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

[Adam Twardoch (Lists)]

I think OFF2 should strive to limit the choices unless really necessary.
This helps creation of test suites. The less you need to test the better,
and also with this approach to add extra fields to the tables, implementers
often internally model their software on the lattest version and somehow
synthesize the missing values.

For example OS/2 v0 & 1 don't have x-height or caps height, but the latest
CSS drafts have provisions for starting the first line at caps-height. So a
font engine has to have a fallback to synth these values even though
99.9999% of fonts has OS/2 newer than v1. And of course those synthesized
fallbacks will differ across implementations.

So why not disallow those old versions altogether in OFF2? Then a browser
maker might be able to safely get rid of that synthesizing code, at least
in the faster OFF2-compliant code path. Fewer lines of code, fewer "ifs",
faster execution and a feasible compliance test suite!

OS/2 is just an example. There are tons of such things in there. :)

A.

On Fri, 11 Sep 2020 at 23:20, David Singer <singer at apple.com> wrote:

>
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> > On Sep 11, 2020, at 14:14 , Adam Twardoch (Lists) <
> list.adam at twardoch.com> wrote:
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> >
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> > Right now am implementer of OT 1.8 has to implement support for all six
> versions of the OS/2 table. OFF2 could get rid of that burden.
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> This is probably a blindingly obvious statement, but a common technique is
> to differentiate the requirements for readers and creators, e.g. writers
> must write version 6 as the default, and may write other versions on
> request; readers must accept version 6, and should accept 3,4,5, and may
> accept earlier versions (the last doesn’t need saying, of course, people
> always may do anything above what the specs mandates or recommends).
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> David Singer
>
> Manager, Software Standards, Apple Inc.
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