'vmtx' tableThe vertical metrics table (tag name: 'vmtx')
allows you to specify the vertical spacing for each glyph in a Quickdraw
GX vertical font. This table consists of either one or two arrays that contain
metric information -- the advance heights and top-side bearings -- for the
vertical layout of each of the glyphs in the font. The vertical metrics
coordinate system is shown in the following figure:
Quickdraw GX vertical fonts require both a vertical
header table (tag name: 'vhea') and a vertical metrics
table. The vertical header table contains information that is general to
the font as a whole. The vertical metrics table contains information that
pertains to specific glyphs.
The overall structure of the vertical metrics table consists of the two arrays shown in the following figure. This table does not have a header, but does require that the number of glyphs included in the two arrays equal the total number of glyphs in the font. A short version of the table may be used to accommodate fonts in which all advance heights are equal.
The vMetrics array contains two values for each entry. These
are the advance height and the top-side bearing for each glyph included
in the array.
The number of entries in the vMetrics array is determined
by the value of the numOfLongVerMetrics field of the vertical header table.
In monospaced fonts, such as Courier or kanji, all glyphs have the same advance height. If the font is monospaced, only one entry need be in the first array, but that one entry is required.
The format of an entry in the vertical metrics array is as follows:
Type |
Name |
Description |
|---|---|---|
| uint16 | advanceHeight | The advance height of the glyph. Signed integer in FUnits. |
| int16 | topSideBearing | The top side bearing of the glyph. Signed integer in FUnits |
The second array is optional and generally is used for a run of monospaced
glyphs in the font. Only one such run is allowed per font, and it must be
located at the end of the font. This array contains the top-side bearings
of glyphs not represented in the first array, and you must assume that all
the glyphs in this array have the same advance height as the last entry
in the vMetrics array. All entries in this array are therefore
monospaced.
The number of entries in this array is calculated by subtracting the
value of numOfLongVerMetrics from the number of glyphs in the
font. The sum of glyphs represented in the first array plus the glyphs represented
in the second array therefore equals the number of glyphs in the font. The
format of the top side bearing array is as follows:
Type |
Name |
Description |
|---|---|---|
| int16 | topSideBearing[] | The top side bearing of the glyph. Signed integer in FUnits |
This example font has 258 glyphs and the value of numOfLongVerMetrics
in the vertical header table is 258. The number of <advance height, top-side
bearing> pairs in the vMetrics Array is therefore 258.
The number of entries in the top-side bearing array is given by the number
of glyphs (258) minus numOfLongVerMetrics (258). For this example,
this corresponds to 258 - 258 = 0; therefore, there aren't any entries in
the second array.
Each entry in the vMetrics array is 2 bytes long. Since
there are 2 entries per glyph and 258 glyphs, the size of this vertical
metrics table is 2 * 2 * 258 = 1032 bytes.
A sample of the vertical metric table data for this font is shown in the following table.
Glyph index |
Advance height |
Top-side bearing |
|---|---|---|
| 0 | 1673 | 102 |
| 1 | 204 | 102 |
| 2 | 204 | 102 |
| ... | ... | ... |
| 257 | 1716 | 102 |
The following table shows the vertical metrics table for this example font.
Offset/ |
Value |
Name |
Comment |
|---|---|---|---|
| 0/2 | 1673 | advanceHeight[1] | The advance height of glyph index 1. |
| 2/2 | 102 | topSideBearing[1] | The top-side bearing of glyph index 1. |
| 4/2 | 204 | advanceHeight[2] | The advance height of glyph index 2. |
| 6/2 | 102 | topSideBearing[2] | The top-side bearing of glyph index 2. |
| 8/2 | 204 | advanceHeight[3] | The advance height of glyph index 3. |
| 10/2 | 102 | topSideBearing[3] | The top-side bearing of glyph index 3. |
| (The array continues for glyphs 4 to 257. This part of the array is not shown here) | |||
| 1028/2 | 1716 | advanceHeight[258] | The advance height of glyph index 258. |
| 1030/2 | 102 | topSideBearing[258] | The top-side bearing of glyph index 258. This is the last glyph in the font. |