GCon above G2 continuity

Draw two straight lines and BlendCrv them with G3 continuity or higher. Check continuity with GCon and it will always report G2.


How can I detect continuity higher than G2?

Hi Duncan,

not a pure Rhino solution, but our plug-in VSR Shape Modeling 2.0 offers analysis (which can also be persistent and updating, if wanted) which analyzes up to G3. Let me know if you should be interested or need more information about it.


Hey Michael,

I’ve heard of VSR before. I am just surprised Rhino itself doesn’t detect this. As it’s not a crucial problem to my project I’m good for now. But would you have a trial I can use w/o having to register?

Sure, if you are using Rhino 5, 64 bit, you can download a trial directly here:


The analysis you were asking for is “Curve Matching Analysis”, third icon from the left on the green toolbar.

Hi Duncan,

During our training we also found out this bug and it should be fixed.
@pascal are you aware of this?

Hi all- while I am not sure this is a bug, it is certainly a limitation in Gcon. Gcon is not lying here, it just is not aware of anything higher than G2 - two co-linear lines are reported as G2, which is also pointless- if it is G anything it must be G infinity.

I would look at the curvature graph for more info about continuity higher than G2, for now at least. If the graph from one curve transitions smoothly to the one on the other, you have G3 - that is, no kink between the graph of one curve and the graph of the next.


@pascal Could be wrong but I think what he means is something like this. Why does it report G2 when I built a G3 blend? Although I guess you can’t really get any higher than G2 because the rate of curvature on straight curve isn’t changing at all???

Hi Dennis- As I see it, GCon reports G2 because that is the most it knows about. The G3 curves are G2, just as they are also G1 and G0, if you see what I mean. If you ask the question, are these curves tangent? the answer is Yes even if they are G3, and the most stringent question GCon can ask in effect is,’ Are these curves G2?’ Does that make any sense?

All this aside, it would be nice if GCon could give info up to G4, since Rhino can create these.


Thanks. I understand a little better, GCon doesn’t show anything above G2 no matter what the curves look like. I agree it would be nice to report it especially like you said since Rhino can build it.

I was thinking because I was blending straight curves that it didn’t show anything above G2 simply because G2 and G3 would be the same, due the rate of curvature being 0.

Yes, working extensively with the curvature graph is what I taught to my trainees this week.
But you have to agree that when GCon command exists, it should be able to discover higher degrees. Or clearly mention it only works up to G2.


I stumbled on this topic when encountering the same issue today in MacRhino (v5.0.1). I wholeheartedly agree with the previous users who are requesting that future versions of Rhino (both Mac and Win) properly report the Geometric Continuity (GCon) between curves.

I fully understand that the Curvature Graph will visually display differences, but these may be slight and not easily perceivable. If one is trying to diagnose/understand various geometric conditions, it seems important that the tool that is supposed to report these (GCon) do so either accurately, or if reporting is not possible higher than G2 (is this really the case, mathematically speaking?) this limitation be noted in the command line report for GCon.

If anyone is curious here’s a screenshot and a .3dm file which illustrates the issue over a range of curvature continuity conditions, from G0 through G4. ~Dave

digiFABlab_Curves_Geometric_Continuity.3dm (86.4 KB)