Shell, as the compulsory keyword, is followed by its identification number. The background surface of the shell is specified after keyword bgsurface. Note that the background surface is also part of the model and will be discretized unless marked as hole or virtual. The list of signed identification numbers of curves bounding the shell (from outside or inside) is preceded by keywords boundary curve. The positive number indicates the anticlockwise orientation of the curve (given by its starting and ending vertex) with respect to the shell when viewing it against the outer normal. The negative number indicates the clockwise orientation of the curve. The sign of identification number of collapsed curve forming the boundary of the shell is irrelevant (as the curve itself). Orientation of the outer normal vector of the shell is given by the background surface. The list of identification numbers of shells surrounded by the currently defined shell may be specified after keyword subshell. Furthermore, identification numbers of vertices and curves inside the shell can be enumerated after appropriate model entity keyword ( vertex or curve) preceded by keyword fixed. The meaning of keywords size, rate, property, and hidden is the same as explained in Section Input Record of a Curve. Only shell shared by two solid physical regions of the same property is allowed to be hidden. The factor specification preceded by keyword factor is effective only if size specification is provided. The meaning of keywords hole, output, coincide, and bassoc is the same as described in Section Input Record of a Surface.
Note that using hidden shells has an impact on the performance because the regions sharing those shells are subjected to an additional smoothing.