ENH: improve surfaceFieldValue sampling and writing (#1999)

- ensure surface writing is time-step and nFields aware.
  This avoids overwriting (ignoring) previous output fields.

- allow sampled surfaces to be used for weight fields as well.
  Not sure why this restriction was still there.

- remove old compatibility reading of orientedFields.
  Last used in v1612, now removed.

- only use face sampling. For surfaceFieldValue we can only do
  something meaningful with face values.

ENH: modify interface methods for surfaceWriter

- replace direct modification of values with setter methods.
  Eg,
     old: writer.isPointData() = true;
     new: writer.isPointData(true);

  This makes it possible to add internal hooks to catch state changes.

ENH: allow post-construction change to sampledSurface interpolation

- rename interpolate() method to isPointData() for consistency with
  other classes and to indicate that it is a query.

- additional isPointData(bool) setter method to change the expected
  representation type after construction

- remove 'interpolate' restriction on isoSurfacePoint which was
  previously flagged as an error but within sampledSurfaces can use
  sampleScheme cellPoint and obtain representative samples.
  Relax this restriction since this particular iso-surface algorithm
  is slated for removal in the foreseeable future.

src/finiteVolume/fvMesh/fvGeometryScheme/averageNeighbour/averageNeighbourFvGeometryScheme.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2020 OpenCFD Ltd.
+    Copyright (C) 2020-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -713,7 +713,7 @@ void Foam::averageNeighbourFvGeometryScheme::movePoints()
             );
 
             // Use outputDir/TIME/surface-name
-            writerPtr->useTimeDir() = true;
+            writerPtr->useTimeDir(true);
 
             writerPtr->beginTime(mesh_.time());
 
@@ -765,7 +765,7 @@ void Foam::averageNeighbourFvGeometryScheme::movePoints()
                     faceWeights
                 );
 
-                if (writerPtr.valid())
+                if (writerPtr)
                 {
                     writerPtr->beginTime(instant(scalar(iter)));
                     writerPtr->write("cosAngles", cosAngles);

src/functionObjects/field/fieldValues/surfaceFieldValue/surfaceFieldValue.C

@@ -971,6 +971,8 @@ bool Foam::functionObjects::fieldValues::surfaceFieldValue::read
     needsUpdate_ = true;
     writeArea_ = dict.getOrDefault("writeArea", false);
     weightFieldNames_.clear();
+    // future?
+    // sampleFaceScheme_ = dict.getOrDefault<word>("sampleScheme", "cell");
 
     totalArea_ = 0;
     nFaces_ = 0;
@@ -1029,15 +1031,8 @@ bool Foam::functionObjects::fieldValues::surfaceFieldValue::read
             dict.subDict("sampledSurfaceDict")
         );
 
-        if (sampledPtr_->interpolate())
+        // Internal consistency. Want face values, never point values!
-        {
+        sampledPtr_->isPointData(false);
-            // Should probably ignore interpolate entirely,
-            // but the oldest isoSurface algorithm requires it!
-            WarningInFunction
-                << type() << ' ' << name() << ": "
-                << "sampledSurface with interpolate = true "
-                << "is likely incorrect" << nl << nl;
-        }
     }
 
     Info<< type() << ' ' << name() << ':' << nl
@@ -1055,15 +1050,6 @@ bool Foam::functionObjects::fieldValues::surfaceFieldValue::read
 
     if (usesWeight())
     {
-        if (stSampled == regionType_)
-        {
-            FatalIOErrorInFunction(dict)
-                << "Cannot use weighted operation '"
-                << operationTypeNames_[operation_]
-                << "' for sampledSurface"
-                << exit(FatalIOError);
-        }
-
         // Can have "weightFields" or "weightField"
 
         bool missing = true;
@@ -1109,18 +1095,6 @@ bool Foam::functionObjects::fieldValues::surfaceFieldValue::read
         }
     }
 
