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Surface Approximation 1241
curvature is greatest, and fewer segments where
its curvature is less.
Surface Approximation
Modify panel > Selec t top-level NURBS object. > Surface
Approximation rollout
Modify panel > Select N URBS surface sub-object. >
Surface 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 Appr oximation rollout controls how
surface sub-objects in the NURBS model are
approximated for purposes of rendering and
view por t display. NURBS can be approximated
differently in viewp orts 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 N URBS model.
Thefirsttwocontrolsonthisrolloutareradio
buttons for selecting the kind of display output,
viewport or renderer, these parameters cont rol.
Surface approximation parameters are
not
animatable.
Note: If the size or shape of a surface changes
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 the disadvantage that you can’t apply
image
motion blur (page 3–1049)
to NURBS objects
whose tessellation changes during animation. The
Regular method of tessel lation is the exception:
it doesn’t change when anim ated, so you can use
image motion blur with Regular tessellation.
Surface Approximation Per Sur face
By default, surface sub-objects use the same
approximationsettingsasthetop-levelNURBS
model. You can overr ide these settings.
Each surface sub-object now has a Surface
Ap proximation rollout of its own. The controls
on this rollout are disabled unless you turn off
the Lock to Top Level toggle. With this toggle
turned off, you can choose approximation settings
specific to this surface sub-object.
Interf ace