GLUTESSPROPERTY() MachTen Programmer’s Manual GLUTESSPROPERTY()

NAME
gluTessProperty - set a tessellation object property

C SPECIFICATION
void gluTessProperty( GLUtesselator* tess,
GLenum which,
GLdouble data )

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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 val-
ues 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 sin-
gle region.

The winding rule classifies a region as
"inside" if its winding number belongs to
the chosen category (odd, nonzero, posi-
tive, 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 separat-
ing the polygon interior and exterior are
returned instead of a tessellation. Exte-
rior contours are oriented CCW with respect
to the normal; interior contours are ori-
ented 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 fea-
ture takes part in several merge opera-
tions, the total distance moved could be
larger.

Feature merging is completely optional; the
tolerance is only a hint. The implementa-
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 ver-
tex 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 end-
points are identical.