bgntri(3G)

NAME
     bgntri, endtri - delimit a NURBS surface trimming loop

FORTRAN 77 SPECIFICATION
     subroutine bgntri

     subroutine endtri

PARAMETERS
     none

DESCRIPTION
     Use bgntri to mark the beginning of a definition for a trimming loop. Use
     endtri 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 bgntri/endtri pair.

     The definition of a single closed trimming loop may consist of multiple
     curve segments, each described as a piecewise linear curve (see pwlcur)
     or as a single NURBS curve (see nurbsc), 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 bgntri and a call to endtri) are
     pwlcur and nurbsc.

     In the following code fragment, we define a single trimming loop that
     consists of one piecewise linear curve and two NURBS curves:

          call bgntri
              call pwlcur(. . .)
              call nurbsc(. . .)
              call nurbsc(. . .)
          call endtri


     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.

NOTE
     On Impact and Infinite Reality lmcolo cannot be called between bgntri and
     endtri


SEE ALSO
     bgnsur, nurbss, nurbsc, pwlcur, setnur, getnur