Down the Kobo Font Kerning Rabbit Hole

Not too long ago, I released version 2.0 of my collection of tweaked fonts, and I was pretty sure I was done tweaking fonts for the rest of the year.

However, this morning I received a message in my inbox from a friendly person who linked me to an interesting post on the MobileRead forums and noted:

One suggestion would be to consider adding ‘old-style’ kern tables so that kerning works correctly when reading kepub files: see e.g. https://www.mobileread.com/forums/showthread.php?t=259102

Okay, that’s cool. Kerning is notoriously bad on Kobo’s kepub renderer, so perhaps it was possible to re-export my fonts and improve them even further?

The email continued:

I’ve had some luck in the past with getting fonts to kern correctly on Kobo by just using Fontforge and exporting them with old-style kerning tables enables.

So, I gave it a try and exported my go-to option, Charter.

It’s hard to see that these are checkboxes on macOS, but they are. Toggling this option is the key to getting proper kerning for the kepub renderer.

Take a look at the results below, I’ve made an interactive component that toggles between both states:

Bad: Font exported without old-style kerning.

Good: Font exported with old-style kerning.

It’s obvious that KC Charter actually seems to render correctly. That’s great! I tried this approach with a few others fonts, and sure enough, things seem to be better this way.

If you’ve taken a look at the repository, you may have noticed that I’ve chosen to keep these fonts as distinct ones from the regular collection. This is done for two reasons:

I want it to be obvious which fonts use Kobo-compatible old-style kerning.
When using old-style kerning, fonts may not render correctly on other operating systems.

In particular, FontForge warns you about Windows when exporting only with old-style kerning.

The warning presented by FontForge when exporting. Depending on how many kern pairs were part of the font, hundreds or even thousands of kern pars may not function correctly on Windows.

There is another toggle you can use (Windows-compatible ‘kern’), but for some fonts this causes issues when exporting due to limitations in the kerning table.

As it turns out, there’s a maximum size that is reached and certain kerning info may be missing beyond a particular point.

If you want to have the best reading experience with your kepub books, using the Kobo-compatible fonts is now the way to go.

I’ll be updating the repository soon with the Kobo Collection, but you can try out a preview already.