[HarfBuzz] Old Korean T jamo that don't conjoin?

[Behdad Esfahbod]

On Thu, Mar 29, 2018 at 1:25 PM, Nathan Willis <nwillis at glyphography.com>
wrote:

> Hi,
>
> I'm trying to understand a comment in the source for the Hangul shaper.
>
> Line 155; after denoting the various jamo blocks, it says "Only <LV,T>
> sequences for T in U+11A8..11C3 combine."
>
> Why is that? The T jamo extend down to U+11FF (all still in the main jamo
> block).
>
> More importantly, perhaps, I can't find a source in Unicode or in the MS
> OpenType docs that mentions that specific limitation, either. The MS docs'
> appendix B lists compositions for the whole T section.[1]
>

That's not relevant. This relates to the precomposed encoded Hangul
Syllable characters. Those only exist for these Ts. See section "Hangul
Syllables" in Unicode Standard. Specifically:

"Modern Hangul syllable blocks can be expressed with either two or three
jamo, either in the
form consonant + vowel or in the form consonant + vowel + consonant. There
are 19 possi-
ble leading (initial) consonants (choseong), 21 vowels (jungseong), and 27
trailing (final)
consonants (jongseong). Thus there are 399 possible two-jamo syllable
blocks and 10,773
possible three-jamo syllable blocks, giving a total of 11,172 modern Hangul
syllable blocks.
This collection of 11,172 modern Hangul syllables encoded in this block is
known as the
Johab set."

So, only those 27 combine.

Other sequences still can combine. Just not as character composition, but
font-level glyph substitutions and positioning.

Is the cut-off from some old standard, or something like that? I can see
> that the Syllables block doesn't have every permutation; I'm just trying to
> understand why T is getting different treatment than L and V....
>
>
> Thanks,
> Nate
>
> [1: https://docs.microsoft.com/en-us/typography/script-
> development/hangul#appendix-b-standard-composition-for-old-hangul-jamos ]
> --
> nathan.p.willis
> nwillis at glyphography.com <http://identi.ca/n8>
>
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>


-- 
behdad
http://behdad.org/
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