[MPEG-OTSPEC] Minor ballot comment - change a field from 16 to 24 bits

[mpsuzuki]

Dear Liam,

I apologize to ask a few questions to your post in 2 months ago...

As far as I could check the current specifications, the field DataOffset
has been 16-bit for a long time, in Microsoft OpenType spec, Apple TrueType
spec, and current 14496-22.

https://learn.microsoft.com/ja-jp/typography/opentype/spec/otvarcommonformats
https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6gvar.html
(in Apple's spec, offsetToData in glyphVariationData structure might be the
field we discuss)

It is reasonable attitude to care about the overflow of this field in 24-bit
GID world, but simple change of this field to 24-bit could be incompatible
with existing implementations. At least, the proposed change in the draft
ballot comment does not mention about a technique to keep the compatibility
with existing fonts & font drivers. Maybe I'm missing something, please
let me ask a few questions.

1) Adding a new version of "gvar" table whose DataOffset is 24-bit is bad option?
Also, reservation of a bit in "flags" field of "gvar" header might be another
option.

2) Some fonts with 24-bit DataOffset are already shipped to the market?

3) Some existing implementations are already capable to handle 24-bit DataOffset?
HarfBuzz was already mentioned, but Behdad commented that it is not enabled
by default.

Regards,
mpsuzuki

On 2025/02/06 12:02, Liam R. E. Quin via mpeg-otspec wrote:
> In 7.2.2.2 GlyphVariationDataHeader, there is a field called
> DataOffset; this is defined to be 16-bit. In testing, people building
> actual fonts found this overflowed and needed to be 24 bits,
> so we have a proposed change, Change dataOffset from Offset16 to
> Offset24.
> 
> My understanding is that this has already been implemented in harfbuzz,
> since its an ISO draft and since it was necessary...
> 
> No other problems were found so far.
> 
> The PDF file enclosed says the same thing, using the ISO template for
> comments (numbered as if made sa a US comment; the Canadian mirror
> committee meeting is tomorrow)
> 
> liam
>