"Blending" Beziers

advice

#1

There has to be a better way than the system I’ve dreamt up and have been using for a while;
When I need a 3D curve, I’ll draw two curves in different planes, extrude and intersect them. Problem is, the result isn’t a Bezier, but a succession of semi-randomly connected segments. This is of course due to a lack of synchronicity in the spacing of the vertices in the original (blue in the screen shot) curves.


How should I go about drawing a precise 3D Bezier directly, in order to obtain a mathematically correct curve?
blended Béziers.skp (59.2 KB)


#2

Does it have to be a Bezier curve? A Catmull spline would be easier.

Do you have a table of offsets for this hull? It would be better to work from that if you’ve got it.


#3

For me, it’d be easier with Beziers, since that’s all I use, aside from the arcs. I’ve seen the word Catmull in passing.
What’s a table of offsets? You mean one of these?:
33%20PM
Hand-drawn from the yard, in 1907, which I’ve learned not to rely on too much, limitations of pen&ink, and perspective distortion from the picture taking…
That’s how I obtained the x,y and z data I’m trying to blend together. I do have a 12mb jpeg that’s perspective corrected, but that doesn’t help much with things like the left and right side outermost curves don’t pass the
“copy-scale-1-and-paste” test…


#4

Ah… I see. So you’d rather limit yourself to only arcs and Bezier curves.

No. That’s a lines drawing. That’s the graphics representation of the table of offsets. The table of offsets would also be used to loft the hull on the loft floor to lay out for the frames used to build the hull.

It’s unlikely that they would since those curves are from different stations on the hull.

Do you have a larger version of the image you posted?


#5

Here’s a table of offsets and the lines drawing for an R-class boat.

Note the outermost half-breadth curve on the left side of the hull is at station 7 while the one on the right is at station 6.

It’s too bad you don’t want to open up to using Catmull splines since they’d be much easier to use for the application.


#6

Have you tried the “BZ_Toolbar” extension? It has some good tools to generate bezier curves, even if you continued with your own current method and re-trace those lines using a bezier tool?


#7

Again, Catmull splines would be better and easier than Bezier curves.The control points for Catmull splines are on the curve, not out in space.


#8

Who said I didn’t want to look into Catmulls?

I was joking in my statement that it’d be easier for me, but I’m perfectly willing to look elsewhere for solutions.
It’s just that I’m barely getting my sea legs with Beziers, and now here’s this Catmull thing.

Just got your last, never occurred to me to take into account the fact that the line count fore and aft was less in the longitudinal projection than in the lateral one, 20 compared to 23. Thanks.

I’ve got to be discreet with the drawings, having gotten them under the condition I not share them. So, no higher-rez than what’s already out there…


#9

If you’re using Fredo’s BZ_Spline toolset, you’ve got the Catmull Spline tool, too.

Consider the audience for which these drawings are made. A boat builder knows that the hull will be symmetrical and only wants the half-breadths. To reduce clutter and improve clarity, usually the stern half of the half-breadths are to the left of the center line while the front half are to the right.

No matter.


#10

Looked it up in SU, yeah, I’ve had that suite of tools for a while, now it’s just a matter of hitting YouTube for the how-to’s. Any further pointers in that regard?


#11

Play with the precision number to adjust the smoothness of the curve. Remember to double click at the last point to end drawing the curve.


#12

By the way, all this boat hull stuff gave me a flashback a prompted a little research:

I got my first Mac in 1985, tried just about every CAD app for the Mac for several years, and by '88~'89, I was becoming the office expert on it. My boss, Bill Grover, said I should come see how a friend of his in the center of Essex was using a Mac to design boats. The guy, it turns out, was Brit Chance, and my memory was he was using a package called MacSurf to design an America’s Cup boat to win back the trophy. A quick search, and it appears the package is still around as MAXSURF now owned by Bentley. Looks serious, as in, “if you have to ask, you can’t afford it” kind of software, but I have no idea, and it seems to be Windows based, no longer Mac based.

About MAXSURF
Brit Chance obit


#13

Awrrr! A pirate needs his own boat!


#14

Looked into Delft-something(ship?), they only run on Windows as well


#15

It’s not the 19th of September today… :slight_smile:


#16

Posting this here as a reply, instead of starting a new thread.
Now that I’ve got a clearer picture as to what it is I’m looking for, I’ll reframe the question;
Is there any way to obtain an editable curve, Bezier preferably, that’s non-planar, or described in 3D.
Up to now, I can obtain these intersections of 2 planar curves, but the result is final unless redrawn manually. If I need regular or progressive lengths of segments, or to change the number of vertices and/or control points, I’m in a bind.
Catmulls have the disadvantage of having a fixed curvature radius at each vertex which, more often than not, produces alternating straight and curved sections.
So, again, is there a resource that allows for the creation and edition of non-planar curves?


#17

Are you using Fredo’s Bezier curves? They can be edited.
Edit%20Bez


#18

Wish I’d thought of that… Tricky part’ll be the placement of the control points.
Thanks.


#19

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