8

1098 Chapter 9: Sur face Modeling
Cur v es for Sweeps
• Besides expect ing cross-sect ion curves to be all
in the same direction,
1-rail (page 1–1205) and
2-rail (page 1 –1205) sweep surfaces work best
if the cross sections intersect the rail or rails. To
achieve this, draw the rai ls first, then draw the
crosssectionsusingtheNURBS
Snaps (page
2–38)
Curve End and Curve Edge turned on.
• 2-ra il sweeps have t he additiona l requirement
that the endpoints of the first cross section
intersect the endpoints of the rails. Again,
NURBS Snaps help you do this.
If the endpoints of the first cross-section don’t
coincide w ith the rail endpoints, the resulting
surface might not follow the rails.
• While you’re editing a sweep, the Edit Curve
button lets you directly transform the CVs of
a rail or cross section, w ithout changing the
sub-object level. Edit Curve also g ives you
access to all the rollouts that control t he curve.
YoucanuseRefineorMakeFirst,forexample,
without changing levels.
Curves on S urf aces a nd Pr oj ected Cur v es
• You can use a viewport to draw a curve on a
surface (page 1–1174)
(COS), but this works
only for visible portions of the surface. To see
theentiresurfaceandthecurveorcurvesonit
projected into a flat plane, use Edit Curve.
Ifyourmousehasamiddlemousebutton,
ALT+middle m ouse button lets you use
arc
rotate (page 3–787)
to change a viewport’s
orientation while you draw the curve on
surface.
• Neither curves on surfaces nor projected cur ves
cancrosstheedgeofasurface.Thisincludes
the seam on surfaces w ith fused CVs. If you
try to project across the seam, only part of the
curve’sprojectioniscreated.
Cr e at in g Bl e n d Su rfaces
• Youcanblendbetweencurvesorbetween
surface edges. (You can’t blend from a trimmed
edge. In that situation, you are blending from
the curve that trimmed the surface.)
• If you want a controllable tangent or tension,
you must blend to a surface edge or a curve on a
surface.Adjustingtensionchangestheflatness
or "bulginess" of that end of the blend.
When a cur ve and a surface (or two surfaces)
are near each other, sometimes it can be hard to
tell w hich edge you are selecting. To assist you ,
the currently selected surface turns yellow, and
the edge that wil l be used for the blend turns
blue. Make sure you have selected the right
surfacebeforeyouchoosetheedge.
• Iftheedgesyouareblendinghavedifferent
numbers of points (usually due to different
surface approximation settings), then
sometimes rendering shows gaps between the
blend and the original surface. If this happens,
go to the
Surface Approximation rollout (page
1–1241)
and increase the va lue 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 curves,
use a
ruled surface (page 1–1194) instead of a U
loft. This is faster.
• Ifloftcreationseemsslow,makesurethe
Display While Creating check box (in t he
ULoft
Surface rollout (page 1–1197)
)isturnedoff.
• If the U loft doesn’t come out as you expected,
try reparameterizing the curves. C lick
Reparam. at the Curve sub-object level. This
button is on the C V Curve rollout. In the
Reparameterize dialog (page 1–1238) ,choose
Chord Length reparameterization.