File:Tracing letter a with Tracy.png

[//pineight.com/pc/#tracy Tracy] is an experimental autotracer written in Python and targeted at TrueType font glyphs. It starts by finding what the Potrace white paper calls "closed paths". But unlike more general tools such as Potrace, Tracy has a distinct philosophy on how to convert the outline to parabolic arcs, inspired by the TrueType spec. Each 45 degrees of curvature gets its own arc, so find local maxima in eight directions and put vertices there, recording their normals. A vertex also goes at inflection points of the outline, which occurs somewhere between consecutive vertices with opposite normals.

This is a visualization of the output when Tracy is run on the 'a' glyph from File:Insular_font_64px.png.
 * Rainbow stairs: The two closed paths. Color indicates the average normal over a sliding window around a point, with the interior of the glyph on the right for the nonzero-rule used by TrueType.
 * Half circles: Local maxima. Color indicates the vertex normal.
 * Gray: Off-curve control points and guessed inflection points.
 * Light gray: The contour defined by the control points.

Limits: inflection points aren't placed optimally, and corners and straight segments aren't detected. But it does approximate the outline close enough for manual tweaking in FontForge or any other app that can edit quadratic splines (that is, not Inkscape, which handles only the cubics of OpenType CFF).