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[doc] Document hb_feature_from_string() syntax 

Copied and edited from the util option documentation. The docbook table
syntax is too verbose, but that is the best I can come up with.
This commit is contained in:
Khaled Hosny 2019-02-19 02:23:58 +02:00 committed by Behdad Esfahbod
parent 3da79dd5b9
commit 6bd4c082e4
1 changed files with 34 additions and 1 deletions
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35
src/hb-common.cc
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@ -974,7 +974,40 @@ parse_one_feature (const char **pp, const char *end, hb_feature_t *feature)
*
* Parses a string into a #hb_feature_t.
*
* TODO: document the syntax here.
* The format for specifying feature strings follows. All valid CSS
* font-feature-settings values other than 'normal' and 'inherited' are also
* accepted, though, not documented below.
*
* The range indices refer to the positions between Unicode characters. The
* position before the first character is always 0.
*
* The format is Python-esque. Here is how it all works:
*
* <informaltable pgwide='1' align='left' frame='none'>
* <tgroup cols='5'>
* <thead>
* <row><entry>Syntax</entry> <entry>Value</entry> <entry>Start</entry> <entry>End</entry></row>
* </thead>
* <tbody>
* <row><entry>Setting value:</entry></row>
* <row><entry>kern</entry> <entry>1</entry> <entry>0</entry> <entry>∞</entry> <entry>Turn feature on</entry></row>
* <row><entry>+kern</entry> <entry>1</entry> <entry>0</entry> <entry>∞</entry> <entry>Turn feature on</entry></row>
* <row><entry>-kern</entry> <entry>0</entry> <entry>0</entry> <entry>∞</entry> <entry>Turn feature off</entry></row>
* <row><entry>kern=0</entry> <entry>0</entry> <entry>0</entry> <entry>∞</entry> <entry>Turn feature off</entry></row>
* <row><entry>kern=1</entry> <entry>1</entry> <entry>0</entry> <entry>∞</entry> <entry>Turn feature on</entry></row>
* <row><entry>aalt=2</entry> <entry>2</entry> <entry>0</entry> <entry>∞</entry> <entry>Choose 2nd alternate</entry></row>
* <row><entry>Setting index:</entry></row>
* <row><entry>kern[]</entry> <entry>1</entry> <entry>0</entry> <entry>∞</entry> <entry>Turn feature on</entry></row>
* <row><entry>kern[:]</entry> <entry>1</entry> <entry>0</entry> <entry>∞</entry> <entry>Turn feature on</entry></row>
* <row><entry>kern[5:]</entry> <entry>1</entry> <entry>5</entry> <entry>∞</entry> <entry>Turn feature on, partial</entry></row>
* <row><entry>kern[:5]</entry> <entry>1</entry> <entry>0</entry> <entry>5</entry> <entry>Turn feature on, partial</entry></row>
* <row><entry>kern[3:5]</entry> <entry>1</entry> <entry>3</entry> <entry>5</entry> <entry>Turn feature on, range</entry></row>
* <row><entry>kern[3]</entry> <entry>1</entry> <entry>3</entry> <entry>3+1</entry> <entry>Turn feature on, single char</entry></row>
* <row><entry>Mixing it all:</entry></row>
* <row><entry>aalt[3:5]=2</entry> <entry>2</entry> <entry>3</entry> <entry>5</entry> <entry>Turn 2nd alternate on for range</entry></row>
* </tbody>
* </tgroup>
* </informaltable>
*
* Return value:
* %true if @str is successfully parsed, %false otherwise.