Topic: vlax-curve-isclosed vs. vlax-curve-isperiodic

[roy_043]

Just wondering: can a closed curve not be periodic?

[ymg]

http://academy.smc.edu/courses/et24/html/et24_wk03b_curves.htm

A closed curve is a loop with coinciding start and end edit points. Where start and end edit points meet is also called a seam. If you move the start edit point, the end edit point moves with it (and vice versa).

A periodic curve is also a loop with a seam, but it has two unseen spans at the end of the curve that overlap the first two spans. For example, suppose you create a circle primitive with four spans (sections). A circle primitive is a periodic curve. If you display the circle's edit points, you'll see only four spans. There are actually six spans, but the last two overlap the first two. The unseen spans provide structure to keep the loop closed and the curvature free of sharp corners when you move a point away from that region.

Don't know if Autocad does the same way but this is the academic definition.

ymg

[roy_043]

Thanks. Somehow your link does not work for me ('server not found').
I am not sure what 'span' means in this context.

Judging from the AutoCAD docs any closed curve is also periodic.

[ribarm]

Judging from the AutoCAD docs any closed curve is also periodic.

But as I may guess any periodic curve isn't also always closed, otherwise why would exist (vlax-curve-isclosed) and (vlax-curve-isperiodic) as two different functions...

Or, maybe : All open curves are non-periodic?

Some of closed curves are periodic and some of them are non-periodic?

As far as I knew by now - those 2 last statements seem to me that are correct... - Are there open curves that are periodic???

"An open curve (non-periodic) has free ends."

http://help.autodesk.com/view/ALIAS/2015/ENU/?guid=GUID-9344F058-3623-474D-9CAB-D866542315B9

Does this mean open=non-periodic?

[Lee Mac]

FWIW, closed 2D (Heavy) polylines & closed 3D polylines are closed but not periodic.

[roy_043]

... otherwise why would exist (vlax-curve-isclosed) and (vlax-curve-isperiodic) as two different functions...

That was my thought as well.

FWIW, closed 2D (Heavy) polylines & closed 3D polylines are closed but not periodic.

Thanks for pointing out those exceptions. I don't understand why they exist if a closed LWPOLYLINE is periodic.

Note this strange behavior (BricsCAD V14):
Test entity: Rectangle (closed LWPOLYLINE with 4 vertices):

Code: [Select]

(vlax-curve-isperiodic (car (entsel))) => T
(vlax-curve-getpointatparam (car (entsel)) 1) => (3598.25 -606.818 0.0)
(vlax-curve-getpointatparam (car (entsel)) 5) => nil

[Lee Mac]

FWIW, closed 2D (Heavy) polylines & closed 3D polylines are closed but not periodic.

Thanks for pointing out those exceptions. I don't understand why they exist if a closed LWPOLYLINE is periodic.

Agreed - I am also confused as to why a closed LWPOLYLINE would be periodic - I originally thought only circular/elliptical objects would be periodic.

[TMoses]

Dear Roy, Dear Lee,
I also cross-checked with AutoCAD 2008/2014 ... same strange "periodic" behaviour for a closed polyline
Indeed, strange, and I would even say, it is simply wrong ...

"Periodic" entities should repeat their values (points) after 1 period, think of a kind of spiral etc.
Many greetings !