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

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:

• Regular
• Italic
• Bold
• BoldItalic

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:

1. 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.
2. Remove any WWS name metadata from the font. This is done because the font is renamed afterwards.
3. 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.)
4. 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.
5. 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.
6. Kern pairs from the GPOS table are copied to the legacy kern table. This only applies to fonts that have a GPOS table, which is used for kerning in modern fonts.
7. The font-line helper 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:

• Skip the kern step
• Use a custom name for a font
• Use a custom name for the prefix
• Remove the GPOS table entirely
• Adjust the percentage of the font-line setting
• Skip running font-line altogether

For debugging purposes, you can run the script with the --verbose flag.

Examples

Generating KF fonts

This applies the KF prefix, applies 20 percent line spacing and adds a Kobo kern table. Ideal if you have an existing TrueType font and you want it on your Kobo device.

The --name parameter is used to change the name of the font family.

./kobofix.py --prefix KF --name="Fonty" --line-percent 20 *.ttf

To process fonts from my ebook-fonts collection which are prefixed with "NV", you can replace the prefix and make adjustments in bulk.

To process all fonts with the "Kobo Fix" preset, simply run:

./kobofix.py --prefix KF --remove-prefix="NV" --line-percent 0 *.ttf

(In this case, we'll set --line-percent to 0 so the line height changes aren't made, because the fonts in the NV Collection should already have those changes applied.)

The expected output is then:

nico@m1ni kobo-font-fix % ./kobofix.py --prefix KF --remove-prefix NV *.ttf --line-percent 0

Processing: NV-Elstob-Bold.ttf
  --remove-prefix enabled: using 'Elstob' as the new family name.
  Renaming the font to: KF Elstob Bold
  PANOSE corrected: bWeight 8->8, bLetterForm 2->2
  Kerning: extracted 342467 pairs; wrote 342467 to legacy 'kern' table.
  Saved: KF_Elstob-Bold.ttf
  Skipping line adjustment step.

Processing: NV-Elstob-BoldItalic.ttf
  --remove-prefix enabled: using 'Elstob' as the new family name.
  Renaming the font to: KF Elstob Bold Italic
  PANOSE corrected: bWeight 8->8, bLetterForm 3->3
  Kerning: extracted 300746 pairs; wrote 300746 to legacy 'kern' table.
  Saved: KF_Elstob-BoldItalic.ttf
  Skipping line adjustment step.

Processing: NV-Elstob-Italic.ttf
  --remove-prefix enabled: using 'Elstob' as the new family name.
  Renaming the font to: KF Elstob Italic
  PANOSE corrected: bWeight 5->5, bLetterForm 3->3
  Kerning: extracted 286857 pairs; wrote 286856 to legacy 'kern' table.
  Saved: KF_Elstob-Italic.ttf
  Skipping line adjustment step.

Processing: NV-Elstob-Regular.ttf
  --remove-prefix enabled: using 'Elstob' as the new family name.
  Renaming the font to: KF Elstob
  PANOSE corrected: bWeight 5->5, bLetterForm 2->2
  Kerning: extracted 313998 pairs; wrote 313998 to legacy 'kern' table.
  Saved: KF_Elstob-Regular.ttf
  Skipping line adjustment step.

==================================================
Processed 4/4 fonts successfully.

Generating NV fonts

Tight spacing, with a custom font family name:

./kobofix.py --prefix NV --name="Fonty" --line-percent 20 --skip-kobo-kern *.ttf

Relaxed spacing, with a custom font family name:

./kobofix.py --prefix NV --name="Fonty" --line-percent 50 --skip-kobo-kern *.ttf

You can play around with --line-percent to see what works for you.