GLUTESSPROPERTY(3G) GLUTESSPROPERTY(3G)
NAME
gluTessProperty - set a tessellation object property
C SPECIFICATION
void gluTessProperty( GLUtesselator* tess,
GLenum which,
GLdouble data )
PARAMETERS
tess Specifies the tessellation object (created with gluNewTess).
which Specifies the property to be set. Valid values are GLU_TESS_WINDING_RULE,
GLU_TESS_BOUNDARY_ONLY, GLU_TESS_TOLERANCE.
data 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 wind-ing winding
ing 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
Is a boolean value ("value" should be set to GL_TRUE or GL_FALSE). When set to
GL_TRUE, a set of closed contours separating the polygon interior and exterior are
returned instead of a tessellation. Exterior contours are oriented CCW with respect to
the normal; interior contours are oriented CW. The GLU_TESS_BEGIN and
GLU_TESS_BEGIN_DATA callbacks use the type GL_LINE_LOOP for each contour.
GLU_TESS_TOLERANCE
Specifies a tolerance for merging features to reduce the size of the output. For
example, two vertices that are very close to each other might be replaced by a single
vertex. The tolerance is multiplied by the largest coordinate magnitude of any input
vertex; this specifies the maximum distance that any feature can move as the result of
a single merge operation. If a single feature takes part in several merge operations,
the total distance moved could be larger.
Feature merging is completely optional; the tolerance is only a hint. The implementa-tion implementation
tion is free to merge in some cases and not in others, or to never merge features at
all. The initial tolerance is 0.
The current implementation merges vertices only if they are exactly coincident,
regardless of the current tolerance. A vertex is spliced into an edge only if the
implementation is unable to distinguish which side of the edge the vertex lies on. Two
edges are merged only when both endpoints are identical.
SEE ALSO
gluGetTessProperty(3G), gluNewTess(3G)
GLUTESSPROPERTY(3G)
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