Re: Draft of feature encoding changes

I updated the PR to reference OFF and added a note that the rest of the
document uses open type links for convenience.

On Wed, Jun 21, 2023 at 1:11 PM Vladimir Levantovsky <
vladimir.levantovsky@gmail.com> wrote:

> The link included there is a direct download link for the 2nd edition of
> the OFF (published 2009). We are now at 4th edition standard with two
> additional amendments finalized, and the 5th edition is now on the way
> (still a working draft though).
> I wouldn't use that link from specref, it's way outdated. Rather, I would
> simply reference the main ISO page for the standard (
> https://www.iso.org/standard/74461.html)  because it's static and will
> always be updated to the most recent version. We can also mention that it
> is available free of charge for download from ITTF.
>
>
> On Wed, Jun 21, 2023 at 2:51 PM Garret Rieger <grieger@google.com> wrote:
>
>> Looks like the ISO standard is in spec ref (
>> https://www.specref.org/?q=open%20font%20format) so I should be able to
>> link it via bikeshed.
>>
>> On Wed, Jun 21, 2023 at 10:58 AM Skef Iterum <siterum@adobe.com> wrote:
>>
>>> Vlad - I was referring specifically to this:
>>> https://www.iso.org/obp/ui/en/#iso:std:iso-iec:14496:-22:ed-4:v1:en:sec:2
>>>
>>> Skef
>>> ------------------------------
>>> *From:* Vladimir Levantovsky <vladimir.levantovsky@gmail.com>
>>> *Sent:* Wednesday, June 21, 2023 6:24 AM
>>> *To:* Skef Iterum <siterum@adobe.com>
>>> *Cc:* Garret Rieger <grieger@google.com>; w3c-webfonts-wg <
>>> public-webfonts-wg@w3.org>
>>> *Subject:* Re: Draft of feature encoding changes
>>>
>>>
>>> *EXTERNAL: Use caution when clicking on links or opening attachments.*
>>>
>>>
>>> I get it that the ISO website is confusing, but you only have to pay for
>>> paper copy of the standard. If you follow the blue box “download” link, it
>>> will bring you to the list of publicly available standards - you scroll
>>> down to ISO/IEC 14496-22 and download the PDF file after you click through
>>> the license dialog box. (Or, just use the link in my previous email.)
>>>
>>> Thank you,
>>> Vlad
>>>
>>>
>>> On Jun 20, 2023, at 10:51 PM, Skef Iterum <siterum@adobe.com> wrote:
>>>
>>> 
>>> The fact that there's an HTML preview with only the early sections and
>>> that asks for money is ... unfortunate.
>>>
>>> Skef
>>> ------------------------------
>>> *From:* Vladimir Levantovsky <vladimir.levantovsky@gmail.com>
>>> *Sent:* Tuesday, June 20, 2023 6:49 PM
>>> *To:* 'Garret Rieger' <grieger@google.com>; 'w3c-webfonts-wg' <
>>> public-webfonts-wg@w3.org>
>>> *Subject:* RE: Draft of feature encoding changes
>>>
>>>
>>> *EXTERNAL: Use caution when clicking on links or opening attachments.*
>>>
>>>
>>> Regarding ISO OFF text – it is publicly available, but it’s a PDF
>>> document that can only be downloaded in full, not making it possible to
>>> link to a particular chapter or feature.
>>>
>>>
>>>
>>> I think we can utilize a dual approach where normative references
>>> section includes the reference to OFF using main ISO link (
>>> https://www.iso.org/standard/74461.html), and we can also use direct
>>> links to various OT pages offering it as informative reference with the
>>> note that the content is identical to ISO OFF standard.
>>>
>>>
>>>
>>> Also, note that the blue box on the ISO OFF page provides the
>>> information and link for free download:
>>> https://standards.iso.org/ittf/PubliclyAvailableStandards/
>>>
>>>
>>>
>>> Vlad
>>>
>>>
>>>
>>>
>>>
>>> *From:* Garret Rieger [mailto:grieger@google.com]
>>> *Sent:* Tuesday, June 20, 2023 8:26 PM
>>> *To:* w3c-webfonts-wg (public-webfonts-wg@w3.org) <
>>> public-webfonts-wg@w3.org>
>>> *Subject:* Draft of feature encoding changes
>>>
>>>
>>>
>>> As discussed on the call this morning I've made a draft of some changes
>>> to how we encode the feature tag set included in the request. The PR can be
>>> found here: https://github.com/w3c/IFT/pull/149
>>>
>>>
>>>
>>> Note, there are a couple deviations from what we discussed:
>>>
>>>    1. We talked about switching to referencing the ISO OFF
>>>    specification instead of Microsoft's OpenType spec. However, it appears
>>>    that ISO OFF isn't publicly available. So I can't easily link to it from
>>>    the IFT specification. Given that, I think it's more useful for
>>>    implementers to continue to link to the public OpenType one instead.
>>>    2. After thinking about the process of how we could actually add
>>>    entries to the list after the initial publication I've decided we'll need
>>>    some versioning mechanism to accomplish future updates since the client and
>>>    server must be speaking the same encoding. For now since there is only one
>>>    version, I haven't actually introduced the mechanism yet, but I did note in
>>>    the appendix that the current list is version 1 and future updates may
>>>    introduce a new version plus a mechanism to communicate it (likely a new
>>>    field in client state and request).
>>>    3. One thing that I forgot to mention during the call which is
>>>    important is that the sorted integer list is delta encoded. That means we
>>>    encode the delta between an entry in the list and the previous entry
>>>    instead of the absolute value. This means generally most entries should
>>>    encode in one byte even if we have IDs that are greater than 127. I made
>>>    one small change to help here and moved the ss01-ss20 and cv01-cv99
>>>    encodings to the end of ID space so they don't add large gaps between other
>>>    unrelated feature tags. With that change everything other than those two
>>>    groups now have ID's less than 127 which guarantees they will encode in one
>>>    byte.
>>>
>>>