Arcs on arbitrary plans

hi all. I don't understand the way autocad draws an arc on arbitrary plans; i'm writing an application that reads the drawing, changes something and draws it again.

 

That ACAD assumes an x-axis given a normal vector, in a consistent way.
Open up the "ActiveX and VBA Reference" -- it's in the Contents of regular
ACAD help -- and search for "Arbitrary Axis Algorithm".

or

http://www.google.com/search?q=arbitrary+axis+algorithm

http://astronomy.swin.edu.au/~pbourke/geomformats/dxf2000/arbitrary_axis_algorithm_dxf_ab.htm


Good luck,

James

 

thank you so much! everything it's clear now; i could never come by my own to the sophistication of choosing the WCS y-axis rather the WCS z-axis according to the closeness of the Normal vector to the positive portion of the WCS z-axis
- - -
by the way, could you explain to me another correlated question? When i pick an arc with UCS/New/Object command, (or in autolisp with netselp)i get two different user coord. system (with the same z-axis in any case), according to the fact i pick close to one end of the arc or to the other.
'bye, antonio

 

by the way, could you explain to me another correlated question?

Actually, yes, I just found your answer. The help file for the UCS command
explains how ACAD calculates the X & Y axis when you choose By Object, and
why you get different UCS depending on where on the arc you choose. I had
to click on a little link on the help page that said "Show all Topics" to
see this stuff (using ACAD 2002).

Good luck,

James