This came up on the METAPOST mailing list:
In the 1980’s I (Peter Wilson) was involved in the TRUCE (Tri-Rational Unified Cubic
Engine) project at the GE Research Laboratories, aimed at automatically
generating NC (Numerically Controlled) tool paths for machining
components from solid (geometric) models. Part of this was trying to
solve offsetting a cubic curve into another one (which would not be a
cubic and in general could be self-intersecting). As far as I am aware
there were no publications about this but useful practical algorithms
were developed about which I have no useful knowledge.
It is a HARD problem.