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The vertical metrics table allows you to specify the vertical spacing for each glyph in a vertical font. This table consists of either one or two arrays that contain metric information (the advance heights and top sidebearings) for the vertical layout of each of the glyphs in the font. The
vertical metrics coordinate system is shown below.
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OpenType vertical fonts require both a vertical header table ('vhea') and the vertical metrics table discussed below. 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 formats of these tables are similar to those for horizontal metrics (hhea and hmtx).
The overall structure of the vertical metrics table consists of two arrays shown below: the vMetrics array followed by an array of top side bearings.
This table does not have a header, but does require that the number of glyphs included in the two arrays equals the total number of glyphs in the font.
The number of entries in the vMetrics array is determined by the value of the numOfLongVerMetrics field of the vertical header table.
The vMetrics array contains two values for each entry. These are the advance height and the top sidebearing for each glyph included in the array.
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 given below.
| Type | Name | Description |
|---|---|---|
| USHORT | advanceHeight | The advance height of the glyph. Unsigned integer in FUnits |
| SHORT | topSideBearing | The top sidebearing 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 sidebearings of glyphs not represented in the first array, and all the glyphs in this array must 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 sidebearing array is given below.
| Type | Name | Description |
|---|---|---|
| SHORT | topSideBearing[] | The top sidebearing of the glyph. Signed integer in FUnits. |