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Surface Approximation 1239
Curve approximation controls are displayed in the
creation parameters for curve objec t s, and on a
Curve Approximation rollout for NURBS models
(top-level NURBS objects). At the mo del level,
approximation controls affect all curve sub-objects
in the model.
Curve approximation is accomplished by segments.
Oneormorelinesegments,orsteps, are used
to approximate each segment of the curve. For
point curves, a segment of a curve is the portion
between one p oint and the next. For CV curves,
the seg ment is determined by the CV’s parametric
knot (page 3–961). The transition from one CV
curve segment to another isn’t visible in viewports.
Curve approximation parameters aren’t
animatable.
Inter face
Steps—The maximum number of line segments
used to approximate each curve segment. If the
curve displays or renders with angles, increase this
value. This control is unavailable when Adaptive
is on. Range=1 to 100.
Optimi ze—Tur n on this check box to optim ize the
curve. When on, interpolation uses the specified
Steps v alue unless two segments are collinear, in
which case they are converted to a single segment.
This control is unavailable when Adaptive is on.
Adaptive—(The default.) Segments t he curve
adaptively, based on its curvature. In other words,
the c u rve is assigned more segments where its
curvature is g reatest, and fewer segments where
its curvature is less.
Surface Approximation
Modify panel > Select top-level NURBS object. > Sur face
Approximation rol lout
Modify panel > Select NURBS sur face sub-object. >
Sur face Approximation rollout
Although NURBS surfaces are analytically
generated, in order to generate and display them
they must be approximated by faces. You use the
controls described in this section to set the type of
approximation used and its parameters.
The Surface Approximation rollout controls how
surfacesub-objectsintheNURBSmodelare
approximated for purposes of rendering and
view por t display. NURBS can b e approximated
differently in viewpor ts and in the renderer.
Typicallyyouwantviewportdisplaytobeclear
and quick, while you want rendered display to be
smooth, accurate, and "realistic." However, the
approximation you choose for viewports creates a
mesh, and the kind of mesh you choose can affect
the behavior of modifiers that you later apply to
the NURBS mo del.
The first two controls on this rollout are radio
buttons for selecting the kind of displa y output,
viewport or renderer, t hese parameters control.
Surface approximation parameters are not
animatable.
Note: Ifthesizeorshapeofasurfacechanges
over time, the tessellation used to approximate it
can change (automatically) as well. This has the
advantage of improving render time in animations.
It has t he disadvantage that you can’t apply image
motion blur (page 3–955) to NURBS objects
whose tessellation changes during animation. The
Regular method of tessellation is the exception:
it doesn’t change when animated, s o you can use
image motion blur with Regular tessellation.