Arcs and Ellipses

[Libbya]

I've been disappointed with the lack of control the dynamic parameters/actions have over arcs and ellipses.  In the interest of gaining more control, I've been playing around with replicating arcs and ellipses using a chained point/stretch on each control point of a spline.  I've gotten it pretty darn close through trial and error.  See attached.  If anyone knows the math for finding the exact control point location let me know.  Thanks.

In the attached file you will find the blue 90° arc (1/4 ellipse with equal lengths) on top of a circle, a couple other eblue 1/4 ellipses on top of ellipses and a red spline ellipse on top of an ellipse.  All of them are stretchable.  

[steven-g]

Nicely done, this opens up a lot of possibilities. The list command will give you coordinates of the control points.

[Libbya]

Finding the coordinates for the control points of the spline is not the issue.  The issue is figuring out exactly where those 5 control points (or rather the 3 internal ones) should be to perfectly replicate a given arc.  At this point, for replicating an ellipse (or 1/4 ellipse) it is close enough for practical purposes, but not quite perfect.  🙂  I'd also like to sort out the internal point locations that will give arcs for angles other than 90°.

[Charles_Shade]

But does the time you spent on this actually make it easier to create an Ellipse within AutoCAD in the long run?

Would you still not need to know the major and minor radii ot the lenght of line you want ot fill?

The Command ELLIPSE is pretty powerful on its own.

But as steven-g said: Nice exploration of the possibilities.

[Libbya]

That's almost like saying the array command in AutoCAD is very powerful and allows on-the-fly adjustment of the column and row offsets so there is no advantage to adding that functionality within the array actions in dynamic blocks.

There is nothing difficult about the ellipse command within autocad and if all you want is an ellipse or ellipse part you are all set.  But if, on the other hand, you want to manipulate that ellipse with it as a part of a larger dynamic block you are SOL.  The only things you can do are scale it, move it and flip it.  You cannot change the axes independently.  With this faux ellipse, you can.

With 1/2 or 1/4 ellipses you can stretch the end points, but it does not adjust the axis (keep the endpoint tangent to vertical/horizontal).  Instead, it 'flattens' or 'bulges' the curve.

I did not spend the time 'just to explore the possibilities' but rather because the above have been desired functions more than once within blocks I have created.  See the attached block which shows the way dynamic actions affect an ellipse (on the right - nothing happens when stretched) and an ellipse 1/4 (on the left - it gets distorted) then compare to the functions within the other block I posted and it becomes clear that the concept expands the functionality of ellipses and part ellipses within dynamic blocks.  

[Charles_Shade]

I find it very interesting what you did and as for me there is no need for such an animal. That does not mean that there is not a use for the functionality and to change Ellipses within a DynBlk as you suggest. You do a great job of solving DynBlk issues outside the box and many here have benfitted from that.

Don't be offended by the query. Just trying to learn what and why.

[Libbya]

No problem Charles.  I'm sorry if my response was overly gruff.  I appreciate your input as well.

[steven-g]

I have been looking around all sorts of sites, but can find no simple, clear explanations of how spline control points are calculated. If you do find anything I would be interested to see it as well.

[Libbya]

I'll do some more looking/thinking and post back if I come up with anything.

[sean.keohane]

Hi,

I needed to create a dynamic block involving an elliptical arc. I worked out a method to stretch the arc using constraints. I did a quick search and didn't find a similar solution so I thought i'd post it. I hope it is useful.

Regards

Sean