RE: ACAD NURBS > RHINO and Gen'lly More Robust 3D CAD

From: Jonathan Chertok (
Date: Fri Apr 11 2003 - 18:00:21 EEST


Thanks for the response.

I am spending a little time trying to get my chops down into NURBS and
Splines and had thought to do this through ACAD as I use it a lot. If I know
what it won't do, then I can make some decisions about what to move into
next. Yesterday I discovered that ACAD will not allow you to control Skew
(left and right) or to control Continuity with Nurbs and Splines - which is
good to know.

Can anybody point me in the right direction for answers:

1. Are NURBS 3D objects and SPLINES 2d? Or can someone straighten out the
terminology for me. Seems like ACAD has a SPLINETYPE variable that controls
a type of SPLINE - either quadratic (5) or cubic (6) - does anyone know if
the "5" and "6" variables have any relation to anything (!) as they would
not seem to relate to the curves' order or polynomial degree.

1A. There is a TOLERANCE for how close to a through point the curve will
sit and there is also a WEIGHT for the individual points. I couldn't figure
out how a tolerance would interact with a weight.

2. Is "Bezier Curve" interchangeable with "B-Spline". If so, how many types
of B-Splines are there (Cubic, Quadratic...)? If not, can you do a Bezier
Curve in ACAD and how?

2A. There is an EDGESURF command in ACAD for 3DMESH which is set to either
Quadratic B-Spline Surface (5), Cubic B-Spline Surface (6) or Bezier Surface
(8). Why can I get a Specific Setting for Bezier Surface with 3DMesh and not
with a SPLINE Command?

3. It would seem that ACAD can - ACIS, SAT, STL, DXF, 3DS - OUT. Is there
information in 2D or 3D NURBS or SPLINES that I need to use a certain format
for? If so, can someone explain to me what that information is and which
file formats would support it...I have essentially 2D ship's curves in the
computer and would like to take a stab at "fairing" them.

Thanks very much,


P.S. Is it your experience that BIM and Parametric Modelling are useful for
a Hybrid Architectural/RP approach to project information? Laserein says
that one of the drawback to REVIT (in a positively euphoric review of the
product) is that it cannot currently do sloping walls or floors...well. I
guess my worry is that the new products will be like some publishing giants
version of a high school textbook.

I think I am hoping to work back from CATIA with the architectural ACAD
question. I know CATIA can do what I am looking to do, and was wondering how
close I could get to it with limited expenditure. Anybody using Mechanical
Desktop for architectural uses...

Universal Joint Design Build
Full Service Design and Construction

Jonathan Chertok. Principal
Austin Texas [512] 407 9628

Buildings + Digital Manufacturing

>From: "Makai Smith" <>
>To: "'Jonathan Chertok'" <>,<>
>Subject: RE: ACAD NURBS > RHINO and Gen'lly More Robust 3D CAD
>Date: Fri, 11 Apr 2003 10:00:30 -0400
>To be really dumb about this, I've noticed that ACAD won't IGES out, but at
>least it will ACIS (*.sat) out. This exports NURBS, but ignores the 2d
>stuff, like splines. Mech desktop I think has IGES.
>Heck, ACAD exports stl, so you *can* get stuff machined without ever using
>IGES. I've only used deskproto and surfcam, mickey mouse stuff maybe, but
>stl was fine. Actually, I think I went VRML into deskproto!
>Architecture trains for ACAD, but its dominance in the marketplace will
>wane. Revit is evidence of this fact: autodesk is now selling a
>"discontinuous" product. In Phil Bernstien's own words, "no one ever got
>into architecture because they liked the autocad interface". When you read
>Laiserin you'll find that BIM *will* kill CAD (or rather fulfill its
>Anyway, acad is great for drawing drawings, but it is beastly when it
>to complex 3d. You have to keep reorienting the UCS (formZ is even a bit
>annoying that way). Try modeling in 3DS or Maya. When you get a taste of
>objects with their own relative coord systems, and a hierarchical database,
>you'll never like acad again, IMHO. There's just a brittleness to acad;
>richness and fluidity of a true 3d toolset blows it away. And let's not
>even get into parametric modeling.
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>-----Original Message-----
>From: Jonathan Chertok []
>Sent: Thursday, April 10, 2003 5:38 PM
>Subject: ACAD NURBS > RHINO and Gen'lly More Robust 3D CAD
>1. Can anyone tell me if there is a special way to get NURBS curves from
>ACAD 2000 into Rhino. I have some curves modelled in ACAD, but they seem to
>come into Rhino without all the through points.
>2. Also, can anyone tell me exactly the limitations of ACAD 2000 are
>particularly with respect to 3D and NURBS modelling. My reading tells me
>that "Autocad is a Non-Uniform Rational B-Spline (NURBS) modeller and uses
>polygon meshing to create 3D surfaces" (ACAD 2000 3D - Matthews). This
>sounds pretty good for my uses. What exactly would this not allow you to do
>with respect to manufacturing (as opposed to rendering)?
>3. I have also run across the statement "Unlike Mechanical Desktop you will
>find Autocad is limited as a surface modeller." Can anyone give me a
>common-sense understanding as to what Mechanical Desktop (or 3D Studio) can
>do for me that Autocad cannot?
>4. If there is a ACAD NURBS (or overall NURBS) Guru out there I wouldn't
>mind picking their brain about a couple other issues as well.
>5. And tangentially (I think) can I go from .stl to IGES relatively easy in
>order to get some things machined? Or is this a tricky bunch of business?
>there a date that this was discussed on RPML, name of person who posted the
>question, etc?
>I'm a monster on ACAD (and hate to put all that "Masonic Knowledge" aside,
>but I would like to step up my 3D Modelling capabilities (like getting
>points in space from 3D objects and being able to projectively work with
>shapes...) as it relates to my architectural - and other - work. Do I go
>with 3D Studio, Mechanical Desktop, Rhino, or look in another direction?
>Charles has specifically mentioned Revit and Nemetscheck's Vectorworks
>Architect - neither of which I am familiar with...has anyone got a handle
>where I can get more information about the state of the state with respect
>to more robust 3D CAD Programs. Speaking of which what does Bentley do for
>I suppose I can pick and choose programs depending on what I am doing, but
>it would be great to get more of a handle on what does what and what is
>coming on strong.
>P.S. I will be taking a closer look at Jerry Laiserin's site with respect
>these issues and also would appreciate any suggested links.
>Universal Joint Design Build
>Full Service Design and Construction
>Jonathan Chertok. Principal
>Austin Texas [512] 407 9628
>Buildings + Digital Manufacturing
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