[OpenType] Allowing DFLT to take languages

[Sairus Patel]

Yes, we’d proposed DFLT. Adam’s description of the rationale sounds about right.

Also, I believe some fonts were also being made then that did not have language-specific behavior, & so the desire was to have only DFLT listed in the font, instead of listing explicit scripts in the font. That is why the spec says: “The 'DFLT' Script table should be used if there is not an explicit entry for the script being formatted”: the OT engine may have detected that the text is kana, say, but if there is no ‘kana’ script entry, then ‘DFLT’ should be used.

Fonts, and OT engines, have moved on since the time this was proposed, and explicit script entries are in most fonts I’ve looked at.

Is there a real-world example of someone desiring an actual language system tag under DFLT script?


Thanks,
Sairus


-----Original Message-----
From: <listmaster at indx.co.uk> on behalf of "list.adam at twardoch.com" <list.adam at twardoch.com>
Reply-To: "opentype-list at indx.co.uk" <opentype-list at indx.co.uk>
Date: Friday, April 8, 2016 at 7:23 PM
To: "listmaster at indx.co.uk" <listmaster at indx.co.uk>
Subject: Re: [OpenType] Allowing DFLT to take languages

>Message from OpenType list:
>
>
>I think "DFLT" originated at Adobe, not Microsoft. It was added to the OT spec in v1.4 in 2002, long after the original set of script tags were added. I *believe* the rationale was that one should be able to have an OT engine such as Adobe's that would NOT perform script itemization, or have otherwise a script that shapers could fall back to if they need to apply discretionary features on some glyphs mapped to Unicode codepoints that are in the "Common" script, say PUA. 
>
>Sent from my mobile phone.

—


Martin Hosken wrote:
...
I would like to propose a content change to the OT spec.

In the section on Opentype Layout Common Table Formats, under Script List Table and Script Record, there is a paragraph that says:

If a Script table with the script tag 'DFLT' (default) is present in the ScriptList table, it must have a non-NULL DefaultLangSys and LangSysCount must be equal to 0. The 'DFLT' Script table should be used if there is not an explicit entry for the script being formatted.

I propose that the words "and LangSysCount must be equal to 0" be removed from the standard.

Rationale:

1. There is no technical reason for this limitation. All shaping engines support multiple languages for the DFLT script.
2. No applications (except OTS now) reject fonts on account of having a non-zero LangSysCount. This implies that this part of the standard is being ignored.
3. If DFLT is being used as a fallback shaper for a given script, as implied by the last sentence in the paragraph, then that text may also have languages associated with it, that need to be handled appropriately in the DFLT script just as they would for the actual script.

In summary, the ongoing inclusion of this restriction only causes problems and adds nothing of value to the standard. Hence I propose it be removed.