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And, if an author decided to use “Selawic New Regular” as a web font and discovered that nothing works as it used to - would that be considered a breaking change?<br>
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<div>Thank you,</div>
Vlad</div>
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<blockquote type="cite">On Sep 11, 2020, at 6:04 PM, Peter Constable <pgcon6@msn.com> wrote:<br>
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<p class="MsoNormal">My usage of the term “breaking change” is what I am familiar with in software engineering projects: some existing protocol that is redefined so that it no longer works as previously assumed — meaning that existing implementations will have
unexpected failures.<o:p></o:p></p>
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<p class="MsoNormal">If there is an entirely new sfnt version, then whatever is included in that format, I would _<i>not</i>_ consider that a _<i>breaking</i>_ change. It’s a new format that doesn’t provide some transitional compatibility/migration with existing
software. In OT1.8x, the ‘glyf’/’gvar’ design allowed for transitional compatibility—the ‘glyf’ table could be read by existing software in a compatible way, but the new functionality wouldn’t be available. CFF2 wasn’t a breaking change; it just didn’t provide
any transitional compatibility for existing software.<o:p></o:p></p>
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<p class="MsoNormal">Now, if a font vendor releases a font, say, “Selawik Regular”, and then publishes a new version of the font that is still called “Selawik Regular” but that uses a new sfnt version/format, I could see that being called a breaking change
since documents formatted with “Selawik Regular” would no longer display correctly in existing software if the older font were replace by the new font: it has broken existing documents. But if instead the vendor published a new “Selawik New Regular” font,
nothing is broken.<o:p></o:p></p>
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<p class="MsoNormal"><b>From:</b> Adam Twardoch (Lists) <list.adam@twardoch.com> <br>
<b>Sent:</b> Friday, September 11, 2020 2:49 PM<br>
<b>To:</b> Peter Constable <pgcon6@msn.com><br>
<b>Cc:</b> Dave Crossland <dcrossland@google.com>; mpeg-otspec <mpeg-otspec@lists.aau.at><br>
<b>Subject:</b> Re: [MPEG-OTSPEC] Introducing breaking changes into the spec (was: RE: [EXTERNAL] Proposal to deprecate derived search values)<o:p></o:p></p>
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<p class="MsoNormal">Well, OK. I guess I misunderstood the notion of “breaking”. Am I right to understand that if a new SFNT version (e.g. "OFF2" or \00\02\00\00) gets introduced, a lot of old assumptions can be removed? <o:p></o:p></p>
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<p class="MsoNormal">Of course existing versions of each table identified by a given tag should retain their meaning, but older versions of tables could be removed, tables could be removed from the spec or even disallowed ("reserved"), and if fields or structures
are changed/removed in tables that we keep, they require a version bump? <o:p></o:p></p>
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<p class="MsoNormal">A.<o:p></o:p></p>
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<p class="MsoNormal">On Fri, 11 Sep 2020 at 23:25, Peter Constable <<a href="mailto:pgcon6@msn.com" target="_blank">pgcon6@msn.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Changing structures to remove fields or redefining field in structures but without a major table version bump would be a breaking change.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Peter<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b>From:</b> mpeg-otspec <<a href="mailto:mpeg-otspec-bounces@lists.aau.at" target="_blank">mpeg-otspec-bounces@lists.aau.at</a>>
<b><br>
<br>
On Behalf Of </b>Adam Twardoch (Lists)<br>
<br>
<br>
<b>Sent:</b> Friday, September 11, 2020 1:22 PM<br>
<br>
<br>
<b>To:</b> Dave Crossland <<a href="mailto:dcrossland@google.com" target="_blank">dcrossland@google.com</a>><br>
<br>
<br>
<b>Cc:</b> mpeg-otspec <<a href="mailto:mpeg-otspec@lists.aau.at" target="_blank">mpeg-otspec@lists.aau.at</a>><br>
<br>
<br>
<b>Subject:</b> Re: [MPEG-OTSPEC] Introducing breaking changes into the spec (was: RE: [EXTERNAL] Proposal to deprecate derived search values)<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Hello Vlad,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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<br>
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them. <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I believe OFF has followed suit, therefore introducing a breaking change to the ISO standard. <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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<br>
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or 1.9.x) should stay in the OFF standard “forever”. <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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. <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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<br>
<br>
container and going with something completely different), but I’d not exclude even that possibility. <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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,<br>
<br>
an “unbreaker” for those de-facto breaking changes that OT 1.8 brought.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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. <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">A.<o:p></o:p></p>
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