HP Workstation zx2000 OpenGL 1.1 Reference for HP-UX 11.x - Page 26

B gluBeginTrim gluBeginTrim gluBeginTrim, gluEndTrim: delimit a NURBS trimming loop definition. C Specification void gluBeginTrim( GLUnurbs* nurb) void gluEndTrim( GLUnurbs* nurb) Parameters nurb Specifies the NURBS object (created with gluNewNurbsRenderer). Description 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 piece wise 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). 26 Chapter 2