ENH: support VTK output of point ids

- can be useful for diagnosing mesh internals and the locations of
  decomposed cells.

applications/utilities/postProcessing/dataConversion/foamToVTK/convertVolumeFields.H

@@ -313,7 +313,13 @@ Description
             internalWriter->beginPointData
             (
                 nVolFields + nDimFields + nPointFields
+              + (withPointIds ? 1 : 0)
             );
+
+            if (withPointIds)
+            {
+                internalWriter->writePointIDs();
+            }
         }
 
         forAll(patchWriters, writeri)

applications/utilities/postProcessing/dataConversion/foamToVTK/foamToVTK.C

@@ -91,6 +91,9 @@ Usage
       - \par -no-point-data
         Suppress conversion of pointFields. No interpolated PointData.
 
+      - \par -with-point-ids
+        Additional pointID field for internal mesh
+
       - \par -poly-decomp
         Decompose polyhedral cells into tets/pyramids
 
@@ -379,6 +382,12 @@ int main(int argc, char *argv[])
     );
     argList::addOptionCompat("no-point-data", {"noPointValues", 1806});
 
+    argList::addBoolOption
+    (
+        "with-point-ids",
+        "Additional pointID field for internal mesh"
+    );
+
     argList::addBoolOption
     (
         "one-boundary",  // allPatches
@@ -495,6 +504,19 @@ int main(int argc, char *argv[])
             << nl;
     }
 
+    const bool withPointIds = args.found("with-point-ids");
+    if (withPointIds)
+    {
+        Info<< "Write point ids requested";
+
+        if (noPointValues)
+        {
+            Info<< ", but ignored due to the '-no-point-data' option";
+        }
+
+        Info<< nl;
+    }
+
     wordRes includePatches, excludePatches;
     if (doBoundary)
     {

src/conversion/vtk/output/foamVtkInternalWriter.C

@@ -730,4 +730,75 @@ bool Foam::vtk::internalWriter::writeProcIDs()
 }
 
 
+void Foam::vtk::internalWriter::writePointIDs()
+{
+    if (isState(outputState::POINT_DATA))
+    {
+        ++nPointData_;
+    }
+    else
+    {
+        FatalErrorInFunction
+            << "Bad writer state (" << stateNames[state_]
+            << ") - should be (" << stateNames[outputState::POINT_DATA]
+            << ") for pointID field" << nl << endl
+            << exit(FatalError);
+    }
+
+    if (format_)
+    {
+        if (legacy())
+        {
+            vtk::legacy::intField<1>(format(), "pointID", numberOfPoints_);
+        }
+        else
+        {
+            const uint64_t payLoad = vtk::sizeofData<label>(numberOfPoints_);
+
+            format().beginDataArray<label>("pointID");
+            format().writeSize(payLoad);
+        }
+    }
+
+
+    // Point offset for regular mesh points (without decomposed)
+    const label pointOffset =
+    (
+        parallel_ ? globalIndex(vtuCells_.nPoints()).localStart() : 0
+    );
+
+    // Cell offset for *regular* mesh cells (without decomposed)
+    const label cellOffset =
+    (
+        parallel_ ? globalIndex(vtuCells_.nCells()).localStart() : 0
+    );
+
+
+    labelList pointIds = identity(vtuCells_.nFieldPoints(), pointOffset);
+
+    // The pointID for added points is the cellID, tag as a negative number
+    label pointi = vtuCells_.nPoints();
+    for (const label celli : vtuCells_.addPointCellLabels())
+    {
+        pointIds[pointi] = (-1 - celli - cellOffset);
+        ++pointi;
+    }
+
+    if (parallel_)
+    {
+        vtk::writeListParallel(format_.ref(), pointIds);
+    }
+    else
+    {
+        vtk::writeList(format(), pointIds);
+    }
+
+    if (format_)
+    {
+        format().flush();
+        format().endDataArray();
+    }
+}
+
+
 // ************************************************************************* //

src/conversion/vtk/output/foamVtkInternalWriter.H

@@ -207,6 +207,10 @@ public:
         //  Must be called within the CELL_DATA state.
         bool writeProcIDs();
 
+        //- Write point ids as PointData.
+        //  Must be called within the POINT_DATA state.
+        void writePointIDs();
+
 
     // Write