Wish: Editing arcs


a customer called: How to edit arcs (like in AutoCAD)?

I haven’t found anything. Any ideas?



First create a curve length dimension or measure the length and then extend.

Hi Martin,

yes, _Extend extends the arc.

She meant pulling arcs by EditPoints to desired shapes like splines (radius value should be fix- or changeable, I think). Perhaps the AutoCAD-gurus can say more about dragging/editing arcs.



what is the user case? you can use ModifyRadius to change it, that will be numerical of course. or you activate center snaps and use scale 2d.

Dear encephalon,

as I understand it: “pulling arcs to shape interactively”.

Thank you for your time


Something like: when you select an arc you see the start-end-center points and you can change their position interactively?

Could be a good wish…

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Many things can be done with the Gumball and / or Orient command

Yes, but, would be good to have an “edit arc/circle” mode where you can move the CP without the need to call several commands (it could be enabled with a double click on the arc or added to the gumball’s options).
We could mix these features:

  • Extend/Shorten by Arc
  • Modify Radius
  • Rotate around center
  • Move Center (to change the radius maintaining the start/end points)
  • Orient Start End (to orient the arc maintaining it’s radius)
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Having all these options in a single command would not save time. ArcBlend command is useful tool for editing arcs.

also all blends work with history, also arc blend, when you turn on history make an arcblend you can move the input curves (control points) and it will adjust. thats why i asked what the specific user case is, there might be some neat adjustment technique without having to revert back to alien commands.

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Are the arcs attached to anything? If not you could “edit” them dynamically with the Gumball set to “by object” - for example scaling with Shift+ a handle…

If they are attached to something like a joined polycurve of course, that becomes much more difficult.

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Hi Mitch,

I have no clue. I’ve hoped some AutoCAD experts are familiar with this, and could explain more.

If this discussion is still in guess mode tomorrow, I will ask my customer for more details about the grant AutoCAD functions. :wink:



Well, my suggestion wasn’t to make a command but a different way to edit arcs and circles maintaining their nature (without turning them into splines).
As Martin and Mitch suggested, Gumball do some of the features like Turning around center (when you have “Align to Object” enabled) and scale 2D (scale with shift pressed) so could be that some extra features are added?
Moving start and end point along the diameter (to change the angle covered by the arc) without turning into a spline isn’t something that could be useful?
Also moving start or end to a new location could be something like Orient 2Pt with Scale enabled.
At the end Rhino recognize Arcs and Circles but I don’t see how to quick edit them (like we do with splines and lines) without “external” commands.

Sorry but I don’t understand how ArcBlend can edit an Arc… you can build an arc between two curves but… i don’t see how to edit a single arc… could be I’m missing something?

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And what about predator commands? :thinking:
Sorry, only jocking :wink:

Please look at command _Polyline, Mode=Arc ,
is not a direct editing, but a overdraw and equally efficient,
so with options ala Tangency, Direction ThroughPoint and Center.

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Hi AutoCAD-lovers,

here are the arc editing AutoCAD-pictures of my customer.

Yes, you are all right, there are a bunch of nice Rhino commands for arcs.

Nethertheless, direct arc editing (pulling end, and smart points) would be a nice improvement for Rhino.

I hope some developers will take a look.

Have a nice and sunny day


P.S.: I would change _EditPtOn, that arcs stay arcs no matter where and which edit-point you pull.

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Hi @Michael_Meyer

Thank you for the update, interesting behaviours, indeed.
Probably focused on specific needs but relevant.

Rodolfo Santos.

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If I’m translating that first series correct - I believe you’re looking to dynamically lengthen or shorten the arc? If that’s the case, I WAS going to say "hey you can just do that with ExtendDynamic - but then I tried it and there’s a bug I think, or at least a use case that’s not been thought through. ExtendDynamic works great for both extending and shortening curves, but in the case of arcs, if you try to shorten it, it thinks you want to complete the whole circle, and overlap your existing arc.

I’ll throw one thing out that folks might find useful - arcs, from a surface modeling perspective are just kinda WEIRD. Any arc over 90 degrees but less than 180 works far more like two different curves that have been smooshed into one object in the background, than one single thing. Over 180 but less than 270? It’s 3 segments, Over 270 but and up to 360 it’s 4 segments. Make any arc over 90 degrees and turn on the points and move them around, you’ll see what I mean. If you take an arc extrusion and try to match it to a NURBS edge, it will often do weird and unpredictable things, because of the way that arcs are defined. When it comes to freeform surface modeling, surfaces made from arcs can be tricky, annoying and frustrating.

As someone who wayyyyyy back in the day transitioned from AutoCAD to Rhino, I didn’t realize this, or why it matters. Lots of AutoCAD folks use arc blends as a way of creating “smooth” transitions between geometry. But in Rhino, unless you have an absolute need for the surface to be a constant radius, you’re far better off using a blend (BlendCrv) to create your linework. I definitely get that there are cases when a true arc constant radius is needed, but in all other cases, they should be avoided - your models will be much easier and better for it.

Hi @sgreenawalt,

thank you for your interesting insights.

My customer was talking about metal bending, pipes, and machines which just can bend arcs, so therefore she needs to create and edit just arcs. She would be happy editing arcs in Rhino, if she could she would kick AutoCAD out. :wink:



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