Hi Fred,
I know this is a two year ago reply. But I have some information which should help understand what's going on with the spiro.
See below:
On Fri, 2020-02-07 at 21:03 +0000, Fred Brennan wrote:
Greetings, I write from the FontForge project. Of particular interest to me is
the Spiro spline feature, which was originated around ten years ago by Raph Levien.
One thing I'd like to add, (which would benefit both our projects,)
is the ability of FontForge to understand the Inkscape Spiro serialization format.
However, there are several things about the format which to me as an
outsider appear to be defects serious enough that I have no idea how to even *import* these splines correctly, much less export our Spiro splines to this format. I would very much like to support the _de facto_ standard Inkscape has originated of supporting Spiro in SVG, but I am lost.
... snip ...
I can probably overcome this, although George Williams was right to
be skeptical of this format. There is no way I can see to define a G2 curve in this strange "original-d" format.
So, anywhere you see `original-d` this means that there is a Live Path Effect (LPE) happening. These are transformational processes which live in their own element in defs and you can see the id contained within the `inkscape:path-effect` attribute.
Following this id you come to an inkscape:path-effect element in <defs> which usually contains all the extra data required for the effect. This does not just control spiros, but also any of the other many many live path effects.
If you were to make a path with just the original-d, you would not have a spiro in inkscape. At least not an editable one. And you only need original-d if you want the spiro to continue to be editable.
This format was created to allow inkscape to render SVG 1.1 paths, while containing much more feature rich path editing. But not intended for importing from other programs. I'm already aware that Blender has co-opted the inkscape:groupmode attribute for their svg files (and never said anything to us) so it's not unheard of to have compatibility.
Best Regards, Martin Owens