We call \textit{hollowing by the ratio $k$} an operation, which for a given
face with the center $G$, executes a dilation on that face with the ratio
$k$, then divides the original face with using this new face.
For example, a cube with a hollow of its top face with a ratio of $0.8$:
The option \Lkeyword{affinage} allows us to hollow a solid's faces either globally or
individually. This option uses the key \Lkeyword{affinagecoeff}
(value $0.8$ by default) which indicates the ratio $k$ used for the
hollow ($0<k<1$).
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\begin{compactitem}
\item \texttt{\Lkeyword{affinage}=\Lkeyval{all}} hollows all the faces;
\item \texttt{\Lkeyword{affinage}=0 1 2 3} hollows the faces 0, 1, 2 and 3;
\end{compactitem}
When a face is hollowed out, the default behaviour suppresses the resulting central
face. However, the option \Lkeyword{affinagerm} allows us to conserve that central face.
When we conserve the centre face, it is---by default---drawn with the same colour
as the original. The option \Lkeyword{fcolor} permits to specify another colour.
