voidgluBeginTrim(GLUnurbsObj *nobj) voidgluEndTrim(GLUnurbsObj *nobj)

*nobj*-
Specifies the NURBS object (created with
**gluNewNurbsRenderer**).

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 **gluBeginTrim** /
**gluEndTrim** pair.

The definition of a single closed trimming loop can consist of multiple curve
segments, each described as a piecewise linear curve (see
**gluPwlCurve**) or as a single NURBS
curve (see **gluNurbsCurve**), 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 **gluBeginTrim** and
**gluEndTrim**) are **gluPwlCurve**
and **gluNurbsCurve**.

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
**gluNurbsCallback**).

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. Trimming curves cannot be self-intersecting, nor can they intersect one another (or an error results).

If no trimming information is given for a NURBS surface, the entire surface is drawn.

gluBeginTrim(nobj); gluPwlCurve(..., GLU_MAP1_TRIM_2); gluNurbsCurve(..., GLU_MAP1_TRIM_2); gluNurbsCurve(..., GLU_MAP1_TRIM_3); gluEndTrim(nobj);

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