107 lines
3.6 KiB
Plaintext
107 lines
3.6 KiB
Plaintext
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'\" e
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'\"! eqn | mmdoc
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'\"macro stdmacro
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.ds Vn Version 1.2
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.ds Dt 6 March 1997
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.ds Re Release 1.2.0
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.ds Dp May 02 11:53
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.ds Dm 37 begintrim
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.ds Xs 15822 6 begintrim.gl
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.TH GLUBEGINTRIM 3G
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.SH NAME
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.B "gluBeginTrim, gluEndTrim
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\- delimit a NURBS trimming loop definition
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.SH C SPECIFICATION
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void \f3gluBeginTrim\fP(
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GLUnurbs* \fInurb\fP )
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.nf
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.fi
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void \f3gluEndTrim\fP(
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GLUnurbs* \fInurb\fP )
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.nf
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.fi
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.EQ
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delim $$
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.EN
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.SH PARAMETERS
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.TP \w'\f2nurb\fP\ \ 'u
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\f2nurb\fP
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Specifies the NURBS object (created with \%\f3gluNewNurbsRenderer\fP).
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.SH DESCRIPTION
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Use \%\f3gluBeginTrim\fP to mark the beginning of a
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trimming loop, and \%\f3gluEndTrim\fP to mark the end
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of a trimming loop. A trimming loop is
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a set of oriented curve segments (forming a closed curve) that
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define boundaries of a NURBS surface. You include these
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trimming loops in the definition of a NURBS
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surface, between calls to \%\f3gluBeginSurface\fP and \%\f3gluEndSurface\fP.
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.P
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The definition for a NURBS surface can contain many
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trimming loops. For example, if you wrote a definition
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for a NURBS surface that resembled a rectangle with
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a hole punched out, the definition would contain two
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trimming loops. One loop would define the outer edge
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of the rectangle; the other would define
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the hole punched out of the rectangle. The definitions
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of each of these trimming loops would be bracketed by a
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\%\f3gluBeginTrim\fP/\%\f3gluEndTrim\fP pair.
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.P
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The definition of a single closed trimming loop can consist
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of multiple curve segments, each described as a piecewise
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linear curve (see \%\f3gluPwlCurve\fP) or as a single NURBS
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curve (see \%\f3gluNurbsCurve\fP), or as a combination of
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both in any order. The only library calls that can
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appear in a trimming loop definition (between the calls
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to \%\f3gluBeginTrim\fP and \%\f3gluEndTrim\fP) are
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\%\f3gluPwlCurve\fP and \%\f3gluNurbsCurve\fP.
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.P
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The area of the NURBS surface that is displayed is the
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region in the domain to the left of the trimming curve
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as the curve parameter increases. Thus, the retained
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region of the NURBS surface is inside a
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counterclockwise trimming loop and outside a clockwise
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trimming loop. For the rectangle mentioned earlier,
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the trimming loop for the outer edge of the rectangle runs
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counterclockwise, while the trimming loop for the punched-out hole
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runs clockwise.
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.P
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If you use more than one curve to define a single trimming
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loop, the curve segments must form a closed loop (that is,
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the endpoint of each curve must be the starting point of the
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next curve, and the endpoint of the final curve must
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be the starting point of the first curve). If the
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endpoints of the curve are sufficiently close together
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but not exactly coincident, they will be coerced to match.
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If the endpoints are not sufficiently close, an error results
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(see \%\f3gluNurbsCallback\fP).
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.P
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If a trimming loop definition contains multiple curves,
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the direction of the curves must be consistent (that is, the
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inside must be to the left of all of the curves). Nested
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trimming loops are legal as long as the curve orientations
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alternate correctly.
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If trimming curves are self-intersecting,
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or intersect one another, an error results.
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.P
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If no trimming information is given
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for a NURBS surface, the entire surface is drawn.
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.SH EXAMPLE
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This code fragment defines a trimming loop that consists of one
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piecewise linear curve, and two NURBS curves:
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.sp
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.Ex
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gluBeginTrim(nobj);
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gluPwlCurve(..., GLU_MAP1_TRIM_2);
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gluNurbsCurve(..., GLU_MAP1_TRIM_2);
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gluNurbsCurve(..., GLU_MAP1_TRIM_3);
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gluEndTrim(nobj);
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.Ee
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.SH SEE ALSO
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\%\f3gluBeginSurface(3G)\fP, \%\f3gluNewNurbsRenderer(3G)\fP, \%\f3gluNurbsCallback(3G)\fP,
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\%\f3gluNurbsCurve(3G)\fP, \%\f3gluPwlCurve(3G)\fP
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