Hi,
to clear the fog a bit more (or thicken it)...
- A Nurbs curve consists of a list of CVs (x0, y0, z0, w0, x1, y1, z1,
w1, ...), and a knot "vector" (0.0, 0.0, 0.0, 1.0, 2.5, 4.0 ,5.0, 5.0,
5.0) in which each item always have to be greater or equal than the one
before.
- Curves can have degree 1, 2 or 3 in Softimage (in other applications,
higher degrees are allowed, but we rarely design ship hulls in Softimage).
- The number of occurrances of a knot value is called it's "multiplicity".
Example: (..., 1.7, 2.1, 2.1, 2.1, 4.0, ...). Knot 2.1 has multiplicity 3.
Maximum: degree of the curve. Such a knot is called a "Bezier" knot - it
has "full multiplicity".
- The first and last knot of an open nurbs curve always have full
multiplicity. Needn't be, but it's nice when the drawn curve's start/end
coincide with the first/last CV.
- Bezier curves, as has been said, are also Nurbs curves. Nurbs are a
superset, a generalization of Bezier curves.
- There's one golden rule for Nurbs curves, which under all
circumstances must be kept (except you want to provoke a crash):
number of knots = number of points (=CVs) + curve degree - 1
Example: create a curve, in the "eye" menu, turn on "points", select a
knot and see what happens when you adjust "Set Knot Multiplicity" - CVs
are added or removed accordingly to keep this rule.
- Bezier knots always coincide with a control vertex. Knots with less
than full multiplicity do not.
- It's a nice convenience feature of the "move component tool" that you
can move not only CVs but also knots.
In fact, that tool is very well designed. Unfortunately, other Nurbs
curve tools are not, or are missing. Same for surfaces.
Back to your question, if it hasn't been answered already:
If you want to "convert" a Bezier curve to a CV curve, simply select all
the Bezier knots (not start and end - you could, but it's useless) and
reduce their multiplicity to 1 ("Set Knot Multiplicity").
The curve shape does not change.
Remember the "Create > Curve > Fit on Curve" tool, if you want to
"re-sample" the curve.
Too much information? ; ]
Best,
Eugen
Am 18.06.2013 23:19, schrieb Ponthieux, Joseph G. (LARC-E1A)[LITES]:
Yes! That would be helpful, didn't realize that was still there.
--
Joey Ponthieux
LaRC Information Technology Enhanced Services (LITES)
Mymic Technical Services
NASA Langley Research Center
__________________________________________________
Opinions stated here-in are strictly those of the author and do not
represent the opinions of NASA or any other party.
*From:*softimage-boun...@listproc.autodesk.com
[mailto:softimage-boun...@listproc.autodesk.com] *On Behalf Of
*Grahame Fuller
*Sent:* Tuesday, June 18, 2013 5:16 PM
*To:* softimage@listproc.autodesk.com
*Subject:* RE: bezier -> nurbs
A Bezier knot is a NURBS knot with multiplicity 3. If you don't want
Bezier-like manipulation, you can use the old Move Point tool (still
available on the Modify > Component menu) instead of the Tweak Curve
tool .
gray
*From:*softimage-boun...@listproc.autodesk.com
<mailto:softimage-boun...@listproc.autodesk.com>
[mailto:softimage-boun...@listproc.autodesk.com] *On Behalf Of
*Ponthieux, Joseph G. (LARC-E1A)[LITES]
*Sent:* Tuesday, June 18, 2013 04:49 PM
*To:* softimage@listproc.autodesk.com
<mailto:softimage@listproc.autodesk.com>
*Subject:* RE: bezier -> nurbs
Yeah, tried that, except when I set the multiplicity to 3 it
apparently converts the curve to Bezier control. Kinda defeats the
purpose as I could just create the curve a Bezier from the get go if I
wanted that.
--
Joey Ponthieux
LaRC Information Technology Enhanced Services (LITES)
Mymic Technical Services
NASA Langley Research Center
__________________________________________________
Opinions stated here-in are strictly those of the author and do not
represent the opinions of NASA or any other party.
*From:*softimage-boun...@listproc.autodesk.com
<mailto:softimage-boun...@listproc.autodesk.com>
[mailto:softimage-boun...@listproc.autodesk.com] *On Behalf Of *Daniel
Brassard
*Sent:* Tuesday, June 18, 2013 4:40 PM
*To:* softimage@listproc.autodesk.com
<mailto:softimage@listproc.autodesk.com>
*Subject:* Re: bezier -> nurbs
Oops, reverse.
From the book
Multiplicity is a property of knots that refers to the number of
control points associated to a knot. On a cubic curve, a knot can have
a multiplicity of 1, 2, or 3. On a surface, each knot curve has two
multiplicities: one in the U direction and one in V. All knots along a
knot curve must have the same multiplicity in the corresponding direction.
Knots with a multiplicity greater than 1 are sometimes called
/multiknots/. Multiknots allow for greater control over the trace of
the curve through the knot, at the expense of smoothness.
·A knot with a multiplicity of 1 has C2 continuity (curvature).
·A knot with multiplicity 2 has C1 continuity (tangency).
·A knot with multiplicity 3 has C0 continuity (position) if the three
control points are not lined up. It is like a Bézier point, with one
control point exactly at the position of the knot on the curve and the
other two control points acting like tangent handles. You can
manipulate these knots on curves in a Bézier-like manner --- see Using
the Tweak Curve Tool
<http://download.autodesk.com/global/docs/softimage2013/en_us/userguide/files/curves_DrawingandManipulatingCurves.htm#WS20FD922AA9DF2240AD94FFADCE73807F-002F>.
On Tue, Jun 18, 2013 at 4:36 PM, Daniel Brassard
<dbrassar...@gmail.com <mailto:dbrassar...@gmail.com>> wrote:
Raise the knot to multiplicity 3 (similar to bezier)
Lower the knots to 2 (curvature) first and second derivative
continuity, smoother curve.
Lower the knots to 1 (tangent) first derivative continuity, tangent
continuity
Lower the knots to 0 (linear), sharp turns, no continuity between knots
On Tue, Jun 18, 2013 at 4:08 PM, Ponthieux, Joseph G.
(LARC-E1A)[LITES] <j.ponthi...@nasa.gov <mailto:j.ponthi...@nasa.gov>>
wrote:
Its been so long since I've tried this in Soft I can't remember...
Is there any logic or formula that will allow you to replicate a
Bezier Knot curve as a CV curve?
I thought all you had to do was make sure the CVs on a Nurbs curve
matched the handle points on a Bezier curve and they would align
perfectly, but the continuity of the Bezier curves is slightly
different than the Nurbs, almost as though the Bezier is a different
degree than the Nurbs curve. Is that the case?
Second, is it possible to convert a Bezier curve to a Nurbs CV curve
and maintain continuity, bias, etc?
--
Joey Ponthieux
LaRC Information Technology Enhanced Services (LITES)
Mymic Technical Services
NASA Langley Research Center
__________________________________________________
Opinions stated here-in are strictly those of the author and do not
represent the opinions of NASA or any other party.