-    // Backwards compatibility for v1612 and older
-    List<word> orientedFields;
-    if (dict.readIfPresent("orientedFields", orientedFields))
-    {
-        fields_.append(orientedFields);
-
-        WarningInFunction
-            << "The 'orientedFields' option is deprecated.  These fields can "
-            << "and have been added to the standard 'fields' list."
-            << endl;
-    }
-
     if (writeFields_)
     {
         const word formatName(dict.get<word>("surfaceFormat"));
@@ -1134,6 +1108,9 @@ bool Foam::functionObjects::fieldValues::surfaceFieldValue::read
             )
         );
 
+        // Propagate field counts (per surface)
+        surfaceWriterPtr_->nFields(fields_.size());
+
         if (debug)
         {
             surfaceWriterPtr_->verbose(true);

src/functionObjects/field/fieldValues/surfaceFieldValue/surfaceFieldValue.H

@@ -177,12 +177,11 @@ Note
         Instead specify the regionType 'functionObjectSurface' and provide
         the name.
     - Using \c sampledSurface:
-        - not available for surface fields
+        - surface fields only supported by some surfaces
-        - if interpolate=true they use \c interpolationCellPoint
+        - default uses sampleScheme \c cell
-          otherwise they use cell values
+        - each face in \c sampledSurface is logically only in one cell
-        - each triangle in \c sampledSurface is logically only in one cell
+          so sampling will be wrong when they are larger than cells.
-          so interpolation will be wrong when triangles are larger than
+          This can only happen for sampling on a \c triSurfaceMesh
-          cells.  This can only happen for sampling on a \c triSurfaceMesh
         - take care when using isoSurfaces - these might have duplicate
           triangles and so integration might be wrong
 

src/functionObjects/field/fieldValues/surfaceFieldValue/surfaceFieldValueTemplates.C

@@ -96,19 +96,14 @@ Foam::functionObjects::fieldValues::surfaceFieldValue::getFieldValues
 
         if (sampledPtr_)
         {
-            if (sampledPtr_->interpolate())
+            // Could be runtime selectable
-            {
+            // auto sampler = interpolation<Type>::New(sampleFaceScheme_, fld);
-                const interpolationCellPoint<Type> interp(fld);
 
-                return sampledPtr_->interpolate(interp);
+            // const interpolationCellPoint<Type> interp(fld);
-            }
-            else
-            {
             const interpolationCell<Type> interp(fld);
 
             return sampledPtr_->sample(interp);
         }
-        }
         else
         {
             return filterField(fld);
@@ -324,6 +319,30 @@ Foam::label Foam::functionObjects::fieldValues::surfaceFieldValue::writeAll
 {
     label nProcessed = 0;
 
+    // If using the surface writer, the points/faces parameters have already
+    // been merged on the master and the writeValues routine will also gather
+    // all data onto the master before calling the writer.
+    // Thus only call the writer on master!
+
+    // Begin writer time step
+    if (Pstream::master() && surfaceWriterPtr_ && surfaceWriterPtr_->enabled())
+    {
+        auto& writer = *surfaceWriterPtr_;
+
+        writer.open
+        (
+            points,
+            faces,
+            (
+                outputDir()
+              / regionTypeNames_[regionType_] + ("_" + regionName_)
+            ),
+            false  // serial - already merged
+        );
+
+        writer.beginTime(time_);
+    }
+
     for (const word& fieldName : fields_)
     {
         if
@@ -349,6 +368,23 @@ Foam::label Foam::functionObjects::fieldValues::surfaceFieldValue::writeAll
         }
     }
 
+    // Finish writer time step
+    if (Pstream::master() && surfaceWriterPtr_ && surfaceWriterPtr_->enabled())
+    {
+        auto& writer = *surfaceWriterPtr_;
+
+        // Write geometry if no fields were written so that we still
+        // can have something to look at.
+
+        if (!writer.wroteData())
+        {
+            writer.write();
+        }
+
+        writer.endTime();
+        writer.clear();
+    }
+
     return nProcessed;
 }
 
