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

[Liam R. E. Quin]

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.


>    1. Should there not be a VARC component flag that, when pulling
> components from a glyf table suppresses the instructions of that
> component?

i belive no, because if someone finds a way to make it work usefully,
they shouldn't be prohibited from doing so.

>    2. Should applications of that flag cancel the instructions of
> nested composites as well?

that sounds like an implementation issue.

>    3. If there is not such a flag, and perhaps if there is, should
> instructions be cancelled when certain total (i.e. top-to-bottom)
> transformations are in play? Or automatically cancelled for the
> portions of the compositing tree in which they are in play?

this also sounds like an implementation issue.

>    4. What are those problematic transformations -- anything but
> translation? Anything but translation and scaling?

rotation by 90 or 180 degrees may be well-defined.

>    5. Do there need to be flags to indicate what transformations a
> set of instructions are "immune" to? Since those instructions will be
> atom-level, where would those flags live?

no, this is something an implementation must determine - the flag would
at best be advisory, and if things go wrong if the flag is set wrongly,
the implementation has to cope.

>    6. Should there be a VARC-component flag indicating that what is
> being loaded is not an individual component but the (limited)
> composite-level TT instructions for this glyph? (The natural home for
> those instructions being in the glyph with the same GID, as same-GID-
> loading is already supported by the spec?)

i don't have an informed opinion here, sorry.

>    7. Whether or not any of the above results in spec changes, should
> there not be guidance about how to hint glyf-based VARC fonts,

Yes, i believe that there should be, but it does not belong in the
spec. The reason i say that is because experience will evolve over time
at a different rate than the ISO spec gets revised, and because the
audience is somewhat different.

Hope this helps; i think it does make sense to ask these questions, by
the way.

liam


-- 
Liam Quin, https://www.delightfulcomputing.com/
Available for XML/Document/Information Architecture/XSLT/
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