SIL Font Development Notes

How to build, modify, and contribute to SIL International font projects.

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2 Source Formats

We keep our font sources in open formats so anyone can have full access to them now and in the long-term future. This also gives us the widest-possible choice of tools, so we can pick the right tool for the right job. Interoperability is a core value of our open font design and production workflow. Being locked into a particular proprietary tool is definitely not what we want, especially long-term.

UFO3 + designspace

Our primary font sources are kept in the UFO3 format. It’s a clearly documented, platform-neutral open format which allows everyone to access the sources with range of tools. Using this format helps future-proof a font project and makes it accessible and maintainable by others. It also helps with keeping font sources in revision control - a key to any collaborative development. Many foundries are also now using UFO as their primary source format. It is directly supported by all the main font design tools (Glyphs, Robofont, FontLab, FontForge, TruFont). We sometimes store other formats in the repository for archival purposes (typically in source/archive/) but the fonts are produced from the canonical UFO sources.

We also occasionally store some font-wide data in the UFO3 lib.plist.

Our font family structures (styles, masters, instances) are defined in designspace files which describe the relationships between individual font UFOs.

We use normalization tools from the pysilfont collection of font utilities to keep the UFOs in a consistent format even after import/export from various other tools and to synchronize them with one another. These keep the font sources clean, vendor-neutral, and friendly to version control systems.

Additional font sources (and tools)

There are some types of data that currently have no canonical place in the UFO3 format. We store and maintain these in additional files:

  • Graphite source code is stored in GDL (Graphite Description Language) format in source/graphite/.
  • Documentation is typically stored in .md (Markdown) or .odt (OpenDocument Text) format in documentation/.
  • Test data and documents are stored in a variety of formats (FTML, SILE, TeX, IDML, text) in tests/.
  • Web font demonstration files (.html, .css) are stored in web/.
  • Tools, such as project-specific python scripts, are stored in tools/.

Auxiliary data sources

There are also some types of data that do have a place in the UFO3 format but are clumsy or awkward to maintain there. We store and edit these in auxiliary files, and use python scripts (usually in preflight) to update the UFOs from them before committing changes to the project repositories:

  • OpenType source code is stored in the .fea format in the UFO (features.fea) but may be defined in a separate file using the more efficient and powerful .feax format. The .feax is compiled into standard .fea during the build process and may in some cases also be used to update the features.fea in the UFOs.
  • Individual glyph data such as production glyph names and glyph orders is stored in a glyph_data.csv file for easy maintenance. The glyph names and orders in the fonts are updated from this data in the preflight script.

For an example of how these sources are organized see Repository Structures.
To learn more about the preflight scripts used to update UFOs from auxiliary sources, see Modifying Font Sources.