Arc Deformable Point Count always 10 points

Rhino
1

How can one change the Point Count setting; one cannot change the value of 10 in Rhino 8 macOS. If you change the value from 10 to 6, the entry is ignored, despite the online manual:

“The Deformable option draws a NURBS approximation of the arc with a specified degree and number of control points.”

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2

Hi @Lagom,

Point Count must be at least Degree + 2. With Degree 5, the smallest value it accepts is 7, so 6 gets rejected and the count stays at 10.

To get 6 points, set Degree to 3 or 4 first, then set Point Count to 6.

– Dale

3

Thanks, I thought, because the manual reads Default degree-2 arc with three control points (1). and then unless there is at least one more control point that Arc with the Deformable option can yield single span curves. So, for a degree 5 single span arc, one must use 7 points, and then remove the extra span with Rebuild.

Would be good if single span would be possible out of the box - or the ambiguous wording in the manual would be changed to that one needs two more points than the degree, not just one.

4

Hi @Lagom,

One thing worth knowing: Deformable doesn’t make a true arc. It makes a non-rational curve that approximates one, which is what lets you push points around without the shape fighting you. A true arc needs weights, i.e. a rational curve, and only the regular Arc gives you that.

For a degree-5 curve with 6 points, the closest you can get is about 0.005 off a circle at R 12. If you need an exact arc, leave Deformable off.

– Dale

5

@dale What is the reason that Arc with Deformable option cannot create a single span curve?

6

HI @davidcockey,

The Degree + 2 minimum is a choice the Arc command makes, not a mathematical limit. A single-span, non-rational curve is just a Bezier curve, and Bezier approximations of arcs are standard.

However, accuracy depends only on degree and sweep angle. You can’t add points to tighten the fit, so you’d raise the degree or split the arc into more spans. Accuracy drops off quickly as the sweep gets larger.

– Dale

7

I’m after degree 5 single span arcs, as often used in the surface modelling of many types of products, not for mechanical parts. I’m used to it from “other software” and hoped it is available here also. The deviation is a negligible 0.0000645372 at the red point indicator :sweat_smile:

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8

Hi @Lagom,

Rebuilding the true arc directly with Degree 5 and Point Count 6 skips that step and should be at least as close.

We can also modify the Arc command to allow for Degree + 1.

arc.3dm (37.5 KB)

– Dale

9

That would be good, also amending the user manual to clarify the current + 2 necessity.

10

The Degree and PointCount options have inconsistent restrictions. I reported this as a bug.

RH-99251 Arc: Restrictions for Degree and PointCount are inconsistent

11

ciao… devo ammettere che questa notizia mi è nuova: ero convintissimo di Grado + “almeno 1” per disegnare con B-Splines (altrimenti Grado + 1 per Bezier).

Trovo corretto poter disegnare un arco deformabile con almeno un punto di controllo in più rispetto al grado.

Ben fatto :handshake:

12

Indeed. It should be degree + 1 like in Alias, ICEM, etc.