If the font file contains only one font, the Offset Table will begin at byte 0 of the file. If the font file is a TrueType collection, the beginning point of the Offset Table for each font is indicated in the TTCHeader. This should only happen when the file format has the extension .ttc however. We will not recognize ttc file formats.
Type Name Description
------ ---------- ----------------------
ULONG sfntVersion 0x00010000 or 'OTTO' — see below.
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.
Total size: 164 + 32 = 166.
The Offset Table is followed immediately by the Table Record entries. Entries in the Table Record must be sorted in ascending order by tag. Offset values in the Table Record 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.
Checksums are calculated using the following C code:
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. In fact, a font is not considered structurally proper without the correct padding. All tables must begin on four byte boundries, and any remaining space between tables is padded with zeros. The length of all tables should be recorded in the table record with their actual length (not their padded length).
Determine which are needed
Type Name Description
------ -------------- -----------------------------
Fixed Version Version of the MATH table, currently set to 0x00010000
Offset MathConstants Offset to MathConstants table - from the beginning of MATH table.
Offset MathGlyphInfo Offset to MathGlyphInfo table - from the beginning of MATH table.
Offset MathVariants Offset to MathVariants table - from the beginning of MATH table.
This table will contain many things called a Math Value Record. This is defined by:
Type Name Description
------ ------------ ---------------------
SHORT Value The X or Y value in design units
Offset DeviceTable Offset to the device table – from the beginning of parent table. May be NULL. Suggested format for device table is 1.
The math constants table is kind of huge... Will need to document this.
TODO: How do you lookup a glyph from this?
This table contains position information defined on a per-glyph basis.
Type Name Description
------ --------------------------- -------------------------------------
Offset MathItalicsCorrectionInfo Offset to MathItalicsCorrectionInfo table - from the beginning of MathGlyphInfo table.
Offset MathTopAccentAttachment Offset to MathTopAccentAttachment table - from the beginning of MathGlyphInfo table.
Offset ExtendedShapeCoverage Offset to coverage table for Extended Shape glyphs - from the beginning of MathGlyphInfo table. When the left or right glyph of a box is an extended shape variant, the (ink) box (and not the default position defined by values in MathConstants table) should be used for vertical positioning purposes. May be NULL..
Offset MathKernInfo Offset to MathKernInfo table - from the beginning of MathGlyphInfo table.
To implement features, a client applies the lookups in the order the lookup definitions occur in the LookupList. As a result, within the GSUB or GPOS table, lookups from several different features may be interleaved during text processing. A lookup is finished when the client locates a target glyph or glyph context and performs a substitution (if specified) or a positioning (if specified).
Note: The substitution (GSUB) lookups always occur before the positioning (GPOS) lookups. The lookup sequencing mechanism in TrueType relies on the font to determine the proper order of text-processing operations.
The headers of the GSUB and GPOS tables contain offsets to Feature List tables (FeatureList) that enumerate all the features in a font.
The Feature List Table
Type Name Description
------ ----------------------- --------------------------------
USHORT FeatureCount Number of FeatureRecords in this table
struct FeatureRecord[FeatureCount] Array of FeatureRecords-zero-based (first feature has FeatureIndex = 0)-listed alphabetically by FeatureTag
The Feature Record
Type Name Description
------ ------------- ----------------------------
Tag FeatureTag 4-byte feature identification tag
Offset Feature Offset to Feature table-from beginning of FeatureList
https://www.microsoft.com/typography/otspec/chapter2.htm
TODO: [ ] Gracefully fail reading version numbers > 1.8. [ ]
- The TTC header yields the number of tables.
- The following, the Offset Table -- beginning of TTF