Patch, as the compulsory keyword, is followed by its identification number. The outer normal vector of the patch plane is specified after keyword normal. The position of the plane may be specified after keyword origin. Otherwise the plane is located in such a way that the plane is exactly in the middle of the extent in the normal direction given by all boundary and fixed vertices of the patch. The planarity of the patch is checked for all boundary and fixed vertices and curves of the patch as the distance from the patch plane. Should this distance exceed a user given tolerance the execution is premeturaly terminated. The tolerance may be specified after keyword epsilon, otherwise the global user defined epsilon given by a command line option (see -e option in Section Command Line Options) or its default value is used. The list of signed identification numbers of curves bounding the patch (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 patch 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 patch is irrelevant (as the curve itself). The list of identification numbers of patches surrounded by the currently defined patch may be specified after keyword subpatch. Each subpatch must be coplanar with the patch, however its normal orientation is arbitrary. If a subpatch is touching the patch along its boundary curve, this curve must be specified also in the list of curves bounding the patch (including proper orientation). Furthermore, identification numbers of vertices and curves inside the patch can be enumerated after appropriate model entity keyword ( vertex or curve) preceded by keyword fixed. The meaning of keywords size, property, and hidden is the same as explained in Section Input Record of a Curve. Only patch 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 patches has an impact on the performance because the regions sharing those patches are subjected to an additional smoothing. Note also, that all the generated nodes classified to the patch are exactly in the patch plane, independently of the position of boundary nodes deviating eventually from the plane (by not more than the user prescribed tolerance).