Re: [ft] What do kerning values mean in RTL text?

[Nelson H. F. Beebe]

Paul Pedriana <address@hidden> asks today about right-to-left
and left-to-right kerning:

>> ...
>>    - Do kerning pairs in RTL refer to two sequential glyphs in logical
>>      order or visual order?
>> 
>>    - If there is a string with the logical order is CAT and a visual
>>      order of TAC, do kerning adjustment pairings apply for CA
>>      (logical order) or for AC (visual order)?
>> ...

The typographical origin of kerning is that when character shapes
would leave a visibly-objectionable space between them (e.g., "A V",
"A W", "A Y", "P J", ...), the (human) typesetter squeezed them
together.  This is reflected today in kerning tables in fonts,
recorded in, e.g., Adobe Font Metric (.afm) files 

        StartKernData
        StartKernPairs 113

        KPX A y -92
        KPX A w -92
        KPX A v -74
        KPX A space -55
        KPX A quoteright -111
        ...
        EndKernPairs
        EndKernData

and TeX font metric (.tfm) files

        (LIGTABLE
           ...
           (LABEL C P)
           (KRN C A R -0.083334)
           (LABEL C y)
           (KRN C o R -0.027779)
           (KRN C e R -0.027779)
           (KRN C a R -0.027779)
           (KRN O 56 R -0.083334)
           (KRN O 54 R -0.083334)
            ...)

The kern is implicitly dependent on writing direction, so an "A q"
kern in left-to-right setting is completely unrelated to that for "q
A" in right-to-left setting.  Neither TeX fonts nor Adobe Type 1 fonts
admit to the notion of a writing direction, but the Metafont book says
that kerns are applied "when a character follows another".  This
suggests that the only reasonable interpretation of kerns is that they
are unidirectional and in logical order: a Hebrew or Arabic font
intended for right-to-left typesetting could have kerns, but they
would apply to the RTL logical order.  The visual order for a reader
of such texts is also RTL, while for people accustomed to the
languages of western Europe and the Americas, it would be
left-to-right.

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