Subject:
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Re: Problem with LDraw-Mode and LDDP Bezier Curve plugin
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Newsgroups:
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lugnet.cad.dev
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Date:
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Mon, 21 Oct 2002 18:03:41 GMT
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Viewed:
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455 times
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In lugnet.cad.dev, Fredrik Glöckner writes:
> "Don Heyse" <dheyse@hotmail.spam.go.away.com> writes:
> > I am also curious how ldraw mode generates the control points.
> It is fairly well described here:
>
> http://folk.uio.no/fredrigl/technic/ldraw-mode/bezier-curve/
>
> Basically, I push the control points back and forth until the length
> of the curve is correct. This is done by numerical integration, and
> is rather CPU intensive.
>
> > I wish Frederic could whip up a short paragraph on how he uses
> > the length to position the control points. I'm not bright enough
> > to digest an algorithm that big. (I read the page on advanced
> > hose generation, but it didn't seem to be up to date with the rest
> > of the documentation).
>
> Oh, you've already read it? It's not true that it's not up to date.
> Rather, I have added a simpler interface after writing the above
> paragraphs, but the old method still works.
Ok, ok! I admit it. I am a moron. I either read something else
that I got out of the google cache, or my eyes glazed over and I
lost focus before I got to the important parts. Thanks for the
short version though. Now I get it.
> On second thoughts, the example Orion provided is sort of not that
> relevant. What he really wants in the example is not a curved
> element, but rather a straight line between the two points. So I
> think that using this example as a bench mark may not be
>
> I think that a totally different method may be needed in this case.
That's why I'm asking. The small initial velocity method generates
a pretty nice (slightly curved) hose in this case. This page:
http://www.ma.usb.ve/~jacob/povgraph/bez.html
suggests a relationship between the bezier curve, a friction
coefficient, and an initial velocity. It makes me think if we
came up with good defaults for friction and velocity for the
various hose materials we'd get curves that match the whe way
the real world parts behave.
> > The code Chris donated can optionally display where the interior
> > control points end up by drawing type 2 lines from the end points
> > to the subsequent interior points. This gives a really nice
> > illustration of how the curve gets shaped, and the 2 extra lines
> > are easily deleted later. It might be nice if this could be an
> > option at in ldraw mode. Something like (setq
> > ldraw-bezier-draw-control-lines t).
>
> It's an interesting idea, but I don't know how useful it is. In all
> the predefined hoses provided by LDraw-mode, the direction towards the
> control points is uniquely defined by the element which you insert.
Yes, I understand the way the direction is calculated. It's the
magnitude of the vector (velocity) in that direction that I'm
interested in showing with the lines. In the example I posted
the magnitude is 50 LDU which gives a smooth curve. If draw it
line by line in LEDIT you'll see the two blue lines don't get all
that close to the middle of the curve. I suspect the control
points auto-generated by ldraw mode are very close to the center
of the curve, resulting in that unsightly zigzag. If you could
see the magnitudes of the control point velocity vectors generated
by different lengths, it might be easier to understand the effect
of choosing different lengths.
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Message has 1 Reply:
Message is in Reply To:
 | | Re: Problem with LDraw-Mode and LDDP Bezier Curve plugin
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| (...) It is fairly well described here: (URL) I push the control points back and forth until the length of the curve is correct. This is done by numerical integration, and is rather CPU intensive. (...) Oh, you've already read it? It's not true that (...) (22 years ago, 21-Oct-02, to lugnet.cad.dev)
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