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1250 Chapter 9: Surface Modeling
For example, if you convert a teapot to NURBS and
set the U and V steps to 15, the Regular method
generates 4470 faces but the Parametric method
generates 204960 faces.
Parametric mesh of the NURBS teapot
Spatial—Generates a uniform tessellation made
of triangular faces.
The Edge parameter specifies the maximum
length of a triangular face in the tessellation. The
valueisapercentageoftheobject’sbounding
box. Decreasing this value increases accuracy but
increases rendering time.
Spatial mesh of t he NURBS teapot
Curvature—Generates a var iable tessellation based
on the curvature of the sur face. The tessellation
has a finer grain w here the surface is more curved.
Changing surface curvature dynamically changes
the curvature tessellation.
The Distance parameter specifies how far the
approximation can deviate from the ac tual NURBS
surface. Distance is a percentage of the diagonal of
each surface’s bounding box. Each surface in an
object is tessellated based on its size, independently
of other surfaces, and scaling a surface doesn’t
change its tessellation. D ecreasing this value
increases accuracy but increases rendering time.
When you set Distance to 0.0, the s oftware ig nores
this parameter and uses the Angle to control
accuracy.
The Angle parameter specifies the m ax imum ang le
between faces in the approximation. Decreasing
this value increases accuracy but increases
rendering time. When you set Angle to 0 .0, the
software ignores this parameter and uses the
Distance to control accuracy.
When b oth Distance and Angle are 0.0, the
surfaces degenerate and can become flat surfaces.
Curvature mesh of the NURBS teapot
Spatial a nd Cur vature—(The default.) Combines
the spatial (edge-length) method and the
cur vature (distance and angle) methods, using all
three values.
TheEdgeparameterspecifiesthemaximum
length of a triangular face in t he tessellation. The