3 Scripting a designspace

It can be useful to build a designspace with a script rather than construct one with an interface like Superpolator or DesignSpaceEditor.

fontTools.designspaceLib offers a some tools for building designspaces in Python. This document shows an example.

Filling-in a DesignSpaceDocument

So, suppose you installed the fontTools package through your favorite git client.

The DesignSpaceDocument object represents the document, whether it already exists or not. Make a new one:

from fontTools.designspaceLib import (DesignSpaceDocument, AxisDescriptor,
                                      SourceDescriptor, InstanceDescriptor)
doc = DesignSpaceDocument()

We want to create definitions for axes, sources and instances. That means there are a lot of attributes to set. The DesignSpaceDocument object uses objects to describe the axes, sources and instances. These are relatively simple objects, think of these as collections of attributes.

Make an axis object

Make a descriptor object and add it to the document.

a1 = AxisDescriptor()
a1.maximum = 1000
a1.minimum = 0
a1.default = 0
a1.name = "weight"
a1.tag = "wght"
  • You can add as many axes as you need. OpenType has a maximum of around 64K. DesignSpaceEditor has a maximum of 5.

  • The name attribute is the name you’ll be using as the axis name in the locations.

  • The tag attribute is the one of the registered OpenType Variation Axis Tags

  • The default master is expected at the intersection of all default values of all axes.

Option: add label names

The labelnames attribute is intended to store localisable, human readable names for this axis if this is not an axis that is registered by OpenType. Think “The label next to the slider”. The attribute is a dictionary. The key is the xml language tag, the value is a unicode string with the name. Whether or not this attribute is used depends on the font building tool, the operating system and the authoring software. This, at least, is the place to record it.

a1.labelNames['fa-IR'] = u"قطر"
a1.labelNames['en'] = u"Wéíght"

Option: add a map

The map attribute is a list of (input, output) mapping values intended for axis variations table of OpenType.

# (user space, design space), (user space, design space)...
a1.map = [(0.0, 10.0), (401.0, 66.0), (1000.0, 990.0)]

Make a source object

A source is an object that points to a UFO file. It provides the outline geometry, kerning and font.info that we want to work with.

s0 = SourceDescriptor()
s0.path = "my/path/to/thin.ufo"
s0.name = "master.thin"
s0.location = dict(weight=0)
  • You’ll need to have at least 2 sources in your document, so go ahead and add another one.

  • The location attribute is a dictionary with the designspace location for this master.

  • The axis names in the location have to match one of the axis.name values you defined before.

  • The path attribute is the absolute path to an existing UFO.

  • The name attribute is a unique name for this source used to keep track it.

  • The layerName attribute is the name of the UFO3 layer. Default None for foreground.

So go ahead and add another master:

s1 = SourceDescriptor()
s1.path = "my/path/to/bold.ufo"
s1.name = "master.bold"
s1.location = dict(weight=1000)

Option: exclude glyphs

By default all glyphs in a source will be processed. If you want to exclude certain glyphs, add their names to the mutedGlyphNames list.

s1.mutedGlyphNames = ["A.test", "A.old"]

Make an instance object

An instance is description of a UFO that you want to generate with the designspace. For an instance you can define more things. If you want to generate UFO instances with MutatorMath then you can define different names and set flags for if you want to generate kerning and font info and so on. You can also set a path where to generate the instance.

i0 = InstanceDescriptor()
i0.familyName = "MyVariableFontPrototype"
i0.styleName = "Medium"
i0.path = os.path.join(root, "instances","MyVariableFontPrototype-Medium.ufo")
i0.location = dict(weight=500)
i0.kerning = True
i0.info = True
  • The path attribute needs to be the absolute (real or intended) path for the instance. When the document is saved this path will written as relative to the path of the document.

  • instance paths should be on the same level as the document, or in a level below.

  • Instances for MutatorMath will generate to UFO.

  • Instances for variable fonts become named instances.

Option: add more names

If you want you can add a PostScript font name, a stylemap familyName and a stylemap styleName.

i0.postScriptFontName = "MyVariableFontPrototype-Medium"
i0.styleMapFamilyName = "MyVarProtoMedium"
i0.styleMapStyleName = "regular"

Option: add glyph specific masters

This bit is not supported by OpenType variable fonts, but it is needed for some designspaces intended for generating instances with MutatorMath. The code becomes a bit verbose, so you’re invited to wrap this into something clever.

# we're making a dict with all sorts of
#(optional) settings for a glyph.
#In this example: the dollar.
glyphData = dict(name="dollar", unicodeValue=0x24)

# you can specify a different location for a glyph
glyphData['instanceLocation'] = dict(weight=500)

# You can specify different masters
# for this specific glyph.
# You can also give those masters new
# locations. It's a miniature designspace.
# Remember the "name" attribute we assigned to the sources?
glyphData['masters'] = [

# With all of that set up, store it in the instance.
i4.glyphs['dollar'] = glyphData


path = "myprototype.designspace"


You can generate the UFOs with MutatorMath:

from mutatorMath.ufo import build
  • Assuming the outline data in the masters is compatible.

Or you can use the file in making a variable font with varLib.

Working with DesignSpace version 5

The new version 5 allows “discrete” axes, which do not interpolate across their values. This is useful to store in one place family-wide data such as the STAT information, however it prevents the usual things done on designspaces that interpolate everywhere:

  • checking that all sources are compatible for interpolation

  • building variable fonts

In order to allow the above in tools that want to handle designspace v5, the fontTools.designspaceLib.split module provides two methods to split a designspace into interpolable sub-spaces, splitInterpolable() and then splitVariableFonts().

Example process diagram to check and build DesignSpace 5

Example process process to check and build Designspace 5.

Also, for older tools that don’t know about the other version 5 additions such as the STAT data fields, the function convert5to4() allows to strip new information from a designspace version 5 to downgrade it to a collection of version 4 documents, one per variable font.