@@ -377,26 +413,6 @@ bool Foam::functionObjects::fieldValues::surfaceFieldValue::writeValues
 
             if (Pstream::master())
             {
-                surfaceWriterPtr_->open
-                (
-                    points,
-                    faces,
-                    (
-                        outputDir()
-                      / regionTypeNames_[regionType_] + ("_" + regionName_)
-                    ),
-                    false  // serial - already merged
-                );
-
-                // Point data? Should probably disallow
-                if (sampledPtr_)
-                {
-                    surfaceWriterPtr_->isPointData() =
-                        sampledPtr_->interpolate();
-                }
-
-                surfaceWriterPtr_->nFields() = 1;  // Needed for VTK legacy
-
                 fileName outputName =
                     surfaceWriterPtr_->write(fieldName, allValues);
 
@@ -404,8 +420,6 @@ bool Foam::functionObjects::fieldValues::surfaceFieldValue::writeValues
                 dictionary propsDict;
                 propsDict.add("file", time_.relativePath(outputName, true));
                 this->setProperty(fieldName, propsDict);
-
-                surfaceWriterPtr_->clear();
             }
         }
 

src/functionObjects/utilities/areaWrite/areaWrite.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2019-2020 OpenCFD Ltd.
+    Copyright (C) 2019-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -306,7 +306,7 @@ bool Foam::areaWrite::write()
 
 
         // Propagate field counts (per surface)
-        outWriter.nFields() = nAreaFields;
+        outWriter.nFields(nAreaFields);
 
 
         // Begin writing
@@ -351,7 +351,7 @@ void Foam::areaWrite::expire()
     {
         surfaceWriter& writer = *(iter.val());
         writer.expire();
-        writer.mergeDim() = mergeDim;
+        writer.mergeDim(mergeDim);
     }
 }
 

src/lagrangian/intermediate/submodels/CloudFunctionObjects/FacePostProcessing/FacePostProcessing.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -215,8 +215,8 @@ void Foam::FacePostProcessing<CloudType>::write()
                     false  // serial - already merged
                 );
 
+                writer->nFields(2); // Legacy VTK
                 writer->write("massTotal", zoneMassTotal[zoneI]);
-
                 writer->write("massFlowRate", zoneMassFlowRate[zoneI]);
             }
         }

src/lagrangian/intermediate/submodels/CloudFunctionObjects/ParticleCollector/ParticleCollector.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2012-2017 OpenFOAM Foundation
-    Copyright (C) 2015-2020 OpenCFD Ltd.
+    Copyright (C) 2015-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -485,8 +485,8 @@ void Foam::ParticleCollector<CloudType>::write()
             false  // serial - already merged
         );
 
+        writer->nFields(2); // Legacy VTK
         writer->write("massFlowRate", faceMassFlowRate);
-
         writer->write("massTotal", faceMassTotal);
     }
 

src/sampling/sampledSurface/isoSurface/sampledIsoSurface.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -652,13 +652,9 @@ Foam::sampledIsoSurface::sampledIsoSurface
         // Not possible for ALGO_POINT
         simpleSubMesh_ = false;
 
-        if (!sampledSurface::interpolate())
+        // Previously emitted an error about using ALGO_POINT with
-        {
+        // non-interpolated, but that was before we had "sampleScheme"
-            FatalIOErrorInFunction(dict)
+        // at the top level
-                << "Non-interpolated iso-surface (point) not supported"
-                << " since triangles span across cells." << nl
-                << exit(FatalIOError);
-        }
 
         if (isoValues_.size() > 1)
         {

src/sampling/sampledSurface/isoSurface/sampledIsoSurface.H

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -70,6 +70,10 @@ Usage
     - triangulate is topo-only
     - simpleSubMesh and multiple isoValues are not available for point.
 
