[MPEG-OTSPEC] Some research on TT instructions and cubics

[Laurence Penney]

> Hin-Tak Leung:
> 
> We still have the ambiguity of 1, 3, 4 (and more) off-curve cubic to deal with. I suppose the 1 case is easy - you just go back to quadratic and ignore the cubic bit. 3,4 or above is ambiguous: is 3 = 1 + 2 or 2 + 1 (with an implied on-curve point in the middle), and is 4 = 2 + 2 (two cubic) or 4 = 1 + 2 + 1 (two quadratics on either side of a cubic)? etc.

There is no ambiguity in the cubic extension to glyf. Please see the Restrictions section here:
https://github.com/harfbuzz/boring-expansion-spec/blob/main/glyf1-cubicOutlines.md

> I think assuming people will start with existing fonts (and do conversions) is dangerous - people will find interesting ways of using the "corners" (rightly or maliciously) and then argue that their newly hacked up fonts should be interpreted / rasterized in such and such way.

I am happy to confirm that my idea was solely intended for Skef to test how well TrueType hinting instructions work with cubic outlines (matching the subject of this thread), no more than that.

- Laurence