First, the new spline is robust. That means that a small change to an input point yields a proportionally small change in the generated spline. By contrast, Spiro was prone to large flips, as a loop would change direction. Even worse, Spiro would sometimes fail to converge (often producing incorrect results resembline particle accelerator tracks). This lack of robustness was a major reason Spiro didn’t catch on more widely.
Second, it uses explicit tangents as a unifying and simple user experience refinement to control a wide range of behaviors. These explicit tangents replace Spiro’s “one-way constraints,” which were powerful but confusing from a user-experience perspective. With explicit tangents, the designer make smooth straight-to-curved transitions, control the curvature when a curve is terminated at a corner, control the exact locations of extrema (especially important for font design), and signal a transition from low curvature to high curvature regions. The new control is intuitive and will be familiar to designers experienced in Bézier drawing.
The new spline should be useful in font design (my original motivation), vector illustration in general, CAD, and other applications such as maps and representation of path centerlines for autonomous vehicles.
Try the online demo.
Read more about the spline in research paper 1.
License and patent grant
All code in this package is released under the terms of the GNU GPL, version 3 or later, at your choice.
Further, there is a provisional patent application filed for the underlying curve technology. The following patent grant applies to any patent which may be issued as a result of that application:
Whereas, Raph Levien (hereinafter “Inventor”) has obtained patent protection for related technology (hereinafter “Patented Technology”), Inventor wishes to aid the the GNU free software project in achieving its goals, and Inventor also wishes to increase public awareness of Patented Technology, Inventor hereby grants a fully paid up, nonexclusive, irrevocable, royalty free license to practice the patents listed below (“the Patents”) if and only if practiced in conjunction with software distributed under the terms of any version of the GNU General Public License as published by the Free Software Foundation, 59 Temple Place, Suite 330, Boston, MA 02111. Inventor reserves all other rights, including without limitation, licensing for software not distributed under the GNU General Public License.
This repo is primarily for my individual research. I am open to collaboration and will accept contributions, but I also want to preserve full ownership of the code so that I can do commercial licensing of code derived from it. Thus, contributions to the repo will require a signed copyright assignment to me.
Please get in touch if this condition is onerous.