OT2KPX(1) Marc Penninga OT2KPX(1)

ot2kpx - extract kerning information from OpenType fonts

ot2kpx [ -afm | -kpx | -lua ] <fontfile>

ot2kpx extract the kerning data from OpenType fonts and prints it to "stdout", either in Adobe's KPX format (for adding to an afm file) or as a Luatex custom feature, for use with the "\directlua" command.

Print a short help text and exit.
Print ot2kpx's version and exit.
Output the kerning data in Adobe's KPX format, as used in afm files. This is the default output format.
Output the kerning data as a Luatex custom font feature, to be included in a "\directlua" command.
<fontfile>
The OpenType font (both otf and ttf format are supported).

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ot2kpx doesn't implement all of the OpenType specification. Things that are missing include: support for font files containing multiple fonts, LookupTables with LookupTypes other than 2, "kern" tables with format other than 0 and ValueRecords with other types of data than just XAdvance data.

Most of these limitations won't matter, since the missing features are rare (the only fonts I know of that use them are the non-western fonts that come with Adobe Reader). Furthermore, many of these features define (according to the OpenType specification) "subtle, device-dependent adjustments at specific font sizes or device resolutions"; since there's no way to express such adjustments in afm format, ignoring them seems to be the only option anyway.

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ot2kpx collects kerning data first from the "kern" table, then from all LookupTables associated with the "kern" feature; if a kerning pair occurs multiple times, the first value seen is chosen. There are (or may be) several issues with this approach:
  • The OpenType specification says that fonts in otf format shouldn't use the "kern" table at all, just the lookups from the "GPOS" table. Many such fonts do, however, contain a "kern" table, but no "GPOS" table; so we use the "kern" table anyway.
  • Instead of reading all LookupTables, it might be better to let the user specify a script and language and process only the LookupTables for those values. However, at least in the fonts I checked, all script/language combinations eventually point to the same "kern" LookupTables, so this approach wouldn't make any difference (apart from further complicating the code).

Marc Penninga <marcpenninga@gmail.com>

Copyright (C) 2005-2023 Marc Penninga.

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version. A copy of the GNU General Public License is included with ot2kpx; see the file GPLv2.txt.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

This document describes ot2kpx version 20231230.

(See the source code for the rest of the story.)

2019-05-20
Added the -version option.
2019-04-15
Added the -lua command-line option to get output in Luatex's custom feature format.
2023-12-30 fontools