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

[William_J_G Overington]

John Hudson wrote
 
> Unless you are anticipating 256 variations of a single character 
> needing to be captured in plain text, ...
 
I am anticipating the possibility that once this feature becomes 
available that new ways of carrying out research may become developed.
 
For example, there was research on the Gutenberg Bible where glyphs for 
a letter are not precisely the same and it is thought that Gutenberg 
used a one-cast matrix system. I have wondered if that is the case, 
would using many ligatures be a cost saving practice? I have further 
wondered if there were unsuspected virtual ligatures such as pi and pl 
and so on made, two unconnected glyphs on the same piece of metal type.
 
Unicode has 256 official Variation Selectors available, some in plane 0, 
most in plane 14, so the same for the user-defined variation selectors 
seems reasonable to me.
 
Unicode traditionally uses blocks of 256 code points, so it seems to me 
that 256 user-defined variation selectors is a good idea.
 
The history of information technology has examples of where a decision 
over what was necessary was later changed.
 
Around 1980 it was decided that two digits was enough to specify a year. 
By the mid 1990s lots of software needed to be altered, at great cost 
and effort, as the year 2000 approached. If only four digits had been 
used from the start.
 
Here is a link to the Unicode roadmap page for plane 14.
 
https://www.unicode.org/roadmaps/ssp/ 
<https://www.unicode.org/roadmaps/ssp/>
 
Having 256 user-defined variation selectors would be a row full. If 
fewer than 256 are encoded, would the rest of the row ever be used for 
something else?
 
Once these user-defined variation selectors are implemented then they 
may well be used in ways not in the proposal.
 
It would be possible to encode in plain text, each of italic glyphs, 
bold glyphs, bold italic glyphs, titling font glyphs (that is larger 
capital letters on the same point size body), Black Letter glyphs, red 
glyphs, and so on, all with a graceful fallback..
 
There is a famous print of the famous This is a Printing Office text 
where most of the text is printed in black ink and one line is printed 
in red ink.
 
With these User-Defined Variation Selectors that text could now be 
typeset in plain text such that with an appropriate font the red 
lettering would be displayed yet with a graceful fallback if the text 
were displayed other than with such a font.
 
William Overington
 
Saturday 22 June 2024
 
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