+Note
+    For the isoMethod \b point should use a "cellPoint" sampleScheme
+    since the triangles generated with this algorithm span across cells.
+
 SourceFiles
     sampledIsoSurface.C
     sampledIsoSurfaceTemplates.C
@@ -215,7 +219,7 @@ protected:
 public:
 
     //- Runtime type information
-    TypeName("samplediIsoSurfacePoint");
+    TypeName("sampledIsoSurface");
 
 
     // Constructors

src/sampling/sampledSurface/isoSurface/sampledIsoSurfacePoint.H

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2020 OpenCFD Ltd.
+    Copyright (C) 2020-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -60,6 +60,10 @@ Usage
         mergeTol | tolerance for merging points             | no  | 1e-6
     \endtable
 
+Note
+    For the isoMethod \b point should use a "cellPoint" sampleScheme
+    since the triangles generated with this algorithm span across cells.
+
 SourceFiles
     sampledIsoSurfacePoint.C
 

src/sampling/sampledSurface/sampledMeshedSurface/sampledMeshedSurface.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -287,7 +287,7 @@ bool Foam::sampledMeshedSurface::update(const meshSearch& meshSearcher)
     // Collect the samplePoints and sampleElements
     // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-    if (sampledSurface::interpolate())
+    if (sampledSurface::isPointData())
     {
         // With point interpolation
 
@@ -422,7 +422,7 @@ bool Foam::sampledMeshedSurface::update(const meshSearch& meshSearcher)
           : mesh().cellCentres()
         );
 
-        if (sampledSurface::interpolate())
+        if (sampledSurface::isPointData())
         {
             label vertI = 0;
             forAll(samplePoints_, pointi)

src/sampling/sampledSurface/sampledSurface/sampledSurface.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2018-2020 OpenCFD Ltd.
+    Copyright (C) 2018-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -97,7 +97,7 @@ Foam::sampledSurface::sampledSurface(const word& name, std::nullptr_t)
     mesh_(NullObjectRef<polyMesh>()),
     enabled_(true),
     invariant_(false),
-    interpolate_(false),
+    isPointData_(false),
     area_(-1)
 {}
 
@@ -106,14 +106,14 @@ Foam::sampledSurface::sampledSurface
 (
     const word& name,
     const polyMesh& mesh,
-    const bool interpolate
+    const bool interpolateToPoints
 )
 :
     name_(name),
     mesh_(mesh),
     enabled_(true),
     invariant_(false),
-    interpolate_(interpolate),
+    isPointData_(interpolateToPoints),
     area_(-1)
 {}
 
@@ -129,7 +129,7 @@ Foam::sampledSurface::sampledSurface
     mesh_(mesh),
     enabled_(dict.getOrDefault("enabled", true)),
     invariant_(dict.getOrDefault("invariant", false)),
-    interpolate_(dict.getOrDefault("interpolate", false)),
+    isPointData_(dict.getOrDefault("interpolate", false)),
     area_(-1)
 {}
 
@@ -155,6 +155,14 @@ Foam::scalar Foam::sampledSurface::area() const
 }
 
 
+bool Foam::sampledSurface::isPointData(const bool on)
+{
+    bool old(isPointData_);
+    isPointData_ = on;
+    return old;
+}
+
+
 bool Foam::sampledSurface::withSurfaceFields() const
 {
     return false;

src/sampling/sampledSurface/sampledSurface/sampledSurface.H

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -121,8 +121,8 @@ private:
         //- Geometry is invariant (never changes)
         bool invariant_;
 
-        //- Interpolate information to the nodes?
+        //- Is point vs cell data
-        bool interpolate_;
+        bool isPointData_;
 
         //- Total surface area (demand-driven)
         mutable scalar area_;
@@ -249,7 +249,7 @@ public:
         (
             const word& name,
             const polyMesh& mesh,
-            const bool interpolate = false
+            const bool interpolateToPoints = false
         );
 
         //- Construct from dictionary
@@ -311,12 +311,16 @@ public:
             return invariant_;
         }
 
-        //- Interpolation to nodes requested for surface
+        //- Using interpolation to surface points
-        bool interpolate() const
+        bool isPointData() const
         {
-            return interpolate_;
+            return isPointData_;
         }
 
+        //- Change point/cell representation, may trigger an expire().
+        //  \return old value
+        virtual bool isPointData(const bool on);
+
         //- Does the surface need an update?
         virtual bool needsUpdate() const = 0;
 
@@ -561,6 +565,12 @@ public:
 
