.WOFF File Format – Header

Fields

Offsets and sizes are in bytes, in decimal. The names are the ones in the W3C specification. The examples are from the header shown at the end of this page.

The table directory follows directly, at offset 44.

Flavour

A TrueType or OpenType font starts with a four-byte sfntVersion, and WOFF copies it into flavor so that a reader knows what kind of outlines the font has before it unpacks anything. The decoder writes it back as the first four bytes of the rebuilt font.

The specification does not make any other value an error, but says these are the ones widely supported. WOFF 1.0 wraps a single font and has no way to describe a font collection (ttcf); WOFF2 added one.

totalSfntSize

A decoder needs to know how much memory the unpacked font takes before it starts, and totalSfntSize tells it. The specification gives the sum:

1. 12 bytes for the font’s offset table (sfntVersion, numTables, searchRange, entrySelector, rangeShift);
2. 16 bytes for each table record;
3. each table’s origLength, rounded up to a multiple of 4.

The result is always a multiple of 4. A file whose totalSfntSize is wrong, or not a multiple of 4, is invalid. It is the size of the font as it was before it was wrapped, if that font was laid out the usual way with no gaps between tables; the specification asks for this sum, not for the original file’s size.

For the example: 12 + 14 × 16 = 236 bytes of header and records, plus the 14 table lengths rounded up (96, 196, 16, 3,596, 8, 5,392, 56, 36, 160, 84, 32, 1,152, 292 and 156, which add up to 11,272), gives 11,508.

Version

majorVersion and minorVersion are the version of the WOFF file, not of the format and not necessarily of the font inside. The specification lets the maker put any version there, which need not match the version in the font’s head or name tables, and it has no effect on how the font is loaded. A reader should not treat any value as an error.

When a header makes the file invalid

A user agent must reject a WOFF file, and not load the font, when:

• signature is not wOFF;
• reserved is not 0;
• totalSfntSize is not the sum above, or not a multiple of 4;
• length, numTables or any offset and length put a table or block past the end of the file, or make two of them overlap;
• a block that is absent does not have both its offset and its length set to 0, or one that is present has them inconsistent with the other blocks.

A metadata block that cannot be read is different: the user agent ignores it, as if it were not there, and still loads the font.

Example

The header of KaTeX’s Size2 Regular font, a TrueType font of 14 tables, as a 6,188-byte WOFF file. It has no metadata and no private data, so the last 20 bytes are zero.

00000000  77 4F 46 46 00 01 00 00  00 00 18 2C 00 0E 00 00   wOFF.......,....
00000010  00 00 2C F4 00 01 00 00  00 00 00 00 00 00 00 00   ..,.............
00000020  00 00 00 00 00 00 00 00  00 00 00 00               ............