The OpenType Font File

An OpenType font file contains data, in table format, that comprises either a TrueType or a PostScript outline font. The rasterizer uses combinations of data from different tables to render the glyph data in the font.

Filenames

OpenType fonts may have the extension .OTF or .TTF, depending on the kind of outlines in the font and the creator's desire for downlevel compatibility.

• In all cases, fonts with only CFF data (no TrueType outlines) always have an .OTF extension.
• Fonts containing TrueType outlines should use the .TTF extension. This enables the fonts to be used under older versions of Windows in which a CFF rasterizer may not be installed. In such cases, the fonts will be identified in the shell with a TrueType icon.

Data Types

The following data types are used in the OpenType font file. All OpenType fonts use Motorola-style byte ordering (Big Endian):

Data Type	Description
BYTE	8-bit unsigned integer.
CHAR	8-bit signed integer.
USHORT	16-bit unsigned integer.
SHORT	16-bit signed integer.
ULONG	32-bit unsigned integer.
LONG	32-bit signed integer.
FIXED	32-bit signed fixed-point number (16.16)
FUNIT	Smallest measurable distance in the em space.
FWORD	16-bit signed integer (SHORT) that describes a quantity in FUnits.
UFWORD	Unsigned 16-bit integer (USHORT) that describes a quantity in FUnits.
F2DOT14	16-bit signed fixed number with the low 14 bits of fraction (2.14).

  

Most tables have version numbers, and the version number for the entire font is contained in the Table Directory. Note that there are two different version number types, each with its own numbering scheme. USHORT version numbers always start at zero (0). Fixed version numbers always start at one (1.0 or 0x00010000).

The Fixed point format consists of a signed, 2's complement mantissa and an unsigned fraction. To compute the actual value, take the mantissa and add the fraction. Examples of 2.14 values are:

Decimal Value	Hex Value	Mantissa	Fraction
1.999939	0x7fff	1	16383/16384
1.75	0x7000	1	0/16384
0.000061	0x0001	0	1/16384
0.0	0x0000	0	0/16384
-0.000061	0xffff	-1	16383/16384
-2.0	0x8000	-2	0/16384

Organization of an OpenType Font

A key characteristic of the OpenType format is the TrueType sfnt "wrapper", which provides organization for a collection of tables in a general and extensible manner.

The OpenType font file begins at byte 0 with the Offset Table.

Type	Name	Description
Fixed	sfnt version	0x00010000 for version 1.0.
USHORT	numTables	Number of tables.
USHORT	searchRange	(Maximum power of 2 <= numTables) x 16.
USHORT	entrySelector	Log2(maximum power of 2 <= numTables).
USHORT	rangeShift	NumTables x 16-searchRange.

  

OpenType fonts that contain TrueType outlines should use the value of 1.0 for the sfnt version. OpenType fonts containing CFF data should use the tag 'OTTO' as the sfnt version number.

The Offset Table is followed at byte 12 by the Table Directory entries. Entries in the Table Directory must be sorted in ascending order by tag. Offset values in the Table Directory are measured from the start of the font file.

Type	Name	Description
ULONG	tag	4 -byte identifier.
ULONG	checkSum	CheckSum for this table.
ULONG	offset	Offset from beginning of TrueType font file.
ULONG	length	Length of this table.

  

The Table Directory makes it possible for a given font to contain only those tables it actually needs. As a result there is no standard value for numTables.

Tags are the names given to tables in the OpenType font file. All tag names consist of four characters. Names with less than four letters are allowed if followed by the necessary trailing spaces. Table checksums are the unsigned sum of the longs of a given table. In C, the following function can be used to determine a checksum:

ULONG
CalcTableChecksum(ULONG *Table, ULONG Length)
{
ULONG Sum = 0L;
ULONG *Endptr = Table+((Length+3) & ~3) / sizeof(ULONG);

while (Table < EndPtr)
	Sum += *Table++;
return Sum;
}

Note: This function implies that the length of a table must be a multiple of four bytes. While this is not a requirement for the TrueType scaler itself, it is suggested that all tables begin on four byte boundries, and pad any remaining space between tables with zeros. The length of all tables should be recorded in the table directory with their actual length.

TrueType Collections

A TrueType Collection (TTC) is a means of delivering multiple OpenType fonts in a single file structure. TrueType Collections are most useful when the fonts to be delivered together share many glyphs in common. By allowing multiple fonts to share glyph sets, TTCs can result in a significant saving of file space.

