Font File Anatomy: Inside OpenType Fonts

Table Directory Structure

OpenType Font File Structure
├── Offset Subtable (12 bytes)
│   ├── sfntVersion: 0x00010000 (TrueType) or "OTTO" (CFF)
│   ├── numTables: number of tables
│   ├── searchRange, entrySelector, rangeShift (for binary search)
│
├── Table Record Array (16 bytes × numTables)
│   ├── [0] tag: "cmap", checksum, offset, length
│   ├── [1] tag: "head", checksum, offset, length
│   ├── [2] tag: "hhea", checksum, offset, length
│   └── ... (one record per table)
│
└── Table Data (varies)
    ├── cmap table data at recorded offset
    ├── head table data at recorded offset
    └── ... (tables can be in any order)

The table directory enables random-access parsing of font tables without sequential file reading. Each table record stores a byte offset and length, so a parser looking for the GPOS table can jump directly to its position without reading through cmap, head, hhea, and other preceding tables. The specification requires table records to be sorted alphabetically by their four-character tag, which enables binary search through the directory. A font with 20 tables requires at most 5 comparisons to locate any specific table using binary search, compared to up to 20 sequential reads without sorted ordering. Tools like fontTools use this binary search optimization when programmatically accessing specific tables.

Each table record also stores a checksum of that table's data, calculated as the sum of all 32-bit words in the table (with the final word padded to a 4-byte boundary with zeros). The head table is special: its own checksum field is zeroed before calculating the global file checksum stored in checkSumAdjustment, which equals 0xB1B0AFBA minus the sum of all table checksums. Validation tools like OTS and Font Bakery verify both per-table checksums and the global head checksum. Any font modified after generation without recalculating checksums will fail validation—this is one of the most common issues introduced by tools that patch font binaries directly rather than using proper font APIs.

Required Tables

Every valid OpenType font must contain these 8 tables:

Outline Tables

Fonts must contain either TrueType or CFF outlines (not both):

Advanced Tables

The OpenType Layout tables—GSUB, GPOS, and GDEF—form an interdependent subsystem for advanced typography. GSUB handles glyph substitutions: replacing one glyph or sequence with another for ligatures, small caps, stylistic alternates, or script-specific required forms. GPOS handles glyph positioning adjustments: kerning between adjacent glyphs, mark-to-base positioning for diacritics above base characters, cursive attachment for connected scripts, and contextual adjustments. Both GSUB and GPOS reference GDEF for glyph class data. GDEF classifies each glyph as a base character, ligature component, combining mark, or multi-component piece, and GSUB and GPOS lookup rules use these classifications to target appropriate glyphs during contextual matching. A font with GSUB or GPOS but lacking GDEF may experience feature failures in text engines that rely on GDEF class information during lookup application.

Variable fonts extend the standard table structure with a second axis-variation layer. The fvar table defines named design axes—weight, width, optical size, or proprietary axes identified by four-letter tags—each with minimum, default, and maximum numeric values. The gvar table stores delta arrays for every glyph, describing how each outline coordinate shifts across the design space. HVAR, VVAR, and MVAR extend variation coverage to horizontal metrics, vertical metrics, and named table values respectively. This architecture allows a single font file to replace an entire static family while remaining more compact than the combined family, since glyph outlines are shared and only the deltas between design instances are stored. Understanding this layer is essential for debugging variable font issues, where unexpected rendering at specific axis positions often traces to glyph delta data or avar normalization.

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