[MPEG-OTSPEC] Cmap format to map 1 char to multiple glyphs?

[Behdad Esfahbod]

On Tue, Dec 12, 2023 at 11:51 AM John Hudson <john at tiro.ca> wrote:

> I proposed that to the OT developer list a long while ago, and recall that
> Kamal had a similar idea, initially in terms of handling Unicode
> decompositions such that fonts would not need precomposed diacritics. At
> the time, Microsoft thought it unlikely to get traction, as it implied
> significant engineering for unclear benefit, but perhaps the benefit is
> clearer now? As you say, being able to decompose a Unicode character to an
> arbitrary sequence of glyphs is very useful for Arabic, and by-passes the
> need to handle such decompositions in GSUB prior to other shaping.
>
> I suppose the question is whether there is a significant benefit to doing
> this outside of GSUB? — or, indeed, if there might be a reason it would be
> preferable in GSUB?
>
> The inconsistency in dot handling in different joining forms of some
> Arabic characters means that one doesn’t always want to up-front decompose
> some characters to base grapheme and combining dots, but those could be
> excluded from the cmap and passed to GSUB form decomposition in the joining
> form features. But that being the case, why not do it all in GSUB?
>
Thanks John. The main benefit in my opinion is not allocating a gid to
every precomposed Unicode character, most of them Latin. The Arabic
use-case is extra.

b


JH
>
>
> On 2023-12-12 9:04 am, Behdad Esfahbod wrote:
>
> Thank you everyone for the very productive meeting.
>
> I like to also bring this issue up. If there is interest, I can work on
> it. I wrote in my reply to Peter earlier:
>
> *This reminds me of another idea we discussed in, I think, 2019, from
> Monotype to introduce a `cmap` subtable that would map individual
> characters to sequences of glyphs. Then the pre-composed Unicode characters
> wouldn't need to have their own glyphs. Back then we dropped the idea for
> backwards-compat reasons. But maybe we can pick it up now?*
>
> This is very useful for Arabic as well...
>
> behdad
> http://behdad.org/
>
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> John Hudson
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