fontsubsetting

Font subsetting is the process of creating a smaller version of a font file that contains only the characters needed for a particular document or application. The technique removes unused glyphs, kerning pairs, and other metadata from a full font, reducing file size and improving loading times. It is commonly used in web design, where large TrueType or OpenType fonts can increase page weight and delay rendering; by sending only the glyphs required for the visible text, browsers can display pages more quickly and efficiently.

The subsetting operation involves parsing the original font’s tables—such as cmap, kern, and glyph outlines—to identify which glyphs are referenced in the source material. Tools then generate a new font file that includes only those glyphs and the necessary supporting tables. Most modern font editors and command‑line utilities support subsetting; popular options include FontTools (with the pyftsubset script), Glyphs App, and commercial software like FontLab.

Benefits of font subsetting include reduced bandwidth consumption, faster font download and parsing, and lower memory footprint on client devices. In addition, subsetting can help with licensing, as it may be permissible to distribute only a subset of a font in certain contexts. However, it also introduces challenges: images or text that use characters not included in the subset will fallback to system fonts, potentially causing visual inconsistency. Maintaining proper Unicode mapping and hinting information is also crucial to preserve typographic quality.

Overall, font subsetting is a practical optimisation technique for digital publishing, enabling efficient use of font resources while ensuring consistent appearance across platforms.