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BubbleKern is a set of new kerning tools for the Glyphs font editor by Georg Seifert. that is based on the shape of surrounding outlines you draw, which is called a bubble. Once you draw bubbles for all letters, BubbleKern automatically kerns the typeface in seconds. That’s right, you kern by drawing it!


A bubble needs to be made as a extra layer in each master, and it has to be named “bubble“. It doesn’t have to be straight segments or single path, and you craw however you want. Components automatically inherit the bubbles of base glyphs, so you don’t have to draw one for an accented glyph (for letters like Eth or Tbar, you just draw a bubble for the bar).

When asked to kern, first of all BubbleKern scans through each letter to make an approximation of bubble shape so that it’s faster to calculate. And the temporary dictionary of bubble sidebearings is built for every letter.

Then for a given pair, BubbleKern looks at every scan height, finds where bubbles touch each other first, and kerns by the distance. In the example below, it finds the closest distance of 30 units, and kerns by -30.

As for pairs to kern, a type designer needs to make a checklist for BubbleKern to work with. Don’t worry, there is a script to help you with that.

But why does it not simply look at all possible pairs, every glyph against every other glyph? Because that takes so much time to check, only to create a huge mess. If your font has 600 glyphs, the maximum number of possible pairs is 600²=360,000 which contains meaningless pairs like %& or some other junk that you won’t enjoy cleaning up.

A good tool is something the user can control. BubbleKern will kern a font for you, but only do the obvious, not do some mysterious calculation in the background (e.g. Adobe’s optical kerning). That’s why I leave bubble shapes and pairs to designers.


  • ShowKernBubbles.glyphsReporter A viewer plugin that shows bubbles in colour.
  • Make Bubble A Python script to help you start drawing bubble layers.
  • Delete Bubble A Python script that deletes bubble layers from selected glyphs.
  • The Python script that does the magic.
  • BubbleKern Manual.pdf Instruction on how to use BubbleKern with more detail than this README.


  1. Install three scripts in ~/Library/Application Support/Glyphs/Scripts folder. Make Bubble Layers and BubbleKern require vanilla and robofab modules to display user interface. You can install them easily from Glyphs (Preferences > Addons > Modules). If you needed to install modules, you will need to restart Glyphs. Installed scripts will appear under Scripts menu.
  2. The Show Kern Bubbles viewer plugin needs to be installed in ~/Library/Application Support/Glyphs/Plugins folder (if there is none, create one). After installation, you need to restart Glyphs. It will show up under View menu as Show Kern Bubbles. Alternatively you can double click the plugin file to install it; confirm Confirm the dialog that appears in Glyphs.
  3. If the script gives me an error, you may have or .pyc in your Scripts folder. In that case, delete them and try running the script again. Restarting Glyphs may be necessary.
  4. Restart Glyphs

Usage Instructions

The essential part is that you have a layer called "bubble" and run BubbleKern script. Every other tool is basically optional.

  1. Make a extra layer and name it "bubble" for every layer you want to kern. Use Make Bubble Layers script to speed up the process.
  2. Bubble layer dictates the kerning, and generally a bubble needs to be larger. Use Show Kern Bubbles viewer plugin to help shaping it. It is important to make the bubble stick to sidebearings somewhere, in order to create zero kerning. (e.g. think about how to avoid creating a kerning value between H and H while using bubbles)
  3. Once you have bubble layer, run BubbleKern, in which you build a list of kerning pairs you want to check. There are two ways to build a list; permutation and plain text.
  4. In BubbleKern, click Kern All Pairs for initial run. When you update a bubble shape of a glyph, run Kern Pairs with Selected Glyphs to update that specific part of kerning data, which is faster.


Copyright 2015 Toshi Omagari (@tosche_e).

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

See the License file included in this repository for further details.