Re: Treatment of Variation Selectors in the Client

IFT patches are always specific to the font, so yes, they'll be based in part on the content of format 14 (if present) and you won't get glyphs that only substitute for other glyphs not in the subset.

Beyond that, though, the question is how flexible we can be in satisfying the glyph closure requirement. It seems like the strategy used for static subsets (again, if I'm reading the code right) is to treat the variation selectors as "extra", not considering them as part of the list of unicodes to be preserved (or not). So, for example, even if VS 1 isn't in the list of codepoints to be preserved, you can still get glyphs only accessible using VS 1. It's not clear to me why that's the case.

Skef
________________________________
From: John Hudson <john@tiro.ca>
Sent: Thursday, October 17, 2024 2:29 PM
To: public-webfonts-wg@w3.org <public-webfonts-wg@w3.org>
Subject: Re: Treatment of Variation Selectors in the Client


EXTERNAL: Use caution when clicking on links or opening attachments.


On 2024-10-17 09:28, Skef Iterum wrote:

I may be reading the code wrong, but both the HarfBuzz and fontations subsetters variation alternates are treated as potential substitutes for "individual" Unicode regardless of the unicode values to be included. That is, the implementation doesn't look to see whether the variation selector itself has been requested to be included. So, in effect, UVS mappings are supported by default (unless I'm misreading).

It's not clear to me why this is the case. I suppose one explanation would be that certain shapers might use variation selectors implicitly, and therefore they aren't considered to be "real" codepoints. But in that case maybe we should treat them like layout features that are used by default, giving the option to a shaper, and therefore a client, to skip them when they aren't needed.

Is it possible to query the format 14 cmap subtable and include only the U+xxxx+UVS mappings found there that correspond to the subset for a given initial font delivery or patch?

JH


--

John Hudson
Tiro Typeworks Ltd    www.tiro.com<http://www.tiro.com/>

Tiro Typeworks is physically located on islands
in the Salish Sea, on the traditional territory
of the Snuneymuxw and Penelakut First Nations.

__________

EMAIL HOUR
In the interests of productivity, I am only dealing
with email towards the end of the day, typically
between 4PM and 5PM. If you need to contact me more
urgently, please use other means.