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.
Daniel Rypl
2008-06-04