Right-to-Left Kerning
RTL kerning can be edited visually rather than treated as a separate code-only task. FontLab's model uses positions on the screen; recall visual order versus reading order in Using Recipes for Composite Building. A left class describes the glyph on the left of the displayed pair even when that glyph comes later in the logical text sequence.
Kerning in Visual Order
Display the intended right-to-left text with the appropriate text direction and script processing. Text > Right to Left controls the display direction; OpenType and Unicode script processing supplies the shaping context needed for scripts whose forms depend on their neighbors.
Reversing a string visually is not a substitute for correct shaping. Before kerning Arabic, for example, confirm that the displayed forms and joining behavior are the intended ones. A value adjusted between the wrong forms will not become correct merely because it is stored as an RTL pair.
Use Kerning mode and the same visual pair controls as for left-to-right work. Select the pair by its displayed glyphs, inspect the left/right class or exception scope, and adjust the value. The left-side and right-side exception modifiers continue to refer to those visual sides.
Work with words and punctuation, not only two isolated glyphs. Digits, parentheses, spaces, and mixed-direction text can produce contexts that are not obvious from a list of strong RTL letters. Test the text in the direction in which it will be used.
The kerning class cloud and Show All Combinations remain useful for checking class members. A single representative joining form is not sufficient evidence for all the forms in a class.
Direction Flags and Storage
Editable visual order and stored layout order are different concerns. FontLab converts its visual kerning into the glyph ordering and positioning records needed by the destination format and script processing.
In OpenType output, FontLab can generate separate LTR and RTL kerning lookups. Its generated source may include a RightToLeft lookup flag. That flag is not a general instruction making pair positioning run right-to-left: OpenType defines its operative effect for cursive attachment, not ordinary pair kerning. Correct RTL kerning also depends on the engine's direction, the glyph order, the applicable script lookups, and the value records.
Keep these storage conventions in mind when exchanging files:
| Source or format | Kerning-order consideration |
|---|---|
| Native VFC/VFJ | FontLab maintains its visual pair model. |
| OpenType | Compiled lookups and script/direction-sensitive positioning replace the editable pair model. |
| UFO 3 | Logical-order conventions require conversion for RTL pairs. |
| Older UFO workflows | Earlier visual-order conventions differ from UFO 3; inspect the format version. |
.glyphs 3 |
Direction-specific kerning stores require import/export conversion. |
.glyphs 2 and VFB |
Logical-order data can require direction inference during conversion. |
Neutral characters make conversion especially important. A punctuation glyph's role cannot always be inferred as simply as the direction of a strongly RTL letter. Do not manually reverse every imported pair as a blanket repair; that can reverse data already converted by FontLab.
Keep the original file and compare a small set of RTL words, punctuation pairs, and mixed-direction cases after import and after export. Inspect effective values in text as well as pair names. A pair-list row that appears in a different order is not, by itself, proof that the resulting typography is wrong.