[MPEG-OTSPEC] CFF2 errata

[Behdad Esfahbod]

On Fri, Nov 1, 2024 at 7:09 PM Skef Iterum via mpeg-otspec <
mpeg-otspec at lists.aau.at> wrote:

> On 11/1/24 10:03 AM, Behdad Esfahbod wrote:
>
>
>>    - We will need to consider whether to allow the change in the
>>    PrivateDict as well, and what that means for fallback (what the first
>>    proposal clarifies). All of this means that this is not the time to be
>>    discussing that change.
>>
>> I think multiple vsindex'es are already permitted in PrivateDict. At
> least, there's no text saying otherwise. The relevant text in the spec is:
>
> "The vsindex operator may be used only once in a CharString and must
> precede the first use of the blend operator."
>
> I suspect many if not all existing implementations would break if you
> tried it
> in a PrivateDICT, and it's just something the authors didn't imagine.
> There are
> a lot of operators that can only be used once to reasonable effect that
> don't
> make that explicit.
>

I won't jump to that conclusion fast. I brought up this issue back in 2018
or 2019 with then Adobe folks at a meeting I had with them at Adobe. Read
Roberts and Dave Arnolds were present, and I remember when I brought this
issue up, Dave said it only applies to CharStrings, and multiple vsindex
are allowed in Private DICT.

That said, I just checked the HarfBuzz code written by Michiharu Ariza, and
for PrivateDict it allows multiple vsindex'es but only processes the first
one.

Note that if multiple operators were allowed, you'd need some language to
> sort out which one applied to the CharStrings (e.g. the first one or the
> last
> one). The language that talks about that relationship is generally
> singular.
>

I would think the last one matters.

behdad


> Skef
>
> Thanks,
> behdad
>
>
>>
>>
>> Perhaps we can discuss the two editorial(esque) changes at our next
>> meeting.
>>
>> Thanks,
>> Skef Iterum
>> Adobe, Inc.
>>
>> On 10/22/24 3:17 PM, Behdad Esfahbod via mpeg-otspec wrote:
>>
>> Hi everyone,
>>
>> CFF2 has a file size advantage over TrueType-flavored variable fonts,
>> specially if one doesn't care about hinting (Android, Apple platforms, etc)
>> but does care about (uncompressed) size.
>>
>> CFF2 also alleviates some other limitations of variable-fonts built
>> against the gvar table. Unfortunately, it imposes its own limitations.
>> These are among the issues I'm going to raise.
>>
>> Please see:
>>
>>   https://github.com/harfbuzz/boring-expansion-spec/issues/155
>>
>> for details of the changes I am proposing. Ideally, Adobe folks choose to
>> turn these ideas into proposals and push it through the OFF standardization
>> / OpenType integration process. Failing that, we might want to go ahead and
>> do it at Google
>>
>> Thanks,
>>
>> behdad
>> http://behdad.org/
>>
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