Users may apply inline kerning using the <?h> processing instruction or the fFormatting.addHorizontalKern() method. This instruction applies a horizontal shift at the point of application to move the formatting progression position by a positive or negative amount. When this processing instruction is applied within a word, as with other style properties inserted inline, Layout Developer will split the word into chunks using the instruction as a boundary between chunks and pass each chunk to the OpenType shaping code.

As an example, if the word “difficult” is being formatted with OpenType on, one would expect the ‘ffi’ ligature to be output rather than the separate characters. However, if <?h> or another style change occurs between, say, the ‘f’ and the ‘i’ character, OpenType would be given the two chunks ‘diff’ and ‘icult’ to shape, which would not, therefore, output the ‘ffi’ ligature glyph. It is impossible to apply the shift or style change half-way through a glyph.

Although Layout Developer provides instructions for applying a percentage of the font’s kerning values, these are still ignored.