[MPEG-OTSPEC] Introducing breaking changes into the spec (was: RE: [EXTERNAL] Proposal to deprecate derived search values)

[Adam Twardoch (Lists)]

Hello Vlad,

OpenType 1.8 introduced a major breaking change: OpenType fonts in which
the only source for glyph rendering is the 'CFF2' table. Those fonts
(variable or not) conform with the OT 1.8 spec but older implementations
can’t do anything about them.

I believe OFF has followed suit, therefore introducing a breaking change to
the ISO standard.

There is nothing in the world that speaks against OFF 1.x and OFF 2.x
co-existing. These versions could be treated as “levels“. H.264 is one
codec in MPEG and H.265 is another, which older implementations know
nothing about.

The OFF can be devised in such a way that both the 1.x “level“ and the new
2.x “level” co-exist in the standard. There should be a clear distinction
between these levels, and the last known 1.x rendition (equivalent to the
last OT 1.8.x or 1.9.x) should stay in the OFF standard “forever”.

Implementers can be free to then make their code work with OFF1 only, or
OFF2 only, or both. Font makers will also be free to release their fonts as
OFF2 and also as OFF1, or both.

WOFF[1] and WOFF2 are technically completely different, and there are
implementations that support both or only one. WOFF2 was a complete
breaking charge towards WOFF. I don't think OFF2 wants to go that far (as
in, abandoning the SFNT container and going with something completely
different), but I’d not exclude even that possibility.

When the Variable OT fonts were introduced, or became clear that for many
scenarios, an intermediate layer of software needs to be introduced (e.g.
for embedding into the print stream). Effectively, a OT 1.8 > OT pre-1.8
conversion step, an “unbreaker” for those de-facto breaking changes that OT
1.8 brought.

Many software vendors are aware of this step being need. Since that
filtering/conversion step is being added everywhere, there should be no
problem of extending that step in future, so it can also convert OFF2 into
OT pre-1.8 or OFF1.

A.
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