On Tue, 30 Oct 2018 02:47:25 +0100 Philippe Verdy via Unicode <[email protected]> wrote:
> We are here at the line between what is pure visual encoding (e.g. > using superscript letters), and logical encoding (as done eveywhere > else in unicode with combining sequences; the most well known > exceptions being for Thai script which uses the visual model). For your information, Thai uses the logical encoding, almost by definition. The logical order is the order used in the backing store (See Section 2.2, Unicode Design Principles <http://www.unicode.org/versions/latest/ch02.pdf#G128>). In the Thai ‘combining sequences’ you have in mind, the vowel symbols you have in mind are classified as letters, so we do not have combining sequences! There were ill-defined preposed logically following combining marks (in the charts, but not the tables) in Unicode 1.0, but the problems with implementing them in the Thai monosyllable เพลา were so great that I wonder if any one succeeded at the time - <U+0E1E THAI LETTER PHO PHAN, U+0E3A THAI VOWEL SIGN PHINTHU, U+0E25 THAI LETTER LO LING, U+0E70 THAI PHONETIC ORDER VOWEL SIGN SARA E, U+0E32 THAI VOWEL SIGN SARA AA> with invisible PHINTHU, as opposed to <U+0E2B THAI LETTER HO HIP, U+0E3A THAI VOWEL SIGN PHINTHU, U+0E25 THAI LETTER LO LING, U+0E32 THAI VOWEL SIGN SARA AA> with visible PHINTHU! The official disinformation source, http://www.unicode.org/glossary, misdefines logical order <http://www.unicode.org/glossary/#logical_order> to be ‘the order in which text is typed on a keyboard’. So much for suggestions that one should design keyboard interfaces to convert visual order to storage order! A striking example is New Tai Lue, whose standard ordering was changed from phonetic order to visual order because it was found that the logical order, even using the Unicode *character* encoding, was visual order rather than phonetic order. Richard.
