OpenType Feature File Specification Additions for Variable Values: Public Review

The Adobe Type Team has been working to extend the OpenType Feature File Specification to add direct support for variable fonts. These additions will provide a new workflow in which a single feature file can be applied to an already-variable "base" font, producing a full OpenType font with layout features (and other additional tables). Compared with the currently predominant "merging" workflow, this new syntax makes it easier to reference designspace locations in kerning or other values that are not associated with a given master, eliminates the ambiguity of 0 values in a master font, and provides other advantages.

We are introducing these specification changes in two stages:

• Stage 1, which is now ready for public review, adds support for variable values.
• Stage 2, which should be ready in the first half of 2025, will add support for feature variations (such as swapping one glyph for another based on axis range conditions).

Supporting these changes in our AFDKO toolset has required an extensive modernization of much of its lower-level implementation, which is one reason this project has taken longer than expected. However, the feature file syntax is also used in many tools not built or provided by Adobe. We have no control over whether our proposals are adopted by other toolsets, of course, but we have tried to take the wider needs of the font development community into account in developing these extensions. It is our hope that feature files will continue to be more or less portable between different font development toolchains.

This specification review provides an opportunity for the Adobe type team to answer questions about our choices, and if necessary to make adjustments to the specification so that it is adopted as widely as possible.

Adobe is now accepting comments on the stage 1 work via this GitHub repository. Start a Discussion about whatever you would like to clarify, or add an Issue to raise the possibility of adding to or changing the specification. (As always, it is good practice to look through the lists of existing discussions and issues to avoid duplication.)

The review period for stage 1 ends on January 15th 2025.

The documentation provided includes:

• A new guide to variable features. (This is probably the best place to start.)
• A revised specification marked with the differences from the previous version.
• An annotated ANTLR 4 format grammar.
• A short list of ideas that were rejected.

In addition to this documentation we are providing an example repository with the sources for the Source Serif 4 Variable roman font reorganized to use the new workflow. It explains how to install a beta version of AFDKO that builds these sources, and how the feature files can be modified to experiment with additional aspects of the grammar. (Note that this beta version of our toolset is only partially tested and should not be used to attempt to build production-quality fonts.)

Who should review?

Anyone is free to comment on and ask questions about the changes to the specification, but here is some guidance on whether and how you might want to participate:

• If you are only vaguely familiar with feature files, perhaps because you use a font editor that produces and compiles them for you "under the hood", you can probably trust that the developer of your editor will be reviewing these changes on your behalf.
• If you author your own feature files but your use of them is fairly routine, you might read through the style guide to understand the new functionality, and reference parts of the revised specification if you have further questions.
• If you make or have made extensive use of feature files to create fonts with unusual layout features or other characteristics, we recommend reviewing the style guide and specification to judge if you will be able to do what you need to and if there are aspects of the process that might be improved.
• If you develop a tool that creates, processes, or consumes feature files, we recommend reviewing the proposed changes—including the ANTLR grammar—with an eye toward adopting the extended grammar in the future, and perhaps to engage with your community of users about it. If there are obstacles to that adoption, please raise them in issues or discussions in the review repository and we will try to work with you.

A note about the scope and role of feature files in font development

In reading the proposals you will notice that there is more than one way of specifying a variable metric or anchor, and several optional features such as named designspace locations. This flexibility is intentional, and reflects the role of feature files as a "fallback" mechanism for less common layout needs that may not be supported by an interactive font development system. The goal of the specification has been, and continues to be, to provide almost all of the functionality of the OpenType shaping tables in a form that can be authored and reviewed by font designers and engineers.