Editing the radius of an arc segment of a pline

My instinct tells me there’s logic to the fact that simply turning on points on a pline provides a means to edit an arc (of a consistent radius) into another arc (with another consistent radius). I can pull on all 3 points and make a parabola. I can pull on the centre point and make a pointed arch. Pulling on a side point turns that side into a sub-arc.

There are two things I’m wondering:

  1. How to simply change the radius of an arc segment - which stays an arc while maintaining the position of its starting and ending points.

  2. What’s the advantage of so easily enabling the irregular deformation of an arc rather than easily enabling the retention of its consistent radius? Seems to me most CAD packages have it the other way around.

Hello - to answer your # 2 first - it is not necessarily an advantage… the thing is, Rhino is not a constrained or parametric modeler - this is very nice in some contexts and not so much in others, as you see. For more predominantly mechanical objects - by which I mean planar/linear and tangent arc sort of constructions, there is no question that while Rhino can make them perfectly accurately and all, parametric modelers can be a great convenience.


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That’s what I thought, thanks. Looking forward to answer number 1.

Create a new arc using Arc > StartPoint > EndPoint > Radius

Arc command needs three pieces of information to create an arc. The options let you select what those pieces of information are.


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THanks David, but I know how to create an arc. My post, “Editing the radius of an arc segment of a pline (polyline)” is about how to edit one while maintaining its characterstics as an arc (having one radius.)

I’ve tried every (practical) method I could think of. In addition to the experiments I relate in my initial post, I’ve tried scaling the arc points in one direction, but that results in inconsistent radii also. (By “practical” method I mean a quick simple transformation of the arc segment; rather than exploding the polyline, deleting the arc, and creating a new one.)

please let me know if you have any suggestion in my quest to (from my original post):

Sometimes it is easier and quicker to create new geometry than to edit old geometry. It those cases my preference is to create new geometry.

I appreciate that, thanks. This might be the only way.

What’s behind the hope of a transformational option here is that - in other CAD packages - point editing to accomplish one or two changes rather than starting from scratch makes this sort of task much faster than recreating a complicated option that differs only slightly. The limitation obstructs quick exploration of variations of ideas for things like architectural mouldings, which doesn’t seem to fully capitlaize on the advantages Rhino can otherwise offer for such a task.

What’s behind # 1 has to do with creating architectural mouldings (as demonstrated in the tutorial mcneel offers). Transforming a pline somewhat irregularly but with arcs remaining arcs enables the transformation of a pline from which one moulding has been extruded into a pline from which a similiair but slightly differently configured moulding will be extruded. (This sort of milled feature uses single radius curves everywhere possible.)

Not creating another moulding profile from scratch for every option capitalizes on a computer’s ability to quickly explore different variations on a similair design idea. If that’s impossible here without a) redrawing / joining-in a new arc every time, or b) dragging points to create an irregular curve approximating an arc, I’ll continue to use b). But if there’s a practical and accurate transformational alternative, I’d like to learn it.

In plain Rhino, there isn’t.

Between the lines, Pascal answered # 1 after he had answered # 2.

# 2:
# 1:

In a limited way, Rhino does offer access to certain object parameters. In your specific example:

As long as the arc segment isn’t part of another curve, you can use ModifyRadius to simply change its radius.

This condition is fulfilled.

This type of constraints is not part of that command - it is designed to keep the center point in the same place.

Note that parametrics with constraints is possible in Rhino through the use of Grasshopper. If you know that you will be making several models with slight variation, or you are in a form-finding stage of development, it can pay off to use some time and build a definition in GH.

Thanks Wim. I was afraid of that. Everything has the shortcomings of its virtues, and clearly rhino is no exception w regard to its power for free-forming.

I don’t know that being able to directly manage the parameters of a few types of simple line geometry might prove quite valuable over Rhino’s development, particularly for mechanical and construction applications. I suspect so though, judging from Mcneel’s tutorials directed toward that use.

I think the above may not be correct as far as my query goes, which is about being able to simply edit an arc when its part of a pline.

It turns out after more research on my part: if the Direction command is used in conjunction w Mode=Arc, the next click is the arc’s tangent to its starting point (the documentation calls this point it’s “first end point”!), and the next click after that will create an arc with only one edge node which will remain adjustable in the way that I’m seeking (adjusting its one radius while keeping its end points the same.)

This is in fact true when it is part of a pline.

So: it seems that this parametric feature is in fact in rhino.