For example, a group of Japanese fonts may each have their own designs for the kana glyphs, but share identical designs for the kanji. With ordinary OpenType font files, the only way to include the common kanji glyphs is to copy their glyph data into each font. Since the kanji represent much more data than the kana, this results in a great deal of wasteful duplication of glyph data. TTCs were defined to solve this problem.

The CFF rasterizer does not currently support TTC files.

The TTC File Structure

The TTC file must contain a complete Table Directory for each different font design. A TTC file Table Directory has exactly the same format as a TTF file Table Directory. The table offsets in all Table Directories within a TTC file are measured from the beginning of the TTC file.

Each OpenType table in a TTC file is referenced through the Table Directories of all fonts which use that table. Some of the OpenType tables must appear multiple times, once for each font included in the TTC; while other tables should be shared by all fonts in the TTC.

As an example, consider a TTC file which combines two Japanese fonts (Font1 and Font2). The fonts have different kana designs (Kana1 and Kana2) but use the same design for kanji. The TTC file contains a single 'glyf' table which includes both designs of kana together with the kanji; both fonts' Table Directories point to this 'glyf' table. But each font's Table Directory points to a different 'cmap' table, which identifies the glyph set to use. Font1's 'cmap' table points to the Kana1 region of the 'loca' and 'glyf' tables for kana glyphs, and to the kanji region for the kanji. Font2's 'cmap' table points to the Kana2 region of the 'loca' and 'glyf' tables for kana glyphs, and to the same kanji region for the kanji.

The tables that should have a unique copy per font are those that are used by the system in identifying the font and its character mapping, including 'cmap', 'name', and 'OS/2'. The tables that should be shared by all fonts in the TTC are those that define glyph and instruction data or use glyph indices to access data: 'glyf', 'loca', 'hmtx', 'hdmx', 'LTSH', 'cvt ', 'fpgm', 'prep', 'EBLC', 'EBDT', 'EBSC', 'maxp', and so on. In practice, any tables which have identical data for two or more fonts may be shared.

Creating a TrueType Collection by combining existing OpenType font files is a non-trivial process, but a tool is available to help. The process involves paying close attention the issue of glyph renumbering in a font and the side effects that can result, in the 'cmap' table and elsewhere. The fonts to be merged must also have compatible TrueType instructions—that is, their preprograms, function definitions, and control values must not conflict.

TrueType Collection files use the filename suffix .TTC.

TTC Header

Font Tables

The rasterizer has a much easier time traversing tables if they are padded so that each table begins on a 4-byte boundary. It is highly recommended that all tables be long aligned and padded with zeroes.

For OpenType fonts based on TrueType outlines, the following tables are used:
 
Tables Related to TrueType Outlines

Tag	Name
cvt	Control Value Table
fpgm	Font program
glyf	Glyph data
loca	Index to location
maxp	Maximum profile
prep	CVT Program

  

The PostScript font extensions define a new set of tables containing data specific to PostScript fonts that are used instead of the tables listed above.

 
Tables Related to PostScript Outlines

Tag	Name
CFF	PostScript font program (compact font format)
fvar	Apple's font variations table
MMSD	Multiple master supplementary data
MMFX	Multiple master font metrics

  

Whether TrueType or PostScript outlines are used in an OpenType font, the following tables are required for the font to function correctly:
 
Required Tables

Tag	Name
cmap	Character to glyph mapping
head	Font header
hhea	Horizontal header
hmtx	Horizontal metrics
name	Naming table
OS/2	OS/2 and Windows specific metrics
post	PostScript information

  

There are also several optional tables that support vertical layout as well as other advanced typographic functions:

 
Advanced Typographic Tables

Tag	Name
BASE	Baseline data
GDEF	Glyph definition data
GPOS	Glyph positioning data
GSUB	Glyph substitution data
JSTF	Justification data
vhea	Vertical Metrics header
vmtx	Vertical Metrics

  

OpenType fonts may also contain bitmaps of glyphs, in addition to outlines. Hand-tuned bitmaps are especially useful in OpenType fonts for representing complex glyphs at very small sizes. If a bitmap for a particular size is provided in a font, it will be used by the system instead of the outline when rendering the glyph. ATM will support hand-tuned bitmaps in OpenType fonts.
 
Tables Related to Bitmap Glyphs

Tag	Name
EBDT	Embedded bitmap data
EBLC	Embedded bitmap location data
EBSC	Embedded bitmap scaling data

  

 
Other OpenType Tables

Tag	Name
DSIG	Digital signature
hdmx	Linear threshhold data
PCLT	PCL 5 data
VDMX	Vertical device metrics