Choosing a tessellation method
Tessellation means that
you create a set of polygons from NURBS geometry. Each tessellation
method provides you with options that let you control the resulting
There are four tessellation methods: “Standard
fit”, “General”, “Count”,
and “Control Points”.
Fit is the default tessellation method. It is “adaptive”
tessellation, meaning that the following options are used to determine
when to stop the tessellation.
For example, the tessellation stops at the Fractional
Tolerance value you set. If there is an edge shorter
than the Minimal Edge Length, the tessellation stops
on that edge. If the surface is flat enough within the edge (the
specified chord/height ratio is small enough), the tessellation
- Chord Height Ratio
Height Ratio is the ratio between the maximum distance
of the curve from the polygon edge used to approximate it and the chord
length. The chord length is the linear distance between two polygon
Valid values range between 0 and 1, where larger values
result in fewer polygon vertices.
For example, the default value, 0.1, means that the
height must be larger than 1/10 of the chord length before additional
edit points are created.
- Fractional Tolerance
Tolerance value determines the degree of accuracy maintained
between the original surface and the interpolated polygonal surfaces.
The default is to be accurate to within 0.01 units, where a unit
refers to the current unit of linear measure (the default unit of
measure is centimeters). Therefore, at no point will the polygonal
surface be more than the tolerance distance away from the original
In this next example, notice how you can enhance
the polygonal surface’s accuracy when you change the Fractional
Tolerance value from 1 to 0.01.
- Minimal Edge Length
a value or use the Minimal Edge Length slider
to set the minimum length of the edges of the triangles or quads
that are created.
- 3D Delta
Delta value determines the 3D spacing for U and V isoparms
on a surface that makes up the initial grid for the tessellation.
In the following example, the 3D Delta value
is changed from the default 0.1 to 1.0.
Set the Tessellation
Method to General to display the following
Setting the initial tessellation controls
Unless Use Chord Height or Use
Chord Height Ratio is turned on, a uniform tessellation
is performed. Each span/surface is split into a number of polygons
depending on the Number U and V values
- U Type/V
Type and V Type pop-up menu items let you
specify whether you want to split the surface based on where the
spans are (then split each span), or based on the parameterization
of the whole surface.
- Number U/Number
Each span or surface is split into the number of polygons
you specify here.
Specifying the secondary tessellation controls
Chord Height or Use Chord Height Ratio is
turned on, you can set a specific value for both the Chord
Height and the Chord Height Ratio.
A value greater than 0 results in fewer polygon vertices if the
ratio on the curve is greater than the current value. For example,
the default value, 0.1, means that the height must be larger than
1/10 of the chord length before additional edit points are created.
Swap on to produce triangles with the opposite orientation
for the final quadrilateral.
the Tessellation Method to Count to
display the following slider.
- Count slider
Use the Count slider
to determine how many polygons the surface can be tessellated into.
See the following examples.
This tessellation method converts the NURBS model
to polygons while matching the CVs of the original NURBS surface.
There are no other options for this operation.
When you use the Control Points Tessellation
Type option you set is ignored and the resulting polygon is in Quads by
you convert trimmed NURBS surfaces the surfaces convert as though they
were not trimmed.
When you use the Attach Multiple Output
Meshes option, the operation may fail to attach surfaces
for a variety of reasons:
attaching a number of surfaces, and some of the surfaces are extremely
small, the tolerance must be set to an appropriately small value or
the attach operation will fail.
attaching multiple surfaces, all of the surfaces must be contiguous to
within the user-defined tolerance. Separated regions cannot be attached
in a single operation. Use the attach operation multiple times for this
case; once for each contiguous region.
surfaces to be attached must have their normals facing in the same direction
or the attach will fail.