Mon 04 Sep 2017 09:15:39 AM UTC, comment #2:
For the record: about phala forms.
The word phala or falaa means "point", primarily, an arrow-tip. It is also used in Bengali writing to mean an annex to a letter. The Unicode standard and the OpenType documents call the special forms of ra and ya phala, but the OpenType standard recommends using the "blwf" table also for the below form of ba.
So, which letters exactly have phala forms? I don't think the answer is simple or easy.
Halhed's 1778 Grammar of the Bengali language lists 12 "P,holaa"s. It's important to know that, at least at that time and to the best of Halhed's knowledge, the notion of "P,holaa" was quite broad.
He lists:
ya ra la na ma
Note that in his book, ma takes a wholly subscript form, while more recent forms have a right vertical to the right of the preceding letter.
He spends a lot of time fussing with the "ba", which is conflated with the Sanskrit "wa" or "va", and earlier Bengali versions of these consonants. This remains a Unicode encoding lacuna at this time. Anyway, he regards these also P,holaa.
He also lists the subscript vocalic r -- well, it's also a below-form.
But he goes on to list left-combinations with sha and nga -- which don't produce below forms now or then... They seem to be a sort of instructional device, referring to sets of consonant combinations, rather than a simple set of subscript glyphs however.
How does this impact the practical implementation of the font? This also isn't clear. The ya has a lookup specially for it: 'pstf' -- but the modern ma and also kha too behave in a similar way -- sometimes. The OpenType docs show the 'blwf' lookups being used for ba and ra phala forms -- but it might be advantageous to handle other consonants here too. However, each entry here has to be taken into account in later tables: since 'blwf' executes befor 'half', it absorbs the intervening halanta so the 'half' form of the preceding consonant does not form.
In any case, for 'bng2', it's essential that every consonant of every conjunct be processed through one of 'blwf' 'half' or 'cjct' in order for the conjuct to be recognized as such, and for a closing i-vowel sign to be placed properly.
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