.OTF File Format – fvar, avar and STAT

fvar

The axis records follow at axesArrayOffset, then the instance records straight after them. Readers must step through both with the sizes given, since later minor versions may make records longer.

Axis tags

Axis tags start with a letter and contain only letters and digits, padded with trailing spaces. Tags the font maker invents must start with an upper-case letter and use only upper-case letters and digits; registered tags never fit that pattern, so the two can never clash.

These axes take over from fixed fields: usWeightClass, usWidthClass and post.italicAngle are not varied by MVAR but follow wght, wdth and slnt; in a font with a slnt axis, post.italicAngle must equal its default.

Normalisation and regions

All variation data works on normalised coordinates, −1 to +1, not the user values in fvar. For each axis the default maps to 0, the minimum to −1 and the maximum to +1, linearly in between:

if (v < default)      n = −(default − v) / (default − min)
else if (v > default) n = (v − default) / (max − default)
else                  n = 0

Then, if there is an avar table, its segment map for the axis bends that line (below). Normalised values are stored as F2DOT14.

Each set of deltas applies to a region: per axis a start, peak and end coordinate. For an instance at normalised position n the per-axis factor is: 1 if the region’s peak on that axis is 0 (the axis does not matter); 0 if n is outside start to end; 1 at the peak; otherwise a straight-line ramp from 0 at start (or end) up to 1 at the peak. The region’s scalar is the product of its axes’ factors, and every delta is multiplied by its region’s scalar and added to the default value. Regions in the item variation store must have start ≤ peak ≤ end, all on one side of 0 unless the peak is 0.

avar

• A map that is used must include −1→−1, 0→0 and 1→1; if any is missing the axis is not changed at all.
• fromCoordinate values must strictly increase and toCoordinate values must not decrease; records breaking this may be ignored.
• Between neighbouring pairs the value is interpolated linearly, so the map is a piecewise-linear curve from −1 to 1.

OpenType 1.9.1 defines only version 1.0. A version 2.0, which adds an item variation store so that axes can affect one another, has been proposed and is supported by some software, but is not part of this specification and is not documented here.

STAT

The style attributes table describes each font’s place in its family: the axes the family varies along (variable or not), and labels such as “Bold” or “Condensed” for values on them. Software uses it to build style names and menus, and to fit large families into older models (regular/bold/italic, or weight-width-slope).

Version 1.2 adds axis value format 4; otherwise 1.1 and 1.2 are the same.

Example

SF Mono (SFNSMono.ttf, macOS) is a variable TrueType font. Its 140-byte fvar starts:

0001D0E0  00 01 00 00 00 10 00 02  00 02 00 14 00 06 00 0E
0001D0F0  59 41 58 53 01 26 AC 50  01 26 AC 50 03 84 00 00
0001D100  00 01 01 02 77 67 68 74  01 26 AC 50 01 26 AC 50
0001D110  03 84 00 00 00 00 01 03  00 02 00 00 01 26 AC 50
0001D120  01 26 AC 50 01 04 01 05  00 00 01 44 55 88 01 90
0001D130  00 00 01 06 …

The default is the lightest weight, so every wght region is on the positive side. Its STAT (version 1.2) has two axis records, YAXS (ordering 0) and wght (ordering 1), six axis value tables and elidedFallbackNameID 2; the first table is format 4, labelling the combination of both axes’ values with name ID 273.