146 lines
7.2 KiB
XML
146 lines
7.2 KiB
XML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
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"http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
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<refentry id="gluBeginTrim">
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<refmeta>
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<refmetainfo>
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<copyright>
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<year>1991-2006</year>
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<holder>Silicon Graphics, Inc.</holder>
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</copyright>
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</refmetainfo>
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<refentrytitle>gluBeginTrim</refentrytitle>
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<manvolnum>3G</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>gluBeginTrim</refname>
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<refpurpose>delimit a NURBS trimming loop definition</refpurpose>
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</refnamediv>
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<refsynopsisdiv><title>C Specification</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>void <function>gluBeginTrim</function></funcdef>
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<paramdef>GLUnurbs* <parameter>nurb</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsynopsisdiv><title>C Specification</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>void <function>gluEndTrim</function></funcdef>
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<paramdef>GLUnurbs* <parameter>nurb</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<!-- eqn: ignoring delim $$ -->
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<refsect1 id="parameters"><title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><parameter>nurb</parameter></term>
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<listitem>
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<para>
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Specifies the NURBS object (created with <citerefentry><refentrytitle>gluNewNurbsRenderer</refentrytitle></citerefentry>).
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1 id="description"><title>Description</title>
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<para>
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Use <function>gluBeginTrim</function> to mark the beginning of a
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trimming loop and <citerefentry><refentrytitle>gluEndTrim</refentrytitle></citerefentry> 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 <citerefentry><refentrytitle>gluBeginSurface</refentrytitle></citerefentry> and <citerefentry><refentrytitle>gluEndSurface</refentrytitle></citerefentry>.
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</para>
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<para>
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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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<function>gluBeginTrim</function>/<citerefentry><refentrytitle>gluEndTrim</refentrytitle></citerefentry> pair.
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</para>
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<para>
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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 <citerefentry><refentrytitle>gluPwlCurve</refentrytitle></citerefentry>) or as a single NURBS
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curve (see <citerefentry><refentrytitle>gluNurbsCurve</refentrytitle></citerefentry>), 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 <function>gluBeginTrim</function> and <citerefentry><refentrytitle>gluEndTrim</refentrytitle></citerefentry>) are
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<citerefentry><refentrytitle>gluPwlCurve</refentrytitle></citerefentry> and <citerefentry><refentrytitle>gluNurbsCurve</refentrytitle></citerefentry>.
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</para>
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<para>
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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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</para>
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<para>
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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 <citerefentry><refentrytitle>gluNurbsCallback</refentrytitle></citerefentry>).
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</para>
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<para>
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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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</para>
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<para>
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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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</para>
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</refsect1>
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<refsect1 id="example"><title>Example</title>
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<para>
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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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<programlisting>
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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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</programlisting>
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</para>
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</refsect1>
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<refsect1 id="seealso"><title>See Also</title>
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<para>
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<citerefentry><refentrytitle>gluBeginSurface</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluNewNurbsRenderer</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluNurbsCallback</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluNurbsCurve</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluPwlCurve</refentrytitle></citerefentry>
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</para>
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</refsect1>
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<refsect1 id="Copyright"><title>Copyright</title>
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<para>
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Copyright <trademark class="copyright"></trademark> 1991-2006
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Silicon Graphics, Inc. This document is licensed under the SGI
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Free Software B License. For details, see
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<ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>.
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</para>
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</refsect1>
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</refentry>
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