9

1096 Chapter 9: Surface Modeling
draw curves in 3D. B egin drawing a cur ve in
one viewport, go to another viewport, and
continue drawing.
Ifyourmousehasamiddlemousebutton,
Alt +middle mouse button lets you use arc
rotate (page 3–744) to change a viewport’s
orientation while you are creating the curve.
• To create a transform curve along a specific
axis,turnontheappropriateaxisconstraints,
and then
Shif t +moveacopyofthetransform
cur ve.
Curves and Direction
• NURBS curves show their direction in
viewports. A smal l circle indicates the first
vertex.Ifthecurveisclosed,aplussign(+)
indicates the direction of the curve.
Beawareofcurvedirectionwhenyouuse
cur ves to construct blend surfaces (page
1–1183), U loft (page 1–1196) and UV loft (page
1–1200) surfaces, and 1-rail (page 1–1204) and
2-rail (page 1–1209) sweeps. If the curves don’t
have the same direction, you can get strange
twisting. Makesurecurveshavethesame
direction before you construct the surface.
On the Curve Common rollout, the controls
Reverse and Make First let you control the
direction of the curve, and where its starting
point or CV is located.
Another good way to make sure curves are
aligned is to draw one curve and then use
Shi f t +Clone to create the others. After
creating the alig ned curves, you can transform
CVs to vary the curves on which the surface
will be based.
Curv es f or Sweeps
• Besides expecting cross-section curves to b e all
in the same direction, 1-rail (page 1–1204) and
2-rail (page 1–1204) sweep surfaces work best if
the cross sections intersect the rail or rai ls. To
achieve this, draw the rails first, then draw the
crosssectionsusingtheNURBSSnaps (page
2–41) Curve End and Curve Edge turned on.
• 2-rail sweeps have t he additional 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 with the rail endpoints, the resulting
surface might not follow the rails.
• While you’re editing a sweep, the E dit Curve
button lets you directly transform the CVs of
a rail or cross section, without changing the
sub-object level. E d it Curve also gives you
access to all the rollouts that control the curve.
YoucanuseRefineorMakeFirst,forexample,
without changing levels.
Curv es on S urfa ces a nd Pr oj ected Cur v es
• You can use a viewport to draw a cur ve on a
surface (page 1–1172) (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 mouse button lets you use arc
rotate (page 3–744) to change a viewport’s
orientation while you draw the curve on
surface.
• Neither curves on surfaces nor projected curves
cancrosstheedgeofasurface.Thisincludes
the seam on surfaces with fused CVs. If you
try to project across the seam, only part of the
curve’sprojectioniscreated.
Cr e at i n g Bl e n d S u 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.)