         //- Print information
         virtual void print(Ostream& os) const;
+
+
+    // Housekeeping
+
+        //- Same as isPointData()
+        bool interpolate() const { return isPointData_; }
 };
 
 

src/sampling/sampledSurface/sampledSurfaces/sampledSurfaces.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -171,14 +171,14 @@ void Foam::sampledSurfaces::countFields()
         const sampledSurface& s = (*this)[surfi];
         surfaceWriter& outWriter = writers_[surfi];
 
-        outWriter.nFields() =
+        outWriter.nFields
         (
             nVolumeFields
           + (s.withSurfaceFields() ? nSurfaceFields : 0)
           +
             (
                 // Face-id field, but not if the writer manages that itself
-                !s.interpolate() && s.hasFaceIds() && !outWriter.usesFaceIds()
+                !s.isPointData() && s.hasFaceIds() && !outWriter.usesFaceIds()
               ? 1 : 0
             )
         );
@@ -360,7 +360,7 @@ bool Foam::sampledSurfaces::read(const dictionary& dict)
                 newWriter(writerType, formatOptions, surfDict)
             );
 
-            writers_[surfi].isPointData() = surfs[surfi].interpolate();
+            writers_[surfi].isPointData(surfs[surfi].isPointData());
 
             // Use outputDir/TIME/surface-name
             writers_[surfi].useTimeDir(true);
@@ -426,7 +426,7 @@ bool Foam::sampledSurfaces::read(const dictionary& dict)
                 newWriter(writerType, formatOptions, surfDict)
             );
 
-            writers_[surfi].isPointData() = surfs[surfi].interpolate();
+            writers_[surfi].isPointData(surfs[surfi].isPointData());
 
             // Use outputDir/TIME/surface-name
             writers_[surfi].useTimeDir(true);
@@ -565,7 +565,7 @@ bool Foam::sampledSurfaces::performAction(unsigned request)
             outWriter.beginTime(obr_.time());
 
             // Face-id field, but not if the writer manages that itself
-            if (!s.interpolate() && s.hasFaceIds() && !outWriter.usesFaceIds())
+            if (!s.isPointData() && s.hasFaceIds() && !outWriter.usesFaceIds())
             {
                 // This is still quite horrible.
 
@@ -723,7 +723,7 @@ bool Foam::sampledSurfaces::expire(const bool force)
         }
 
         writers_[surfi].expire();
-        writers_[surfi].mergeDim() = mergeDim;
+        writers_[surfi].mergeDim(mergeDim);
         nFaces_[surfi] = 0;
     }
 

src/sampling/sampledSurface/sampledSurfaces/sampledSurfacesTemplates.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2015-2020 OpenCFD Ltd.
+    Copyright (C) 2015-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -108,7 +108,7 @@ void Foam::sampledSurfaces::performAction
 
         Field<Type> values;
 
-        if (s.interpolate())
+        if (s.isPointData())
         {
             if (!interpPtr)
             {
@@ -151,7 +151,7 @@ void Foam::sampledSurfaces::performAction
 
         if ((request & actions_[surfi]) & ACTION_STORE)
         {
-            if (s.interpolate())
+            if (s.isPointData())
             {
                 storeRegistryField<Type, polySurfacePointGeoMesh>
                 (

src/surfMesh/writers/surfaceWriter.H

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2012 OpenFOAM Foundation
-    Copyright (C) 2015-2020 OpenCFD Ltd.
+    Copyright (C) 2015-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -83,6 +83,9 @@ SourceFiles
 #include "InfoProxy.H"
 #include "runTimeSelectionTables.H"
 
+// Deprecated methods
+// #define Foam_surfaceWriter_directAccess
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 namespace Foam
@@ -254,10 +257,10 @@ public:
 
     // Constructors
 
-        //- Construct null
+        //- Default construct
         surfaceWriter();
 
-        //- Construct null with specified options
+        //- Default construct with specified options
         explicit surfaceWriter(const dictionary& options);
 
