Hello,



This picture shows a screenshot of two circles made in Rhino. The left one is deformable and has much more points then the right one which is not deformable (which is a true circle and not only a NURBS approximation). You can make the left one in MoI too, but not the right, can't you? If not, why? It's even not possible to import those circle from Rhino into MoI.

Circular array 4 of 90 degree arc, join.

Hi 3image,

In order to create that type of circle inside Moi, here's a simple way.

1 - In the Top view create a 90 degrees arc by "arcenter" command, with the center point placed at world origin (0,0,0)
2 - Select the newly created arc
3 - Run the "arraycircular" command
4 - As the center point pick the world origin
5 - In the command options enter these parameters : "item count" = 4, "Angle to fill" = 360
6 - Push "done"
7 - Now you have 4 90 degrees arcs
8 - Select all the arcs
9 - Join all the arcs with the "join" command
10 - Now you have a circle object that is almost the same as the one available in Rhino with the "Circle" command

Ciao,

- Marco (mkdm)

P.S. posted some minutes after the bemfarmer's reply.

That works, I just put in 12 arcs of 30 degrees each for even more control points.

Hi 3image - the circles in MoI actually combine both of those types you are talking about in Rhino.

When you draw a circle in MoI, it will be the exact non-approximated one like the one you show on the right.

When you turn on the control points of that circle, it will at that time set up a companion deformable circle and show you the control points for the deformable one. The exact circle will only actually be replaced by the deformable one if you move any of the points though. If you just turn off points without editing any of them the circle will remain as the exact one.

At some point in the future I'll probably have an option in a properties dialog to enable or disable this "auto conversion to deformable" and to control how many points the deformable version will have.


> If not, why? It's even not possible to import those circle from Rhino into MoI.

They will get imported fine - the circles that you import from Rhino will be exact circles in MoI too, just when you turn on their control points you'll see the control points of the deformable version.

The reason for this behavior in MoI is that editing the control points of the exact circle usually surprises people since it will introduce sharp corners in it.

- Michael

@ Brian & Marco - it's not really necessary to do that type of arc construction, the circles that you create in MoI will already be made of 4 precise non-approximated arcs.

But when you turn on control points, it is assumed that you want to squish those points around and MoI will show you the points of a rebuilt circle that deforms smoothly rather than devolving into 4 arcs with sharp cusps between them.

The exact 4-arc circle will only be replaced by the deformable version if you actually edit any of the control points.

The same thing happens with surfaces of revolution as well.

- Michael

Hi Michael,

> The exact 4-arc circle will only be replaced by the deformable version if you actually edit any of the control points.

I didn't know about this behavior.
Thank you very much for your explanations!

Ciao.

- Marco (mkdm).

:-)

Hello 3image,

at the end every circle is represented as a nurb curve in modern CAD. The number of control points isn't a criteria to decide if it is exact or not. The magic here is the knot vector. You can find a very good desciption for that on page 9 in the following document: http://digiitalarchfab.com/portal/wp-content/uploads/2011/03/Nurbs-Curve-A-Guide-for-the-Uninitiated.pdf

Have a nice day
Karsten

Enlighting!

A more legible copy of the article is here:

http://www.hcs.harvard.edu/~lynders/cs275/NURBCurvesForUninitiated.ps

- Brian