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

Laurence Penney lorp at lorp.org
Fri Sep 22 14:54:46 CEST 2023

> Jonathan Kew:
> I don't think this is correct. It's a very long time since I worked directly with TrueType instructions, so I may be mis-remembering, but my understanding is that IUP *would* be expected to adjust both control points in such a case. The text says "...fall sequentially between..." but does not require the untouched point to be *immediately* between the touched points.

Ah, indeed I think the misunderstanding is confusion "topological between" and "geometrical between". In IUP, the initial decision of which points to move is all about point order around a contour, thus topological. But IUP also has a rule that states if (in the ABCD example) point B or C is *geometrically* outside the range AD, then it moves relative to A (if A is closer) or D (if D is closer). So IUP involves a topological between followed by a geometrical between :)

- L

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