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

Laurence Penney lorp at lorp.org
Fri Sep 22 14:45:14 CEST 2023

I agree with Georg there’s no issue here. Specifically I don't follow this statement:

> If neither cubic control point is hinted, neither will fall sequentially between a pair of touched points, and therefore cubic control points will typically not move in relation to how their adjacent on-curve points move.

In an outline segment ABCD (where A and D are on-curve, B and C are off-curve), when A and D are moved, then B and C will move according to IUP interpolation rules, each preserving its proportional distance from A and D in whichever dimension (x or y) is being processed. Whether they are touched or not, B and C remain "sequentially between" A and D. This behaviour will be invariant whether we’re in quadratic or cubic; the hinting interpreter already ignores on/off curve flags (so those on/off labels at the beginning of this paragraph are actually irrelevant) and would also ignore cubic vs quadratic flags.

- Laurence

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