Ipe does not support closed spiro splines

[Willem3141]

I really like using spiro splines for smooth curves in my drawings. However, I'd often want to use closed spiro splines, which Ipe doesn't support (falls back to a closed B-spline).

As a proof of concept, I tried replacing false by true in the SpiroCPsToBezier2 call on line 796 of ipegeo.cpp. This shows that editing closed spiro splines could in principle work just fine without further changes:

Screencast.From.2026-04-30.14-34-18.mp4

Of course this is not a proper solution: now all non-closed spiro splines are rendered as closed ones! I suppose a backwards-incompatible change of the file format would be necessary, with a new path type for closed spiro splines.

If you agree this would be a useful feature to have, I would be willing to try implementing this properly, and open a pull request if I can get it to work.

[otfried]

Open splines and closed splines are really quite different things in Ipe: open splines (all four variations of them) appear a segments in a Curve (in ipeshape.cpp). Closed splines, however, need their own implementation of SubPath.

I don't remember anymore, but my guess is that I mostly wanted to see what the clothoid splines would look like, so I implemented them as part of curves, and left the closed version until such time as users would ask for them.

So please go ahead and implement a new subclass SpiroSpline of SubPath. I suggest we use the letter R for closed clothoid splines in the XML representation (https://otfried.github.io/ipe/90_file_format.html#ipe-object-elements).