Nico Verbruggen
6cd3e2b20b
This is a big commit that fixes a variety of issues and prevents fonts
from growing far too large without any benefits. Previously, converted
fonts would use multiple subtables for the `kern` table, but sadly those
are usually not read by renderers, so we now don't save al kern pairs,
but we prioritize.
The full list of changes can be found below.
---
Replace individual boolean flags with modal --kern and --hint options:
- --hint {skip,additive,overwrite,strip}: controls hinting behavior,
including new ttfautohint support (additive/overwrite modes)
- --kern {add-legacy-kern,legacy-kern-only,skip}: replaces the old
--skip-kobo-kern and --remove-gpos flags
- --preset {nv,kf}: bundled configurations for common workflows
Add upfront dependency checking (`ttfautohint`, `font-line`) so missing
tools are caught before any processing begins.
Fix GPOS Extension lookup (type 9) support: kern pairs were silently
missed in fonts that wrap PairPos subtables in Extension lookups.
Rework legacy kern table writing to respect format 0 size constraints.
The subtable length field is uint16, limiting a single subtable to
10,920 pairs. Since most renderers (including Kobo) only read the first
subtable, we write exactly one and prioritize pairs by Unicode range.
=> Basic Latin > Latin-1 Supplement > Latin Extended > rest
This way, the most commonly encountered kerning pairs are preserved
when truncation is needed (and it usually is, for quality fonts).
4.3 KiB
Kobo Font Fix
Overview
kobofix.py is a Python script designed to process and adjust TTF fonts for Kobo e-readers for a better reading experience with the default kepub renderer.
It generates a renamed font, fixes PANOSE information based on the filename, adjusts the baseline with the font-line utility, and adds a legacy kern table which allows the kepub engine for improved rendering of kerned pairs.
You can use this to modify or fix your own, legally acquired fonts (assuming you are permitted to do so).
License
Licensed under the MIT License.
Requirements
Python 3, FontTools, font-line.
You can install them like so:
pip3 install fonttools
pip3 install font-line
If you want to use the --hint additive or --hint overwrite options, you also need ttfautohint:
brew install ttfautohint # macOS
On macOS, if you're using the built-in version of Python (via Xcode), you may need to first add a folder to your PATH to make font-line available, like:
echo 'export PATH="$HOME/Library/Python/3.9/bin:$PATH"' >> ~/.zshrc
source ~/.zshrc
Usage
Open a terminal and navigate to the directory containing your font files. Make sure your font files are named correctly. The script will process files that contain the string:
RegularItalicBoldBoldItalic
This is the naming convention used on Kobo devices for proper compatibility with both the epub and kepub renderer.
You can then run:
python3 kobofix.py ./src/*.ttf
By default, the script will:
- Validate all filenames. If there are any invalid filenames, you will be prompted and can continue with all valid filenames, but it is recommended that you fix the invalid files.
- Remove any WWS name metadata from the font. This is done because the font is renamed afterwards.
- Modify the internal name of the font. Unless a new name was specified, this is merely a prefix that is applied. (By default, this is
KF.) - PANOSE metadata is checked and fixed. Sometimes, the PANOSE information does not match the font style. This is often an oversight but it causes issues on Kobo devices, so this fixes that.
- Font weight metadata is updated. There's other metadata that is part of the font that reflects the weight of the font. In case this information needs to be modified, it is adjusted.
- Kern pairs from the GPOS table are copied to the legacy
kerntable. This only applies to fonts that have a GPOS table, which is used for kerning in modern fonts. - The
font-linehelper is used to apply a 20% line-height setting. This generates a new file which is immediately renamed to the desired output format.
The modified fonts are saved in the directory where the original fonts are located.
Customization
You can customize what the script does. For more information, consult:
./kobofix.py -h
Given the right arguments, you can:
- Control kerning behavior (
--kern): add a legacy kern table (default), remove GPOS after extraction, or skip entirely - Control hinting (
--hint): strip hints, apply ttfautohint to unhinted fonts, apply ttfautohint to all fonts, or skip (default) - Use a custom name for a font
- Use a custom name for the prefix
- Adjust the percentage of the
font-linesetting - Skip running
font-linealtogether
For debugging purposes, you can run the script with the --verbose flag.
Presets
The script includes presets for common workflows. If no preset or flags are provided, you will be prompted to choose one.
NV preset
Prepares fonts for the ebook-fonts repository. Applies the NV prefix and 20% line spacing. Does not modify kerning or hinting.
./kobofix.py --preset nv *.ttf
You can override individual settings, for example to use relaxed spacing:
./kobofix.py --preset nv --line-percent 50 *.ttf
KF preset
Prepares KF fonts from NV fonts for use on Kobo devices. Applies the KF prefix, replaces the NV prefix, and adds a legacy kern table. No line spacing changes are made (since NV fonts already have those applied).
./kobofix.py --preset kf *.ttf
Custom processing
You can also specify all flags manually:
./kobofix.py --prefix KF --name="Fonty" --line-percent 20 --kern add-legacy-kern *.ttf