AutoCAD 2004/2005/2006 DWG Format

[#1]

My company is currently running two different software platforms, both AutoCAD and ProE. We have been using AutoCAD for many years now and just started to use ProE. All ellipses that we have created before in the past in AutoCAD were done using the ellipse variable set to 1 so we have programmable ellipses. The ellipse command in ProE unfortunately creates an ellipse as if it would in AutoCAD with the pellipse set to 0, splines and as true ellipse.

I have had some success with some old school drafting techniques creating ellipses in ProE as a series of arcs, however, these are still different then the ones created with the pellipse set to 1 in AutoCAD. They are programmable, but everything we have created before in the past for ellipses that could be used in our current and future products no longer can. That forces some big headaches for us.

My question is: does anyone know the math behind the ellipse created in AutoCAD when the pellipse is set to 1? If I can recreate that in ProE, it would solve our problems. I know that the ellipse is made up of tangent arcs, but I think there are a few other pieces of the puzzle that I am missing.

[#2]

I would like to know this logic as well, for manual parametric programming that I do for large CNC machines.

[#3]

I've been in the process of writing a vba program and I needed to be able to programmatically draw the arcs to approximate the ellipse the same way that autoCAD does.  I searched and searched and couldn't find anything that says how autoCAD comes up with the approximation that it does.  I found this post in my search.

After a lot of messing with the problem I finally figured out how autoCAD does it.  So if anyone is still interested in this I've attached a drawing file (it's a 2007 version) that shows the math and geometry behind autoCAD's pellipse =1 ellipse approximation. 

Hope this helps someone besides me.

Let me know if you have any questions.