vtkSurfaceWriter bug

In v1806 and prior, a bug exists in the vtkSurfaceWriter.C where NaN values are occasionally output to the vtk files when interpolating a field onto the isoSurface with a functionObject such as this:

flame
{
    type surfaces;
    functionObjectLibs ("libsampling.so");
    enabled         true;
    writeControl   adjustableRunTime;
    writeInterval   $...animationOutputTime;
    interpolationScheme cellPoint;
    surfaceFormat vtk;
    // Fields to be sampled
    fields
    (
            T
    );
    surfaces
    (
            flame
            {
                type            isoSurfaceCell;
                isoField        ft;
                isoValue        0.06;
                interpolate     true;
                regularise      false;
            }
     );
}

Unfortunately, I have been unable to reproduce this issue on a simple case. These NaN values are only output occasionally and so far, only for large, complicated cases.

I have been able to apply a temporary bandage to the problem by using this patch (attached). vtkSurfaceWriter.patch

Karl