SET PARAMETRIC SURFACE CHARACTERISTICS specifies the appearance of parametric surfaces. The aspect consists of a type and a data record. The content of the data record is unique for each type.
The defined parametric surface characteristics types are :
1 NONE : No surface characteristics beyond those aspects defined to fill area sets are used. The data record is empty for this type.
2 WORKSTATION DEPENDENT : A workstation dependent representation that displays the shape of the surface is drawn. This method does not distinguish between front and back facing portions of the surface. The appearance of the representation is controlled by the appropriate set of output primitive attributes for the representation. It is workstation dependent how the representation interacts with any interior rendering indicated by the interior aspects. There are no data record contents for this type.
3 ISOPARAMETRIC CURVES : Isoparametric curves are drawn on the surface. The data record contains the number of curves to draw in each of the parameter dimensions and their placement. If the placement is UNIFORM the specified number of curves are evenly spaced within the parameter range of the surface ; curves are also drawn at the limits of the parameter range. If the placement is NON-UNIFORM the specified number of curves are evenly spaced between each pair of knots ; curves are also drawn at the knots. In both cases only the portions of isoparametric curves are drawn that are within the interior of the surface as defined by any trimming loops.
This method does not distinguish between front and back facing portions of the surface. The appearance of the isoparametric curves is controlled by the polyline attributes. The approximation of the isoparametric curves is controlled by the surface approximation criteria. The isoparametric curves shall lie on the approximated surface. The isoparametric curves are drawn in addition to any interior rendering indicated by the interior style or back interior style. Isoparametric curves have higher visual priority than the primitive's filled or hollow interiors, but lower priority than the primitive's edges.
4 LEVEL CURVES in MC : Level curves are drawn on the surface. The curves
correspond to the intersections of the surface and a finite set of planes
perpendicular to a modelling coordinate direction vector. The positions of the
planes are specified by a sequence of intersection points along an infinite
line defined by a modelling coordinate origin point Po and a
direction vector, 
The ti are a sequence of parameters in
increasing order specifying the intersection points, they are in the range
-[infinity] < ti < [infinity]. The Pi are
the intersection points of the perpendicular planes with the infinite line.
Po is a specified origin point in modelling coordinates, and
is the specified direction vector in modelling
coordinates. The i-th plane is perpendicular to the direction vector
, and intersects the infinite line at point
Pi. The data record consists of the origin point,
Po ; the direction vector 
; and the
ordered list of parameters, ti. Only the portions of the
level curves are drawn that are within the interior of the surface as defined
by any trimming loops.
This method does not distinguish between front and back facing portions of the surface. The appearance of the level curves is controlled by the polyline attributes. The approximation of level curves is controlled by the surface approximation criteria. The level curves shall lie on the approximated surface. The curves are drawn in addition to any interior rendering indicated by the interior style or back interior. Level curves have higher visual priority than the primitive's filled or hollow interiors, but lower priority than the primitive's edges.
5 LEVEL CURVES in WC : Level curves are drawn on the surface. The curve
correspond to the intersections of the surface and a finite set of planes
perpendicular to a world coordinate direction vector. The positions of the
planes are specified by a sequence of intersection points along an infinite
line defined by a world coordinate origin point Po and a
direction vector, 
The ti are a sequence of parameters in
increasing order specifying the intersection points, they are in the range
-[infinity] < ti < [infinity]. The
Pi are the intersection points of the perpendicular planes with the
infinite line. Po is a specified origin point in world
coordinates, and 
is the specified direction vector in
world coordinates. The i-th plane is perpendicular to the direction
vector 
, and intersects the infinite line
at point Pi. The data record consists of the origin point,
Po ; the direction vector 
; and the
ordered list of parameters, ti. Only the portions of the
level curves are drawn that are within the interior of the surface as defined
by any trimming loops.
This method does not distinguish between front and back facing portions of the surface. The appearance of the level curves is controlled by the polyline attributes. The approximation of level curves is controlled by the surface approximation criteria. The level curves shall lie on the approximated surface. The curves are drawn in addition to any interior rendering indicated by the interior style or back interior. Level curves have higher visual priority than the primitive's filled or hollow interiors, but lower priority than the primitive's edges.
Lighting and shading shall be applied to parametric surfaces. The interior shading method shall be applied to each facet.