Thanks Behdad, Very useful and informative. Thanks.
Enjoy your tacos. Andrew On 29/12/2012 5:41 PM, "Behdad Esfahbod" <beh...@behdad.org> wrote: > On 12-12-26 08:02 PM, Andrew Cunningham wrote: > > Hi Behdad, > > Hi Andrew, everyone, > > Sorry for being slow on the list or on fixing bugs. I'm in Mexico City and > exploring the amazing tacos around town. Hopefully I'll get back to fixing > all reported issues next week. > > > > Will the approach of using DFLT script work with a multi a multiscript > font? > > I.e. if I need to support Arabic (Jawi), Western Cham and Khmer? > > > > And if I need to support reordering and ligatures DFLT will be ok? > > > > Are all OT features supported by DFLT or only some? > > Let me just explain what HarfBuzz exactly does: > > Based on the Unicode script of the text and a set of hardcoded rules, we > choose which shaper to use. The current rules are in this piece of code: > > > > http://cgit.freedesktop.org/harfbuzz/tree/src/hb-ot-shape-complex-private.hh#n143 > > To summarize: > > - For Arabic, Mongolian, Syriac, N'ko, 'Phags-pa, and Mandaic, we always > use > the Arabic shaper, > > - For Thai and Lao we use the Thai shaper, > > - For Brahmi-derived scripts that have a left-matra kind of character we > use > the Indic shaper if and only if a non-DFLT GSUB table is found, > > - For Khmer we use the Indic shaper if and only if the GSUB table has a > 'pref' feature, > > - For Myanmar we only use the Indic shaper if the 'mym2' OpenType script > is > present, and NOT if it's 'mymr', > > - Otherwise, use the default shaper. > > > After this, we do our custom normalization (which also handles Hangul Jamo > (de)composition as well as matra decompositions. Then we map to glyphs, > and > apply script-specific OpenType shaping. > > > Now, here's what each shaper does: > > > - Default shaper does this: > > * Enables these features, in both GSUB and GPOS: ccmp, liga, locl, > mark, > mkmk, rlig, > > * Depending on the text direction, enable for horizontal: calt, clig, > curs, kern, rclt, and for vertical: valt, vert, vkrn, vpal, vtrt2, > > * For horizontal text, enable either ltra and ltrm, or rtla and rtlm > depending on the direction, > > * Enables a few features based on the script: For Hangul, enable ljmo, > vjmo, and tjmo. For Tibetan enable abvs, blws, abvm, and blvm, > > * All these features are enabled globally except for rtlm which is only > enabled for RTL runs and characters that do NOT have a Unicode mirroring > character, and applied together. > > > - The Thai shaper does two things: > > * Use PUA-encoded shaping (using MS and Mac Unicode encodings) if > there is > no GSUB found, > > * Do SARA AM decomposition and reordering, > > * Apply all features from the default shaper. > > > - The Arabic shaper does Arabic joining analysis and do: > > * Apply ccmp and locl features together, > > * Apply one of init, medi, fina, iso, med2, fin2, or fin3 based on the > analysis result, > > * Apply rlig, then calt, then the three of cswh, dlig, and mset > together, > as well as other features from the default shaper. > > > - The Indic shaper does a bunch of things, all based on the MS Indic > OpenType spec, extended to support more scripts. In particular, it breaks > text into syllables based on a grammar, and for each syllable does: > > * Apply these features globally: locl and ccmp, > > * Do initial-reordering, > > * Apply the following features in this order: nukt, akhn, rphf, rkrf, > pref, half, blwf, abvf, pstf, cfar, cjct, and vatu. Of which, these ones > are > applied globally: nukt, akhn, rkrf, cjct, and vatu, and the rest based on > analysis does on the syllable (and the font tables), > > * Do final-reordering, > > * Apply the following features all at the same time: init, pres, abvs, > blws, psts, haln, dist, abvm, blwm. Of which, all are applied globally > except > for init, as well as other features from default shaper, except for liga > which > is turned off. > > > Indic initial-reordering consists of (omitting lots of details): > > - Find base character. Different rules are used for different > categories of > scripts, > > - Reorder characters based on their desired position in the syllable, > > - Make Reph sequence to be reordered later. > > > Indic final-reordering consists of (omitting lots of details): > > - Reorder left matra to the desired position, > > - Reorder formed Reph to the desired position after base, > > - Reorder pre-base reordering consonants. > > > > That's about it, at a high level. So, to answer your question, if you > depend > on Indic reordering, you cannot rely on the DFLT script. If you do NOT > depend > on such reordering, it is best to use the DFLT script such that the > reordering > rules do NOT interfere with your GSUB rules. It is fine to mix tables for > multiple scripts all under DFLT as long as the shaper-selection rules above > work for you. > > Hope that answers your questions. > > Cheers, > behdad >
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