cvf::Vec3f direction, int perpFaceIdx, int elmIdx );
 

ApplicationLibCode/GeoMech/GeoMechFileInterface/RifVtkReader.cpp

@@ -191,6 +191,10 @@ bool RifVtkReader::readFemParts( RigFemPartCollection* femParts )
             femPart->addElementSet( setName, elementSet );
         }
 
+        // VTK coordinates are negated to match ResInsight internal convention (z positive downward),
+        // which flips the IJK directions. Invert all so IJK=0 is at the correct corner.
+        femPart->setInvertIJKInStructGrid( true );
+
         femPart->setElementPartId( femParts->partCount() );
         femParts->addFemPart( femPart );