fgluBeginTrim(3G) OpenGL Reference fgluBeginTrim(3G)NAME
fgluBeginTrim, fgluEndTrim - delimit a NURBS trimming loop definition
FORTRAN SPECIFICATION
SUBROUTINE fgluBeginTrim( CHARACTER*8 nurb )
SUBROUTINE fgluEndTrim( CHARACTER*8 nurb )
PARAMETERS
nurb Specifies the NURBS object (created with fgluNewNurbsRenderer).
DESCRIPTION
Use fgluBeginTrim to mark the beginning of a trimming loop, and
fgluEndTrim to mark the end of a trimming loop. A trimming loop is a set
of oriented curve segments (forming a closed curve) that define
boundaries of a NURBS surface. You include these trimming loops in the
definition of a NURBS surface, between calls to fgluBeginSurface and
fgluEndSurface.
The definition for a NURBS surface can contain many trimming loops. For
example, if you wrote a definition for a 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 would define the hole punched out of the rectangle. The
definitions of each of these trimming loops would be bracketed by a
fgluBeginTrim/fgluEndTrim pair.
The definition of a single closed trimming loop can consist of multiple
curve segments, each described as a piecewise linear curve (see
fgluPwlCurve) or as a single NURBS curve (see fgluNurbsCurve), or as a
combination of both in any order. The only library calls that can appear
in a trimming loop definition (between the calls to fgluBeginTrim and
fgluEndTrim) are fgluPwlCurve and fgluNurbsCurve.
The area of the NURBS surface that is displayed is the region in the
domain to the left of the trimming curve as the curve parameter
increases. Thus, the retained region of the NURBS surface is inside a
counterclockwise trimming loop and outside a clockwise trimming loop. For
the rectangle mentioned earlier, the trimming loop for the outer edge of
the rectangle runs counterclockwise, while the trimming loop for the
punched-out hole runs clockwise.
If you use more than one curve to define a single trimming loop, the
curve segments must form a closed loop (that is, 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, they will be coerced to match. If the endpoints are not
sufficiently close, an error results (see fgluNurbsCallback).
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fgluBeginTrim(3G) OpenGL Reference fgluBeginTrim(3G)
If a trimming loop definition contains multiple curves, the direction of
the curves must be consistent (that is, the inside must be to the left of
all of the curves). Nested trimming loops are legal as long as the curve
orientations alternate correctly. If trimming curves are self-
intersecting, or intersect one another, an error results.
If no trimming information is given for a NURBS surface, the entire
surface is drawn.
EXAMPLE
This code fragment defines a trimming loop that consists of one piecewise
linear curve, and two NURBS curves:
gluBeginTrim(nobj);
gluPwlCurve(..., GLU_MAP1_TRIM_2);
gluNurbsCurve(..., GLU_MAP1_TRIM_2);
gluNurbsCurve(..., GLU_MAP1_TRIM_3);
gluEndTrim(nobj);
SEE ALSO
fgluBeginSurface, fgluNewNurbsRenderer, fgluNurbsCallback,
fgluNurbsCurve, fgluPwlCurve
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