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Vertical Substitution Segment
The Vertical Substitution Segment contains pairs of glyph
IDs. Each pair specifies the horizontal and vertical glyph ID
for a character. The segment can be built directly from a
TrueType mort table which contains a vertical substitution
array. The segment definition is shown in the table below.
Byte 15 (MSB) 8 7 (LSB) 0 Byte
0 VT (22100) 1
2
4
Data Segment Size (4*n+4) * 3
5
6 Horizontal Glyph ID #1 7
8 Vertical Glyph ID #1 9
. . . . . .
4*n+2 Horizontal Glyph ID #n 4*n+3
4*n+4 Vertical Glyph ID #n 4*n+5
4*n+6 End of table mark #1 = 0xFFFF 4*n+7
4*n+8 End of table mark #2 = 0xFFFF 4*n+9
The Horizontal Glyph ID field is used by TrueType as an ID
number for the horizontal glyph data associated with a
given character. The Vertical Glyph ID field contains the ID
number for the vertical glyph data associated with the
same character.
The vertical glyphs can be downloaded using the PCL
Character Definition Command using a character code =
0xFFFF.
A TrueType mort table typically contains a header of 76
bytes, followed by the vertical substitution array which
follows the segment format described here. However, the
mort table header is designed to be variable-length, and the
location of the vertical substitution data may be located
elsewhere in mort tables in future fonts.
If the Font Type is not Type 3 (16-bit fonts), this data
segment is ignored.
Printer-Specific Differences 2-41