8

1090 Chapter 9: Sur face Modeling
The arrow points to the segment indicating an error
condition.
S eed V al ues
Some kinds of dependent sub-objects depend on
geometry that might have more than one solution.
For example, if you want to create a surface-curve
intersection point, and the curve intersects the
surfacemorethanonce,thesoftwaremustdecide
which intersection is to be the location of the point.
For these kinds of objects,
seed value (page
3–1102)
parameters control the decis ion. The seed
location is on a parent object, and the software
chooses the location nearest to the seed value
that satisfies the creation condition. You can alter
the seed value when you edit these dependent
sub-objects. The seed location is displayed as a
yellow square.
For example, the seed location for a surface-curve
intersection point is a U position along the length
of the parent curve. The surface-cu rve intersection
closest to the seed is chosen as the location of the
dependent point.
The seed location for a surface is a pair of UV
coordinates in the surface’s
parameter space (page
3–1082)
.
Replacing Parent Sub-Objects
Dependent sub-objects have controls that let
you replace the object or objects on which they
depend. For example, Offset Surface has a button
called Replace Base Surface. You can click this
buttonandthenclickadifferentsurfacetoactas
thebaseoftheoffset.
This capability lets you replace a trimmed surface
with its untrimmed version, or vice versa. To do
so, you need to use t he Select-By-Name dialog. For
example, select the trimmed surface sub-object
andturnonthe
Keyboard Shortcuts Override
Toggle (page 3–912)
on the status bar. Click the
replacement button, press the H key , expand the
surface’s t ree, and then hig hli ght the name of the
untrimmed version.
Rigid Surfaces
To i mp r ove p e r f or m a n c e , y ou c a n m a ke a ny k i n d
of surface sub-object into a rigid surface. The only
editing allowed on a rig id surface is to transform
it at the Surface sub-object level. You can’t move
a rigid surface’s points or CVs, or change the
number of points or CVs.
Rigid surfaces reduce the amount of memory
used by the NURBS model. Making surfaces rigid
improves performance, especially for large and
complex models.
When a surface is rigid, you can’t see its points
or CVs when you are at the P oint or Surface CV
sub-object levels. If the model has only rigid
surfaces and no point curves, the Point and Surface
CV sub-object levels aren’t available at all.
Tomakearigidsurfaceeditableagain,clickMake
Point, Make Independent, Make Lof t, or Convert
Surface.