         //- Construct from components
@@ -378,15 +381,17 @@ public:
         //  Currently only used by the legacy VTK format.
         inline label nFields() const;
 
-        //- The number of expected output fields.
+        //- Set the number of expected output fields
         //  Currently only used by the legacy VTK format.
-        inline label& nFields();
+        //  \return old value
+        inline label nFields(const label n);
 
         //- Are the field data to be treated as point data?
         inline bool isPointData() const;
 
-        //- Set handling of field data to face/point data.
+        //- Set handling of field data to face/point data
-        inline bool& isPointData();
+        //  \return old value
+        inline bool isPointData(const bool on);
 
         //- Should a time directory be spliced into the output path?
         inline bool useTimeDir() const;
@@ -395,9 +400,6 @@ public:
         //  \return old value
         inline bool useTimeDir(const bool on);
 
-        //- Change handling of spliced output path.
-        inline bool& useTimeDir();
-
         //- Get output verbosity
         inline bool verbose() const;
 
@@ -405,14 +407,12 @@ public:
         //  \return old value
         inline bool verbose(const bool on);
 
-        //- Access output verbosity
-        inline bool& verbose();
-
         //- The current value of the point merge dimension (metre)
         inline scalar mergeDim() const;
 
-        //- The current value of the point merge dimension (metre)
+        //- Change the point merge dimension (metre)
-        inline scalar& mergeDim();
+        //  \return old value
+        inline scalar mergeDim(const scalar dist);
 
 
     // Time
@@ -539,6 +539,28 @@ public:
             Ostream& os,
             const InfoProxy<surfaceWriter>& ip
         );
+
+
+    // Housekeeping
+
+        #ifdef Foam_surfaceWriter_directAccess
+
+        //- Access number of expected output fields.
+        label& nFields() { return nFields_; }
+
+        //- Access handling of face/point data
+        bool& isPointData() { return isPointData_; }
+
+        //- Access handling of spliced output path
+        bool& useTimeDir() { return useTimeDir_; }
+
+        //- Access output verbosity
+        bool& verbose() { return verbose_; }
+
+        //- Access value of the point merge dimension
+        scalar& mergeDim() { return mergeDim_; }
+
+        #endif
 };
 
 

src/surfMesh/writers/surfaceWriterI.H

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2019-2020 OpenCFD Ltd.
+    Copyright (C) 2019-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -33,9 +33,11 @@ inline Foam::label Foam::surfaceWriter::nFields() const
 }
 
 
-inline Foam::label& Foam::surfaceWriter::nFields()
+inline Foam::label Foam::surfaceWriter::nFields(const label n)
 {
-    return nFields_;
+    label old(nFields_);
+    nFields_ = n;
+    return old;
 }
 
 
@@ -45,9 +47,11 @@ inline bool Foam::surfaceWriter::isPointData() const
 }
 
 
-inline bool& Foam::surfaceWriter::isPointData()
+inline bool Foam::surfaceWriter::isPointData(const bool on)
 {
-    return isPointData_;
+    bool old(isPointData_);
+    isPointData_ = on;
+    return old;
 }
 
 
@@ -65,12 +69,6 @@ inline bool Foam::surfaceWriter::useTimeDir(const bool on)
 }
 
 
-inline bool& Foam::surfaceWriter::useTimeDir()
-{
-    return useTimeDir_;
-}
-
-
 inline bool Foam::surfaceWriter::verbose() const
 {
     return verbose_;
@@ -85,21 +83,17 @@ inline bool Foam::surfaceWriter::verbose(const bool on)
 }
 
 
-inline bool& Foam::surfaceWriter::verbose()
-{
-    return verbose_;
-}
-
-
 inline Foam::scalar Foam::surfaceWriter::mergeDim() const
 {
     return mergeDim_;
 }
 
 
-inline Foam::scalar& Foam::surfaceWriter::mergeDim()
+inline Foam::scalar Foam::surfaceWriter::mergeDim(const scalar dist)
 {
-    return mergeDim_;
+    scalar old(mergeDim_);
+    mergeDim_ = dist;
+    return old;
 }