Glu.BeginTrim (gb.opengl.glu)

Static Sub BeginTrim ( Nurb As GluNurb )

Delimit a NURBS trimming loop definition.



Specifies the NURBS object (created with Glu.NewNurbsRenderer).


Use Glu.BeginTrim to mark the beginning of a trimming loop and Glu.EndTrim 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 Glu.BeginSurface and Glu.EndSurface.

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 Glu.BeginTrim*/Glu.EndTrim pair.

The definition of a single closed trimming loop can consist of multiple curve segments, each described as a piecewise linear curve (see Glu.PwlCurve) or as a single NURBS curve (see Glu.NurbsCurve), 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 Glu.BeginTrim and Glu.EndTrim) are Glu.PwlCurve and Glu.NurbsCurve.

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 Glu.NurbsCallback).

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.


This code fragment defines a trimming loop that consists of one piecewise linear curve, and two NURBS curves:
gluPwlCurve(..., GLU_MAP1_TRIM_2);
gluNurbsCurve(..., GLU_MAP1_TRIM_2);
gluNurbsCurve(..., GLU_MAP1_TRIM_3);

See also

Glu.NurbsCallback, Glu.NurbsCurve, Glu.PwlCurve

See original documentation on OpenGL website