Arc can also be a 3 degree curve?

I have a curve which is a 3 degree curve and also an arc according to SelCrvByDegree and SelArc.
If I check the cruve in rhino 6 it is only an 3 degree curve and not an arc with the same scripts.
What is going on, (I know it is a 3 degree whith a constant / same radius) because I get some problems when I save it as dwg.

arc and 3 degree curve.3dm (22.4 KB)

Hi Edward -

There is no SelArc command in Rhino - you’ll want to ask the author of that script.

Hi Wim,

That’s probably Pascal, although Mitch did a Python one too. Mitch’s shows the same difference between R6 and R7, and I think it may be attributable to a change in the Rhinocommon Curve.isArc() method between versions (it isn’t a big script and that looks like the only candidate). I think you’ll need to ask a McNeel dev if that is so.


Hello- that curve is selected in 7 with my version of SelArc, and not in 6… let me tinker a moment…
@E.O.Stam - see if this works better -
CurveAndArcUtilities.rhp (66.5 KB)

I don’t know how you had the script but here it is inside a plug-in that has a bunch of arc-related tools, some of which may even be useful.


Unblock then drag and drop the rhp file onto Rhino.


An arc can be exactly represented by a rational degree 2 NURBS curve with appropriate control point locations and weights. This is the form Rhino uses for arcs and circles.

Any NURBS curve can be exactly represented by a NURBS curve of any higher degree. So an arc can be exactly represented by a degree 3 curve. The degree 3 form of an arc will have more control points than the degree 2 form.

A user can transform an exact arc which is a rational degree 2 NURBS curve into an exact rational degree 3 NURBS curve using ChangeDegree with Deformable=No. The degree 3 form of an arc will have multi-knots even though curvature is continuous. It is not obvious to me when the rational degree 3 NURBS form of an arc would be preferable to the rational degree 2 NURBS curve form.

Removed due to error


Hi David,

Two points: for full disclosure, Rhino identifies a degree 3 arc as a “Nurbs Arc” as distinct from an Arc

And in Rhino 7 the SelArc scripts referred to, using the facilities provided by RhinoCommon, do now identify the degree 3 Nurbs arcs as arcs, without distinction from degree 2 arcs, whereas in R6 they didn’t. That was the gist of the original post. So, for the moment at least, I’m sorry, but your statement is incorrect.

Hi Pascal,

I test it again but selarc is still telling me that this:
still is a arc but it isn’t according to my nesting program and acad when I export it

regards Edward
t042.dwg (93.5 KB)

Hi Edward - see if SimplifyCrv sorts it out.


Hi Pascal,

every day we have a few hundred off those and most of them are oke, hide the correct one’s and the one’s that are not arcs I can convert


Hi Edward, you don’t say if that helps or not, but if so, I don’t think there is any harm in simplifying accross the board.


We have had discussions about this kind of stuff in the past - I have generally argued for a strict interpretation of rs.IsArc() - which is what my script uses - but others have wanted curves that are near-arcs within tolerance to also be selected - which looks like it won in this case. This is the same with circles IIRC. Your ‘arc’ does select with my SelArc script in this case.

It calls the underlying code:

The method doesn’t say anything about making sure that the object is of type Rhino.Geometry.ArcCurve…

hi Pascal,
O sorry, yes SimplifyCrv it is doing the job and also convert to arc.
I will do all the arc’s in the futuru to be sure.
But when there is a command I can select arcs with (the real ones for me) it would be great.
Thanks Pascal

Here you go… (edit, small modification) (701 Bytes)

It will select your original curve after it is simplified to a true arc, but the original curve does not select.

SelArcStrict.3dm (1.6 MB)

Here is one that does the same thing for circles: (703 Bytes)

Another object to consider is a 1-segment PolyCurve containing an ArcCurve. It can be created by deleting segments from a PolyCurve.

SelArcStrict_WithPolyCurve.3dm (32.5 KB)

_What reports it as an “Arc”.
_SimplifyCrv does not convert it to an ArcCurve.

I would say that’s an bug in how Rhino handles this case - if you run What on your object, you get this:


Whereas rs.coercegeometry() still thinks it’s a polycurve:

Don’t know who’s at fault here…

Thank you Helvetosaur,

it is working only the one’s in the upload the script is not selecting.


ID: 6f4b5017-e2da-4c9d-9f73-d0cd5b8ebef6 (1006)
Object name: (not named)
Layer name: cut
Render Material:
source = from layer
index = -1
Attribute UserData:
UserData ID: 563238F9-C201-411d-A7B1-13895A0317AD
Plug-in: Rhino
description: AutoPointsOn
saved in file: no
copy count: 1

Valid curve.
start = (145.342,0.163,0.000)
end = (145.339,8.242,0.000)
center = (145.578,4.203,0.000)
radius = 4.047
angle = 186.734 degrees

arc not selecting.3dm (19.9 KB)

I can reproduce this here by creating a polycurve containing one or more arc segments, then deleting some segments to leave just the arc portion(s). They will not select with the script because RhinoCommon still thinks they are polycurves. @pascal or @Alain ?

The following is a workaround to the problem, but really shouldn’t be necessary.

"""Selects only "true" arcs, not ones that can be converted via SimplifyCrv."""

import rhinoscriptsyntax as rs
import Rhino

err_msg="No {} objects added to selection".format(entity) 
if obj_ids:
    for obj_id in obj_ids:
        if isinstance(obj,Rhino.Geometry.ArcCurve):
            if not obj.IsClosed: select.append(obj_id)
        #workaround code (shouldn't be necessary)
        elif isinstance(obj,Rhino.Geometry.PolyCurve):
            if obj.SegmentCount==1:
                if isinstance(obj.SegmentCurve(0),Rhino.Geometry.ArcCurve):
                    if not obj.SegmentCurve(0).IsClosed: select.append(obj_id)
    if select:
        if len(select)>1: s="s"
        else: s=""
        print "{} {} object{} added to selection".format(len(select),entity,s)
    else: print err_msg
else: print err_msg

I see that it is one segment polycurve, that seems wrong… but, overall, I need a recap - @E.O.Stam - what, exactly, would you like to happen with arcs?

fwiw, my SelArc is this, no idea if that helps anything or not -

import Rhino
import rhinoscriptsyntax as rs
import scriptcontext as sc

def SelArc():
    tol = sc.doc.ModelAbsoluteTolerance
    atol = sc.doc.ModelAngleToleranceRadians
    ids = rs.ObjectsByType(4)
    if not ids:
    count = 0
    for id in ids:
        if rs.IsCircle(id): continue
        ARC = False
        crv = rs.coercecurve(id)
        if crv.IsArc():
            ARC = True
            sim = crv.Simplify(Rhino.Geometry.CurveSimplifyOptions.All, tol, atol)
            if sim.IsArc():
                ARC = True
        if not rs.IsObjectSelected(id) and ARC:
            count += 1

    strSel = " arcs added to the selection."
    if count == 1:
        strSel = " arc added to the selection."
    print str(count) + strSel
if __name__ == '__main__': SelArc()



I just want to select all the arc’s and no other curves that looks like a arc or have the excact same shape as a arc.
And when they are selected I just want to hide the arc’s who where in a curve contour so I can join the other crv’s without the arc’s
I hope I explained it correct so you understand.

Hi Edward - thanks - I guess Mitch’s script ought to do it then - he’s ;looking for objects that are Rhino.Geometry.ArcCurve()