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 nurb:BeginTrim() / nurb:EndTrim() pair.
The definition of a single closed trimming loop can consist of multiple curve segments, each described as a piecewise linear curve (See nurb:PwlCurve for details.) or as a single NURBS curve (See nurb:Curve for details.), 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 nurb:BeginTrim() and nurb:EndTrim()) are nurb:PwlCurve() and nurb:Curve().
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 counter-clockwise, 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 nurb:Callback for details.).
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.
Please consult an OpenGL reference manual for more information.