.OTF File Format – CFF2 table

Differences from CFF

• One font only: there is one Top DICT, stored directly, with no Top DICT INDEX.
• No Name INDEX, String INDEX, encoding or charset. Names come from the name and post tables, characters from cmap.
• No advance widths in charstrings: widths come only from hmtx and vmtx, varied by HVAR and VVAR.
• INDEX counts are 32-bit.
• Charstrings have no endchar or return, and none of the arithmetic, logic or storage operators; they gain blend and vsindex.
• The argument stack holds 513 values instead of 48.
• Paths are filled with the non-zero winding rule, so contours may overlap (needed for variable fonts, where overlaps cannot practically be removed); CFF  outlines may not overlap.
• Every Font DICT and Private DICT pair applies to a group of glyphs chosen by FontDICTSelect, as in CID-keyed CFF fonts, but any font may use it.

Layout

The first three must be at the start in this order; the rest may be anywhere, found by offsets from the start of the table (except the local subroutines, which are found from their PrivateDICT).

Header

INDEX

An empty INDEX is a 4-byte zero count and nothing else.

Numbers

DICTs and charstrings use the same number forms as CFF, with this split:

Charstring coordinates therefore cannot reach 32768 or more, or below −32768. The encodings are described on the CFF page.

TopDICT

Every other CFF Top DICT operator (version, Notice, FontBBox, charset, Encoding, Private, ROS and so on) is gone. An operator not defined for a DICT has unspecified behaviour.

Font DICT

PrivateDICT

CFF’s ForceBold, initialRandomSeed, defaultWidthX and nominalWidthX are gone. Note the codes: vsindex and blend are 22 and 23 in a PrivateDICT but 15 and 16 in charstrings.

FontDICTSelect

VariationStore

Only the regions are used from it. The VariationRegionList lists every region used anywhere in the table; each ItemVariationData selects a subset through its regionIndexes, and must have itemCount and its word-delta count 0 with no delta sets, because the deltas themselves are written inline as blend operands. Axes are numbered in their fvar order, and a font with a VariationStore must have an fvar table. The store is not shared with other tables’ item variation stores, even if identical.

CharStrings

A CFF2 charstring is a Type 2 charstring (see Type 2 charstrings) with these changes:

• No width before the first operator.
• No endchar: the glyph ends where its data ends. No return: a subroutine ends where its data ends. Subroutines may nest 10 deep and may not recurse. callsubr and callgsubr pop only the subroutine number.
• None of and, or, not, abs, add, sub, div, neg, eq, drop, put, get, ifelse, random, mul, sqrt, dup, exch, index, roll, nor the deprecated dotsection and seac form of endchar.
• Every contour is closed by a line back to its start, implied by the next moveto or the end of the charstring. Outer contours run anticlockwise (filled area on the left), the opposite of glyf.

Byte 28 and 32–255 are numbers; every other code is undefined. Blendable means the operator’s arguments may be produced by blend.

Hints work as in Type 2, with the mask bits always referring to stems in the order they were declared. In a variable font, blended stems may overlap in some instances even if they do not by default, so if any stems overlap anywhere in the design space all should be declared with hstemhm/vstemhm and selected with hintmask. The second value of a stem pair must stay zero or more after blending, and the edge-hint widths −20 and −21 must not be blended.

blend and vsindex

vsindex takes one operand, ivd: the index of the ItemVariationData whose region list applies. Its default is 0 or the PrivateDICT’s vsindex; in a charstring it may be used once, before any blend, and overrides the PrivateDICT. The number of regions in that list is k.

blend takes n + n × k + 1 operands: n default values, then k deltas for the first value, k for the second and so on, then n. It replaces them with n results: each default value plus the sum of its deltas, each multiplied by the scalar (from 0 to 1) that the current axis settings give its region. A blend need not cover all of the next operator’s arguments; several blends can each cover some of them. For example, with one region and a scalar of 0.75, 120 52 1 blend hlineto draws a line of 120 + 52 × 0.75 = 159.

Because k changes the operand count, a subroutine that uses blend or vsindex must agree with every glyph that calls it. Metrics are not varied here but in HVAR and VVAR; there is no gvar table and no inferred deltas, but moving a contour’s first rmoveto moves the whole contour.

Other tables

• maxp must be version 0.5 (6 bytes) and its numGlyphs must equal the CharStringINDEX count.
• head: indexToLocFormat and glyphDataFormat are 0; unitsPerEm must agree with FontMatrix.
• hmtx/HVAR and vmtx/VVAR give all metrics; fvar is required in a variable font; gvar is not used.

Limits

Example

The OpenType specification’s sample CFF2 table is 226 bytes: a variable font with one axis, two glyphs and one local subroutine that draws a square. Its first 16 bytes and the subroutine:

0000  02 00 05 00 07 CF 0C 24  C3 11 9B 18 00 00 00 00
...
00C8  BD BD EF 8C 10 8B 15 F8  88 27 FB 5C 8C 10 06 F8
00D8  88 07 FC 88 EF F7 5C 8C  10 06

Between them, the VariationStore (length 38) defines two regions on the one axis, from −1 to 0 peaking at −0.5, and from −1 to −0.5 peaking at −1, and one ItemVariationData with itemCount 0 that selects both. Both glyphs are just 20 0A, −107 callsubr: with one local subroutine the bias is 107, so they call subroutine 0. At the default the square is 500 units wide; towards −1 on the axis it narrows.