fgluTessProperty(3G) OpenGL Reference fgluTessProperty(3G)NAME
fgluTessProperty - set a tessellation object property
FORTRAN SPECIFICATION
SUBROUTINE fgluTessProperty( CHARACTER*8 tess,
INTEGER*4 which,
REAL*4 data )
PARAMETERS
tess Specifies the tessellation object (created with fgluNewTess).
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
fgluTessProperty 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.
Page 1
fgluTessProperty(3G) OpenGL Reference fgluTessProperty(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
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 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, 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
fgluGetTessProperty, fgluNewTess
Page 2
