Surface, as the compulsory keyword, is followed by its identification number. If surface orders are not provided and the surface is not fixed to any surface the orders are extracted from the curves bounding the surface. When the surface is fixed to a surface, the orders may but need not be specified. In the former case, order in any direction must be greater or equal to the order of the parent surface or set to zero (the order of the parent surface is used in that case). Identification numbers of the four bounding curves in clockwise order with respect to the surface outer side (normal) are specified after keyword curve. The first and second curves determine the and directions, respectively, on the surface with the origin at their common vertex. The surface outer normal is then given by the vector product of vectors tangent to the parametric curves in and directions and oriented in the positive and directions, respectively. The orientation of bounding curves is not relevant. No two adjacent curves are allowed to be degenerated into a single point and no two opposite or adjacent curves are allowed to be the same or coinciding. This disables creation of surfaces degenerated into a curve or point. When the surface is fixed to a surface its bounding curves must be fixed to the parent surface itself. The ordering of bounding curves of the fixed surface must be in agreement with bounding curves ordering of the parent surface. Since physical curves fixed to a physical solid surface are not allowed to intersect each other, the physical surfaces fixed to the same physical solid surface cannot overlap. The meaning of keywords size, factor, rate, property, virtual, and hidden is the same as explained in Section Input Record of a Curve. It should be mentioned however that the smallest of both principal radii of curvature on the surface is considered when the rate is specified. Only surface shared by two solid physical regions of the same property may be hidden. Also note that for surfaces and curves with nonzero size the (nonzero) value of factor is recursively propagated to the boundary entities of the next lower dimension untill a smaller (nonzero) factor is encountered. Keyword hole is used to specify that the currently defined surface forms a hole. The output of triangles on a surface may be enforced or suppressed by setting yes or no after output keyword. By default, surfaces bounding a physical solid region are not designated for output. Also the meaning of keyword polysize is the same as explained in Section Input Record of a Curve. Note that the control points on a surface are ordered in such a way that the first index (corresponding to direction) is running faster. Similarly as in Section Input Record of a Vertex, if generation of elements on a model entity between the surface just being defined and other close surfaces, patches, or shells (already defined) bounding that entity is to be prevented, the other surfaces, patches, or shells should be specified after appropriate model entity keyword ( surface, patch, or shell) preceded by keyword coincide. Keyword bassoc followed by setting yes or no controls separate output of tetrahedrons (ids) classified to region bounded by the surface. Note that a specific command line option controls whether all or none of the physical surfaces will be by default designated for output of associated tetrahedrons.
A set of internal control points must be provided for each non-fixed surface of degrees greater than 1 in both directions. It means that input record of such a surface of degree must be followed by records for each internal control point of the surface. The numbers of the control point (in direction ranging from to and in direction ranging from to ) specified after keyword Polygon are used as the control point sequence numbers and can be referenced only in the context of the current surface. The control point may be specified by three coordinates preceded by keyword xyz or may be associated with any already specified polygon control point using keyword poly followed by the sequence numbers of that control point. Keywords size and weight have the same meaning as explained in Section Input Record of a Curve.
Planar surface must be convex. Note that using hidden surfaces has an impact on the performance because the regions sharing those surfaces are subjected to an additional smoothing.