Font Typesetting Function 03: “Mechanism of Typesetting with Narrowed Spacing Between Characters”

It has been previously explained that the OpenType features have GPOS (making adjustments by moving the glyph position) and GSUB (replacing the glyph). Starting here, and over the next several entries, GPOS will be explained.

In Latin fonts, GPOS is used to arrange the accent marks, etc. in the appropriate positions in addition to kerning. In Japanese fonts, GPOS is mainly used for typesetting with narrowed spacing between characters.

As the subject of typesetting with narrowed spacing between characters is a little complicated, I would like to establish the terms first. In Japanese fonts, Kanji and Kana are designed within a square frame as a rule. This frame is called the body or imaginary body.

The rectangle in which the character within the body is tightly housed is called the bounding box, and the spaces between the top, bottom, left, and right of the bounding box are called side bearings. The space on the left side is left side bearing.

There are various functions for narrowed spacing between characters, but roughly speaking, in terms of appearance apart from internal logics, all functions are used to remove side bearings. In the next entry, I will talk about the functions for narrowed spacing between characters in InDesign/Illustrator.

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