bgntrim, endtrim - delimit a NURBS surface trimming loop
Use bgntrim to mark the beginning of a definition for a trimming loop.
Use endtrim to mark the end of a definition for a trimming loop. A
trimming loop is a set of oriented curves (forming a closed curve) that
defines boundaries of a NURBS surface. You include these trimming loop
definitions in the definition of a NURBS surface.
The definition for a NURBS surface may contain many trimming loops. For
example, if you wrote a definition for NURBS surface that resembled a
rectangle with a hole punched out, the definition would contain two
trimming loops. One loop would define the outer edge of the rectangle.
The other trimming loop would define the hole punched out of the
rectangle. The definitions of each of these trimming loops would be
bracketed by a bgntrim/endtrim pair.
The definition of a single closed trimming loop may consist of multiple
curve segments, each described as a piecewise linear curve (see pwlcurve)
or as a single NURBS curve (see nurbscurve), or as a combination of both
in any order. The only Graphics library calls that can appear in a
trimming loop definition (between a call to bgntrim and a call to
endtrim) are pwlcurve and nurbscurve.
In the following code fragment, we define a single trimming loop that
consists of one piecewise linear curve and two NURBS curves:
pwlcurve(. . .);
nurbscurve(. . .);
nurbscurve(. . .);
The area of the NURBS surface that the system displays is the region in
the domain to the left of the trimming curve as the curve parameter
increases. Thus, the resultant visible region of the NURBS surface is
inside for a counter-clockwise trimming loop and outside for a clockwise
trimming loop. So for the rectangle mentioned earlier, the trimming loop
for the outer edge of the rectangle should run counter-clockwise, and the
trimming loop for the hole punched out should run clockwise.
If you use more than one curve to define a single trimming loop, the
curve segements must form a closed loop (i.e, the endpoint of each curve
must be the starting point of the next curve, and the endpoint of the
final curve must be the starting point of the first curve). If the
endpoints of the curve are sufficiently close together but not exactly
coincident, the system coerces the them to match. If the endpoints are
not sufficiently close, the system generates an error message and ignores
the entire trimming loop.
If a trimming loop definition contains multiple curves, the direction of
the curves must be consistent (i.e., the inside must be to the left of
the curves). Nested trimming loops are legal as long as the curve
orientations alternate correctly. If no trimming information is given
for a NURBS surface, the entire surface is drawn.
On Impact and Infinite Reality lmcolor cannot be called between bgntrim
bgnsurface, nurbssurface, nurbscurve, pwlcurve, setnurbsproperty,
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