[MPEG-OTSPEC] Does a rendering system know if a variation selector requested glyph is not available in a font?

[John Hudson]

The font wouldn’t need to do anything special in this scenario. The 
behaviour you describe is the sort of thing that is usually found at the 
application level, which is where text highlighting/colouring takes 
place, in concert with the shaping engine. It seems easy enough to do: 
check for presence of variation selector sequences, check for format 14 
cmap mappings for those sequences in the current font, highlight any 
sequences without mappings.

[In the context of the runology queries, I don’t think variation 
selectors are the best solution. There are similar issues in the study 
of historical texts in many writing systems, and the set of variants is 
usually too large and too open-ended to be suitable for Unicode’s strict 
definition of variation selector forms. The current version of the Brill 
Epichoric font, for instance, includes 31 forms of Alpha—not including 
RTL boustrophedon variants and stoichidon spacing forms—and it only 
takes discovery and publication of one more inscription to introduce one 
or more additional variants.]

JH


On 2024-06-07 4:11 am, William_J_G Overington via mpeg-otspec wrote:
>
> There is a document
>
> (R)Unicode: Encoding and Sustainability Issues in Runology
>
> https://www.unicode.org/L2/L2024/24129-runology.pdf
>
> I have no expertise at all in Runology but I did notice one thing in 
> the document that has prompted me to make an observation.
>
> On page 14 the document has the following.
>
> > Fonts supporting that form would display the appropriate glyph, recognising the string of 
> base-character + variation selector, but fonts without such support 
> would ignore the variation selector and fall back to a generic form 
> for the base character.
>
> I suggest that it would be possible to have a version of the program 
> that displays the glyphs to be such that if the fall back glyph is 
> displayed due to the requested glyph not being available in the font, 
> then that fall back glyph could be displayed in, say, red, or some 
> other way, so that it would be clear that that was the situation 
> rather than it being just an unreported fallback situation. Is that 
> possible with fonts and rendering systems as they are now?
>
> So it would not be a situation of the variation selector request being 
> ignored, but a situation of the variation selector request not being 
> acted upon yet a notification that the request had not been acted upon 
> notified to the researcher.
>
> I appreciate that this would not be a feature requiring encoding in 
> Unicode, but would involve the font and the rendering system. So how 
> could this be implemented please, indeed are there any programs that 
> already implement such a feature? If the font returns the default 
> glyph when asked for the variation sequence requested glyph, does the 
> rendering system know that this is the case? If so, how? If not, can a 
> feature be added to the font specification so that the rendering 
> system will know please?
>
> I am thinking that such a feature could b useful in various 
> situations, not only runology.
>
> William Overington
>
>
> Friday 7 June 2024
>
>
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-- 

John Hudson
Tiro Typeworks Ltdwww.tiro.com

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

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