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

[Peter Constable]

That’s not a breaking change. It’s a new feature that requires new software for support. It’s no more breaking than introduction of the colour tables.

Changing structures to remove fields or redefining field in structures but without a major table version bump would be a breaking change.


Peter

From: mpeg-otspec <mpeg-otspec-bounces at lists.aau.at> On Behalf Of Adam Twardoch (Lists)
Sent: Friday, September 11, 2020 1:22 PM
To: Dave Crossland <dcrossland at google.com>
Cc: mpeg-otspec <mpeg-otspec at lists.aau.at>
Subject: Re: [MPEG-OTSPEC] Introducing breaking changes into the spec (was: RE: [EXTERNAL] Proposal to deprecate derived search values)

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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