How to Define Custom OpenType Features in FontForge

FontForge fully supports creating and applying custom OpenType feature tags, allowing font designers to create specialized layout behavior beyond the default feature lists. While FontForge gives you the interface to input arbitrary four-character tags into GSUB (substitution) and GPOS (positioning) lookup tables, the actual rendering of these custom features depends entirely on the text-shaping engine used by the host application. This article explains how to define custom tags in FontForge, rules for formatting them, and practical considerations regarding application support.

How to Create Custom Feature Tags in FontForge

Custom OpenType features are added through the Lookups section of FontForge's Font Info window.

  1. Open your font file in FontForge.
  2. Go to Element > Font Info from the main menu, or press Ctrl + Shift + F (Cmd + Shift + F on macOS).
  3. Select the Lookups tab on the left panel.
  4. Select either GSUB (for glyph substitution) or GPOS (for glyph positioning), depending on what action the feature performs.
  5. Click Add Lookup.
  6. Choose the Type of lookup (e.g., Single Substitution, Ligature Substitution, Pair Position).
  7. In the feature configuration area, double-click an empty row under the Feature list or select the dropdown. Instead of choosing a predefined tag (like liga or kern), manually type your custom four-character tag into the field.
  8. Choose your target scripts and languages (such as latn{dflt}) to define when the feature applies.
  9. Click OK, then add subtables to specify the glyph mapping rules associated with this tag.
  10. Generate the font (File > Generate Fonts) as an OpenType (CFF) or TrueType font to compile the tables.

Tag Formatting Rules

All OpenType feature tags must comply with standard OpenType specifications:

  • Length: Tags must consist of exactly four ASCII characters. If a identifier has fewer than four characters, it must be padded with trailing spaces.
  • Characters: Use printable ASCII characters within the range of 0x20 to 0x7E. Typically, lowercase alphanumeric characters are preferred for custom additions.
  • Collisions: Avoid using existing standardized tags defined by Microsoft and Adobe (such as calt, dlig, smcp) unless you intend to follow their standard layout behaviors.

Rendering Engine Support and Compatibility

Defining a custom tag inside a font does not guarantee that software will execute it. Most general-purpose software relies on text layout engines such as HarfBuzz, CoreText, or DirectWrite. These shaping engines only process standard registered OpenType features automatically.

However, custom tags can still be used in specific environments:

  • Web Browsers: Modern browsers using CSS can trigger custom tags via low-level font controls, provided the layout engine does not strip unrecognized tags:

    font-feature-settings: "cust" 1;

  • Custom Software and Game Engines: Applications using custom text rendering pipelines or low-level HarfBuzz integrations can query and activate non-standard feature tags directly.

If your font needs custom behavior that must work across standard design applications (such as Adobe InDesign, Illustrator, or Affinity Designer), do not use arbitrary custom tags. Instead, map your custom behaviors to the standardized Stylistic Sets (ss01 through ss20) or Character Variants (cv01 through cv99). These tags are widely recognized by desktop publishing software and provide native user-interface controls for end users.