[MPEG-OTSPEC] VARC, glyf, and TT-instructions

[Laurence Penney]

> On 25 Jan 2024, at 01:25, Skef Iterum via mpeg-otspec <mpeg-otspec at lists.aau.at> wrote:
> 
> On 1/24/24 16:39, Liam R. E. Quin wrote:
>>> On Wed, 2024-01-24 at 14:50 -0800, Skef Iterum via mpeg-otspec wrote:
>>>  At the same time, component instructions are very unlikely to work
>>> in the face of skews or rotations, and may not even work in the face
>>> of scaling. 
>> My understanding has always been that renderers turn off hinting when
>> text is skewed or rotated (other than by a multiple of 90°). I think
>> the onlky font format i know of that defined hinting for rotated text
>> was Folio’s F3, later co-owned i think by Morisawa and Sun, which had
>> built-in drop-out hinting.
>> 
>> So i’d expect a renderer to do the same with variable components that
>> are skewed or rotated.
> There is a distinction between whether the text path itself is skewed or
> rotated and whether a component in a composite is skewed or rotated.
> Asking around it seems as though with the existing glyf components
> instructions are not automatically turned off when "compositing", but
> perhaps that info is wrong.
> Either way, though, that seems like something the specification should
> clarify.
> 
I asked similar questions when I was getting my head around TT hinting, and recall being told that skewed or rotated components were not hinted. The person I asked would most likely have been Greg Hitchcock.

A 2x2 transform can easily be tested for niceness by the presence of a non-zero diagonal, with the other two values at 0. Wingdings was originally built to use 90°, -90° and 180° rotations for corner ornaments and arrows. However I understand it was rebuilt to avoid them because of Microsoft and Apple’s conflicting implementations of the interaction with translation: before or after the transform.

That conflict led to system builders and font editing apps avoiding transforms beyond scale (maybe not even scale) – the glyph would simply be decomposed – so there were no real life examples. Later, control bits were added to the spec to fix the translation ambiguity, but I’m not sure whether font apps started allowing such composites. The number of real fonts using transforms remains extremely low AFAIK so I don’t think the issue is high on any agenda.

My memory on this is somewhat hazy. Better authorities would be Sampo Kaasila, Greg Hitchcock, Ned Holbrook, Yuri Yarmola, Adam Twardoch, and Georg Seifert.

- Laurence
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