This page serves a a place to collect the UFO team’s thoughts on potential future features of the specification.

UFO 3 Additions

Over the last 13 years the UFO specification has been developed in major steps. This has been the case because when the format was started the binary font formats that the specification superset were not rapidly changing. The UFO specification could go several years without a major upgrade and still only need a few minor additions. The binary font specifications are changing rapidly now and the UFO specification development process needs to adapt to this.

The problem with updating the UFO specification rapidly is that the format has been designed to be authoring tool neutral. If a new bit of data is added to fontinfo.plist, authoring tools must be updated to handle this new data otherwise the data will be lost. In UFO 3 we developed a way to make official additions to the specification that won’t result in data loss. This is done by using the various lib locations and the data directory with standard storage keys and formats. This works well for very small and very big changes, but not so well for changes that fall between. Unfortunately, these “in between” changes are the kinds that are being made to the OpenType specification–new tables, new fields, new data. This new data is too complex to store in the lib locations. So, we need to develop a way to handle this. We’re going to do this by making a small tweak to the way that authoring tools interpret the data in fontinfo.plist.

The fontinfo.plist specification is going to gain a new section that addresses how an authoring tool should handle unknown key/value pairs. Previously, unknown pairs were considered illegal. Going forward, when an unknown pair is encountered the tool should preserve the pair and should write it, unchanged, back into fontinfo.plist during a subsequent save operation. This does not mean that anyone can add anything that they want to fontinfo.plist. (lib.plist must still be used for that.) The authoring tool should not attempt to validate the unknown data apart from ensuring that it conforms to standard Property List specifications.

This change will allow the specification to be updated incrementally without users encountering data loss. Major changes, such as additions to GLIF, will still require a complete version jump.

SVG Table

The following is a sketch for how “SVG in OpenType” could be stored in UFO.

There must be a layer named “public.openTypeSVG” (note, this is a UFO file layer name, not necessarily the name that must be shown to the user in an interface). That will contain a layer of glyphs that should be represented in the SVG table.

The GLIF may contain anything except an element that contains data. In the glyph lib, there will be this key:

This will specify the file name to use as the SVG data.

The default outline data for use in the CFF or glyf table (as required in the SVG table specification) should be in the UFO’s default layer. If this glyph does not exist, it is up to the authoring tool to determine how to handle the situation. This is because the data may not yet exist during the design phase, but it is, obviously, required when compiling an OpenType font.

The SVG files will be located in:

There is no file naming specification for the SVG files except that they must be file system legal as defined elsewhere in the UFO spec.

SVG documents should conform to the rules defined in the “Glyph Semantics and Metrics” of the SVG Table specification.

COLR Table

Color palettes ([CPAL Table]) will be located in fontinfo.plist.

Single File UFO

UFO has two possible storage formats. The first, “UFO Package”, is a multi-file, file system directory structure. The second, “UFO ZIP”, is a ZIP archive containing the same directory structure as defined for UFO Package.

A UFO ZIP must be a ZIP archive that follows these specifications:

  1. ZIP64 format is allowed, but not required.
  2. File names must be encoded to UTF-8.
  3. All contents of the UFO must be contained within a single directory. For example, rootdirectory/metainfo.plist and rootdirectory/kerning.plist. The name of this directory will be arbitrary and no assumptions should be made about it’s name.
  4. The file may or may not be compressed.
  5. The file must not be encrypted.

File Name Extensions

.ufo UFO Package
.ufoz UFO ZIP


ZIP has been chosen over several other viable candidates because of its broad interoperability and ease of implementation for developers. Most, if not all, modern programming languages have existing code for reading and writing ZIP. Additionally, ZIPs are also easy for users to create on their own. For example in OS X a user can take a UFO Package and create a UFO ZIP directly in the Finder.

Unified Font Set

We are in the early phase of designing a new file structure that will represent multiple UFOs. We are currently calling these “Unified Font Set” (see below for other names). This is the current file structure:

It is hopefully obvious that this is largely based on the structure of UFO. That is by design. We want clear associations between UFS and UFO data. This will allow for significant code reuse.

Unless specified otherwise, the files are optional.


Format version, creator, etc. for the UFS. This file is required.


The name of this file has not been determined. “documents” is too ambiguous.

The data in a UFS can represent many things and this file will define the intended purpose(s) of the UFS. The goal is to establish common purposes and data structures that can be used across tools. For example, interpolation rules have a wide variety of uses beyond simple outline generation. These rules can be useful in kerning, font mastering, hinting and proofing tools. We want to establish publicly available locations and data structures for this type of data. On the surface, this file appears similar to lib.plist. However, it’s intent is different. lib.plist originated as a way to store shallow, private data with only occasional data defined for public use. The purpose of this new file is to define public data that may potentially be deeply structured.

The concepts behind and structure of this file are still being developed. We currently have two ideas for the structure of this file.

Category : Items

The first idea is based on categorizing purposes and storing data for each item within a category.

    "com.something.category" : [
            "name" : "string",
            # category specific details

The top level key would be a category definition that defines what the value for the key represents. A category is an abstraction that would represent universal data like interpolation rules. They could also represent something privately defined. These keys would follow the reverse domain naming scheme we use for the various lib locations. We would reserve public.* keys for use by official UFS spec defined categories.

The values would be a list of items in the category. This would allow for more than one definition within a category. The items in the list would be dicts that follow a defined (either publicly or privately) specification. The only requirement would be a name key/value pair that defines a name for the item. Everything else would be defined on an as needed basis.

UFO Reference Data

The other idea is to store an ordered list of external UFO references with app-defined data defining the purpose of the reference.

    ('master_regular', {'public.source': "sources/MyFontRegular.ufo",
                       'com.something.blah': { ..anything.. }}),
    ('master_bold',    {'public.source': "sources/MyFontBold.ufo",
                       'com.something.blah': { ..anything.. }}),


Global font information data. The potential contents correspond to the UFO fontinfo.plist specification.


Global glyph group definitions. The potential contents correspond to the UFO groups.plist specification.


Global OpenType feature definitions. The potential contents correspond to the UFO features.fea specification.


Arbitrary custom data for the UFS. The potential contents correspond to the UFO libs.plist specification.


A MutatorMath .designspace file defining interpolation rules for the UFS. The .designspace format would need to be modified to allow more than one design space per file.

Global and Local Data

Global data is data defined in the UFS. Local data is data defined in a UFO. Defining different data for the same location/key/record is allowed. Data collation rules are defined below.


If a top level key is in a UFO’s fontinfo.plist, the UFO’s value for the key overrides the value defined in the UFS. The following are exceptions to this rule.

openTypeNameRecords When compiling a name table, the local entries are appended to the global entries.

guidelines Local guidelines are appended to the global guidelines. The identifiers for all guidelines must be unique in the combined list.


If a group is defined locally, it overrides a group with the same name defined globally.


The local feature text is appended to the global feature text. The .fea syntax combines, not overrides, the contents of features defined more than once.


The UFS lib.plist is specific to the UFS. There is no resolution order necessary.

Location of UFOs

UFOs references by the UFS must be stored next to the UFS. Path resolution rules will not be defined.

Other Contents

Not all portions of the UFO file structure are applicable to UFS. Obviously, there is no need for layer or glyph data. Additionally, these are the current thoughts on the other files and directories:

Single File UFS

Just like UFO, there will be “UFS Package” and “UFS ZIP” formats.

Other Names

Unified Font Set is not the only name we’ve come up with for this new structure. Here are some others:

Magazine *.magazine
Unified Font Bundle *.ufb


The following ideas are being considered for the UFO 4 specification.