The Table Directory

The TrueType 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.

This is followed at byte 12 by the Table Directory entries. Entries in the Table Directory must be sorted in ascending order by tag.

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 TrueType font file. At present, all tag names consist of four characters, though this need not be the case. Names with less than four letters are allowed if followed by the necessary trailing spaces. A list of the currently defined tags follows.

Required Tables

Tag	Name
cmap	character to glyph mapping
glyf	glyph data
head	font header
hhea	horizontal header
hmtx	horizontal metrics
loca	index to location
maxp	maximum profile
name	naming table
post	PostScript information
OS/2	OS/2 and Windows specific metrics

Optional Tables

Tag	Name
cvt	Control Value Table
EBDT	Embedded bitmap data
EBLC	Embedded bitmap location data
EBSC	Embedded bitmap scaling data
fpgm	font program
gasp	grid-fitting and scan conversion procedure (grayscale)
hdmx	horizontal device metrics
kern	kerning
LTSH	Linear threshold table
prep	CVT Program
PCLT	PCL5
VDMX	Vertical Device Metrics table
vhea	Vertical Metrics header
vmtx	Vertical Metrics

Other tables may be defined for other platforms and for future expansion. Note that these tables will not have any effect on the scan converter. Tags for these tables must be registered with Apple Developer Technical Support. Tag names consisting of all lower case letters are reserved for Apple's use. The number 0 is never a valid tag name.

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.

Note that the offset in the Table Directory is measured from the start of the TrueType font file.