Use **gluBeginTrim** to mark the beginning of a
trimming loop, and **gluEndTrim** 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 **gluBeginSurface** and **gluEndSurface**.
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.
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.