Font Development Best Practices

Technical guidance regarding font development and production

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2.2 UFO

UFO (Unified Font Object) is our recommended format for new font projects, and can offer advantages for existing projects. The UFO specification calls it a “cross-platform, cross-application, human readable, future proof format for storing font data”. It is a text-based format that draws on XML and plist structures. A UFO is a folder with a .ufo extension that contains files for font-wide metadata and a ‘glyphs’ folder that contains each glyph as a separate .glif file.

UFO versions, or ‘flavors’

UFO is actually not one single standard. There are multiple versions, or ‘flavors’ of UFO:

  • UFO 1 - The original from 2004. Rarely used now.
  • UFO 2 - The most common version (since 2009), with the broadest application support. Only slightly different from UFO 1. Still lacks some basic features, such as explicit glyph anchor elements.
  • UFO 3 - A much-improved and expanded spec from 2012. However adoption has been poor. Only FontForge offers UFO 3 support.

In practice, most foundries use UFO2, but may store application- or foundry-specific information in a lib.plist file. This can be a way of working around some of UFO2’s deficiencies, and some applications even respect (don’t touch) or read (use) lib.plist data added to a UFO by some other application. The pysilfont project includes a UFO normalizer and converter.

We currently recommend storing UFO sources in UFO2 format, and relying on a UFO converter tool to move data between formats.

There are various discussions going on concerning future UFO versions beyond 3, by both the original UFO authors and others, but there is no clear successor at this point.

Application support

Every major font design program supports UFO, but to varying degrees.

  • Robofont provides the most seamless and native support (requires Mac OS X 10.9 or later, though older versions are available for OS X 10.6 or later; commercial).
  • Glyphs can use UFO as a native format (requires Mac OS X 10.9 or later; commercial).
  • FontForge can import and export UFOs, including the UFO3 format (available for Windows, Mac, Linux; free).

We have used each of these successfully in UFO development, and can recommend their use. The key to successful use is to store UFOs in a normalized format then re-normalize them after editing them.

  • Trufont uses UFO as a native format, but is still early in development
  • Fontlab Studio 5 does not read or write UFO, but they provide a vfb2ufo converter that can be useful. It provides limited two-way conversion, but is mainly useful for one-time conversion into long-term UFO. They intend to support UFO more fully in version 6.

In addition to these visual design programs there are other applications and utilities that support UFO:

UFO Normalization

A weakness of the UFO format is that there is no normalized form. The ordering and normal default settings for various elements can differ between applications. The unfortunate result is that opening a UFO with an app, changing one point coordinate, then saving, can result in hundreds of changes, even to files seemingly unrelated to the single coordinate change. This makes source code version control very difficult.

The solution is to define a normalized form and always normalize the UFO prior to committing any changes to a source code repository (such as one on Github). This will ensure that the master version is always in normalized form, and any changes can be easily compared.

The pysilfont project provides UFO normalization through the psfnormalize command. After installing pysilfont, the workflow then becomes:

  • Check out (or clone) project from source code repository
  • Open UFO in editing application
  • Save changes
  • Run psfnormalize on the UFO with the command psfnormalize filename.ufo
  • Review changes via your version control client
  • Commit your changes

The normalization scheme imposed by psfnormalize has been reviewed by many people and although it is not a standard, it is reasonably well accepted by those who care about UFO normalization. We recommend that any UFO-based font projects stored in version control repositories always store the UFO in normalized form as set by psfnormalize.

Example projects

Adobe has a number of public projects that are based on UFO. Source Sans Pro is a good example.

SIL also has two example projects that we’ve used in testing UFO workflows: Nokyung and Andika Mtihani.

Why use UFO?

Our main motivation is that it is a well-supported, open, application-neutral format. We do not want to require that someone use a particular commercial program on a particular platform in order to access and use our font source. It also allows us to use common industry tools, and frees us from being locked into any one tool. It is handled well by version control systems, and has strong long-term readability.