[HarfBuzz] A problem in thai shaper

datao zhang dataozhang at hotmail.com
Fri Apr 13 06:41:50 PDT 2012

Problem 1:
An common Thai character is composed of 5 parts:
Preceding vowel [1 optional] + consonant [1 mandatory]+ diacritic vowel [1 optional] + tone mark [1 optional]+ following vowel [2 optional]
When move cursor, we can move to after “Preceding vowel” or before “following vowel”.  You can’t move to the middle of consonant and diacritic vowel( or tone mark or the composition). 
Special case:
SARA_AM can be treated as a special diacritic vowel, it can follow the tone mark.
MAITAIKHU can be treated as diacritic vowel but can’t follow the tone mark
THANTHAKHAT can be treated as tone mark
So I think for the new Thai shaper, the valid composition of “consonant [1 mandatory]+ diacritic vowel [1 optional] + tone mark [1 optional] “ should be set as same cluster.
Problem 2:
When there is no consonant exist, the dotted circle should be inserted as base character.  The logic should be the first step for the shaping engine to find the invalid combing marks. Refer to http://www.microsoft.com/typography/otfntdev/thaiot/shaping.aspx#comb
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