Michael,

Thanks for clearing things about control points. However I'm still confused with circular objects in MoI. When I display control points of a newly created circle primitive in MoI and Rhino, I see different results. I already read your posts about this and you are saying unlike Rhino MoI evaluates control points in a different way so that we can edit objects more smoothly.

Now please look at my example screenshot below. The shape on right is a rotated elliptical closed curve and left one is a perfect circle created from draw curve/circles.
It's obvious that right one is an ellipse, we can tell that just by looking at the shape. But let's just assume, we can't tell the difference just by looking. Then how do we know that it's a circle in MoI?





1. Both have 12 control points.
2. Both of their dimensions are 20x20mm
3. Both of them are closed curves.

So how do we know it's an ellipse or a circle in MoI?
Is there a built-in command or a script which circularizes any closed curve?

Thanks in advance.

Unwrap the curve = you have length = L
then draw a circle : L= 2 *pi * R so Radius = L / (2*pi)
you can enter directly formula inside the box dialog of the circle for Radius! :) enter pi as the two letters pi !

unwrap: http://moi3d.com/forum/messages.php?webtag=MOI&msg=5136.1

Ps Click on the Size box number for have the exact length for make your copy / past of the numeric L !!!
Because that you show on the up corner screen is troncated or approach value!

The object properties of a circle will be "arc" with a radius value. The ellipses wont.

A circle is an ellipse :-)

Frenchy Pilou,

Those tips are very helpful. Thank you.

BurrMan,

I totally missed that radius property. Thank you. But still, in some cases the object is supposed to be a circle but it does not have a radius property.

Let me give you an example.

1. Create 2 circles.
2. Move one of them up in Z direction.
3. Create loft between them. There you have a cylinder.
4. Delete circles.
5. Select top or bottom edge boundary of the cylinder.
6. Copy and paste that boundary (Ctrl+C and Ctrl+V)
7. There you have a closed curve which looks identical to the circle we created at first step.

Radius property is missing. Why? This is what I'm talking about.

Hi Milkywaif,


> When I display control points of a newly created circle primitive in MoI and Rhino, I see
> different results. I already read your posts about this and you are saying unlike Rhino MoI
> evaluates control points in a different way so that we can edit objects more smoothly.

Yeah, it's a frequent area of confusion in Rhino that when you try to edit the points of a circle it behaves strangely, it basically gets kinks formed in it. If you grab any of those points in Rhino and drag them around you'll see that happening.

To make that kind of point editing easier in MoI there is an automatic rebuild that happens behind the scenes. In MoI when you turn on control points you actually see the control points of the rebuilt version which will smoothly deform if you grab any of the points and move them around, rather than devolving into something with sharp corners in it.



> Now please look at my example screenshot below. The shape on right is a rotated elliptical
> closed curve and left one is a perfect circle created from draw curve/circles.
> It's obvious that right one is an ellipse, we can tell that just by looking at the shape. But let's just
> assume, we can't tell the difference just by looking. Then how do we know that it's a circle in MoI?

Well, first of all you can't be sure that the shape you see in Rhino there is a circle just by glancing at it either, you would have to measure each of those points and also look at the weights of each point as well. It's not really good to judge "circularity" just by control point arrangement alone.

I've attached an example file here, which is not a circle despite having control points arranged exactly the same as in your screenshot. A NURBS circle needs not only the control points arranged in a certain way but also the weight values to have particular values as well.



> But let's just assume, we can't tell the difference just by looking. Then how do we know that it's a circle in MoI?

Well, normally you'd know it because you just used the Circle command to draw it... But if you somehow forgot how you created it, like Burr writes above if you select a circle or an arc then the size area of the properties panel will read a "Radius" value (which you can change by clicking on that line of the properties panel) rather than a generic bounding box "X by Y" size.


> Is there a built-in command or a script which circularizes any closed curve?

No there's nothing specifically for that currently, probably your best bet would be to just draw a new circle.


I'm not really sure _why_ you would have this situation of having curves that you were worried about being circular or not, are you doing some kind of reverse engineering task where you're trying to rebuild things from scanned data or something similar to that?


- Michael

Hi milkywaif,

> Radius property is missing. Why? This is what I'm talking about.

When you do those steps, switch the "Profiles" option in the Loft to be "Profiles = Exact".

Then you should get an exact circle when you repeat those steps.


Basically when you do a Loft if you leave the default "Auto" mode for profiles, it will do an rebuild of the profiles if it sees that they have interior fully multiple knots (the juncture areas that form kinks when you drag them in Rhino).

The steps that you did results in a surface that is not quite 100% circular, it's off by something like 0.001 units.

The properties panel will only label a curve as circular (by displaying a radius value) if it's within a very tight tolerance of being a circle, more like within 0.0000001 units of circular.


It's better to make an exact cylinder by doing Extrude of just one curve, rather than doing a Loft between 2 curves. Loft has to combine multiple curves together into one single surface structure, and it tries to incorporate some simplification/rebuilding of the curves involved in order to avoid making a surface that ends up inheriting the combined structure of different curves all together.


- Michael

Michael,

> I'm not really sure _why_ you would have this situation of having curves that you were worried about being circular or not, are you doing some kind of reverse engineering task where you're trying to rebuild things from scanned data or something similar to that?

Not really. I'm building my models from scratch. I'm a long time polygon/subd modeler but I have absolutely no experience in NURBS modeling. I tried almost all of NURBS modeling software out there and decided to purchase MoI because it's very simple, yet powerful and user friendly. I'm trying to understand the logic between the tools and output geometry. As I mentioned earlier, I had some problems with my previous NURBS models. They had broken edges, distorted looking faces, bad fillets or sometimes they did not export correctly and I couldn't fix them easily. I believe it's because I used wrong choice of tools at some point. You know, sometimes you can do loft, revolve or sweep and you get the same looking result. I just want to know that I'm using the right tool when I need.

Thank you very much for your responses. You've been a great help.

Hi milkywaif, you're welcome!

Also check out here for some links to discussions and general tips for people who are coming from a sub-d / poly modeling background:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4865.2

The overall method of construction with NURBS modeling is fairly different than poly modeling, and some of those discussions are about the differences and how to sort of avoid trying to only do poly modeling type techniques in MoI.

I hope that MoI will be useful for you!

- Michael