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

Hin-Tak Leung htl10 at users.sourceforge.net
Sat Sep 23 12:53:18 CEST 2023

 That's quite unacceptable, to be honest, with a lot of "undefined behaviour". What will happen, if it takes off, is that quirks and limitations of one particular implementation will come to be expected, then get formalised as "limitations/quirks as specification" :-(.
Just telling people not to have not-2 consecutive off-curve points, doesn't mean they won't try. Then, they expect it to behave like their favourite implementation does, and try to push it into the spec... I think I can say there is such a past instance where maths were done in 16-bit due to tech limitations, then people start to expect, and * demand * that maths must overflow in specific ways.
You can say it is restricted now, but pretty sure that some will not only ignore the restriction, but also * demand * that the restriction is to be broken in quite specific ways, to  their preference / convenience.
It is also a hugh step backwards - in quadratics, contours consisting of entirely off-curve points are legal, and in fact beneficial for a few reasons.
    On Saturday, 23 September 2023 at 00:17:45 BST, Laurence Penney <lorp at lorp.org> wrote:  
 > 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:

> 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

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