The "full" filtering option in isoSurface now no longer attempts to
remove non-manifold faces. As a result, this filtering level is now
robust, but it may leave small imperfections in the surface. This has
been reinstated as the default filtering level as it has advantageous
properties over "partial" filtering in both smoothness of visualisation
and in the size of the files it generates.
A new "clean" level has been added, which does try and remove
imperfections. This is equivalent to the previous operation of the
"full" option. This is not guaranteed to be robust. In certain
configurations the removal process can propagate and delete an entire
section of the surface.
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