HP Workstation zx2000 OpenGL 1.1 Reference for HP-UX 11.x - Page 418

T gluTessProperty gluTessProperty gluTessProperty: set a tessellation object property. C Specification void gluTessProperty( GLUtesselator* tess, GLenum which, GLdouble data) Parameters tess which data Specifies the tessellation object (created with gluNewTess). Specifies the property to be set. Valid values are GLU_TESS_WINDING_RULE, GLU_TESS_BOUNDARY_ONLY, GLU_TESS_TOLERANCE. Specifies the value of the indicated property. Description gluTessProperty is used to control properties stored in a tessellation object. These properties affect the way that the polygons are interpreted and rendered. The legal values for which are as follows: GLU_TESS_WINDING_RULE Determines which parts of the polygon are on the "interior". data may be set to one of GLU_TESS_WINDING_ODD, GLU_TESS_WINDING_NONZERO, GLU_TESS_WINDING_POSITIVE, or GLU_TESS_WINDING_NEGATIVE, or GLU_TESS_WINDING_ABS_GEQ_TWO. To understand how the winding rule works, consider that the input contours partition the plane into regions. The winding rule determines which of these regions are inside the polygon. For a single contour C, the winding number of a point x is simply the signed number of revolutions we make around x as we travel once around C (where CCW is positive). When there are several contours, the individual winding numbers are summed. This procedure associates a signed integer value with each point x in the plane. Note that the winding number is the same for all points in a single region. The winding rule classifies a region as "inside" if its winding number belongs to the chosen category (odd, nonzero, positive, negative, or absolute value of at least two). The previous GLU tessellator (prior to GLU 1.2) used the "odd" rule. The "nonzero" rule is another common way to define the interior. The other three rules are useful for polygon CSG operations. GLU_TESS_BOUNDARY_ONLY 418 Chapter 18