GLUTESSPROPERTY(3G) UNIX Programmer's Manual GLUTESSPROPERTY(3G)
gluTessProperty - set a tessellation object property
void gluTessProperty( GLUtesselator* tess,
GLenum which,
GLdouble data )
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.
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.
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GLUTESSPROPERTY(3G) UNIX Programmer's Manual GLUTESSPROPERTY(3G)
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 call-
backs use the type GL_LINE_LOOP for each con-
tour.
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 implementation
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, regard-
less of the current tolerance. A vertex is
spliced into an edge only if the implementa-
tion is unable to distinguish which side of
the edge the vertex lies on. Two edges are
merged only when both endpoints are identi-
cal.
gluGetTessProperty(3G), gluNewTess(3G)
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