9

How to Make Objects with NURBS Modeling 1097
• If you want a controllable tangent or tension,
you must blend to a surface edge or a curve on a
surface. Adjusting tension changes the flatness
or "bulginess" of that end of the blend.
When a curve and a surface (or two surfaces)
are near each other, s ometimes it can be hard to
tellwhichedgeyouareselecting.Toassistyou,
the currently selected surface turns yellow, and
theedgethatwillbeusedfortheblendturns
blue. Make sure you have selected the right
surfacebeforeyouchoosetheedge.
• Iftheedgesyouareblendinghavedifferent
numbers of points (usually due to different
surface approximation settings), then
sometimesrenderingshowsgapsbetweenthe
blend and the original surface. If this happens,
go to the Surface Approximation rollout (page
1–1239) and increase the value of Merge until
the gaps disappear when you render.
The Merge setting affects only the production
renderer. It has no effect on viewport display.
Lof ts
• If you need a surface between only two cur ves,
use a r uled surface (page 1–1193) instead of a U
loft. Thisisfaster.
• Ifloftcreationseemsslow,makesurethe
Display While Creating check box (in the U
Loft Surface rollout (page 1–1196))isturnedoff.
• If the U loft doesn’t come out as you expected,
try reparameterizing the curves. Click
Reparam. at the Curve sub-object level. This
button is on the C V Curve rollout. In the
Reparameterize dialog (page 1–1237),choose
Chord Length reparameterization.
If a curve is dependent or a point curve, first
youwillhavetomakeitindependent(thisalso
improves performance).
Curves that are made of two joined curves
have this problem more often than others.
Ifyouhaveajoinedcurveasoneofthe
cur ves to construct the loft, reparameterize it
before you create the loft, or set the curves to
reparameterize automatically.
•TheEditCurvebuttonletsyoudirectly
transform the CVs of a curve within a U loft
or UV loft (page 1–1200),withoutchanging
the sub-object level. Edit Curve a lso g ives you
access to all the rollouts that control the curve.
YoucanuseRefineorMakeFirst,forexample,
without changing levels.
• To close a UV loft, you c an pick the first V
curveagaintomakeitthelastcurveintheloft.
Sometimes a seam is visible at this location in
the U V loft.
Multisided B lend Sur faces
•Iftheprogramdoesn’tcreatethemultisided
blend (page 1–1213), fuse t he CVs at the three
or four corners. Snap ping CVs to each other
doesn’t always succeed, because of rounding
off.
Multicur ve Tr immed Sur faces
• Multicur ve trimmed surfaces are the only way
to create a trimmed hole that contains sharp
angles.
Displ acement M apping
• In general, the default tessellation settings
aren’t suitable for displaced surfaces. With
these default settings, displacem ent mapping
can create an extremely high face count,
which performs very slowly. Change the
surface approximation to the lowest necessary
resolution. A good rule of th umb is to start
with Spatial approximation and an Edge value
of 20. If that is too low, reduce the Edge value
untilthemodellooksasitshould.
•UsetheDisplace NURBS world space modifier
(page 1–515) to convert the displacement map