Reverse chaining contextual lookup

[Mansour, Kamal]

Use of Reverse Chaining

The current definition of reverse-chaining single substitution recognizes that for some situations it is best to search backwards for a pattern in a run of text. This type of lookup was introduced specifically to cope with the complexities of the Nastaliq style of Arabic script. It turns out that a full implementation of other styles of Arabic writing, including Naskh, could also benefit from such a scan of a run of text.

Upon matching a pattern, the current definition of the reverse-chaining lookup allows only a simple one-to-one substitution to take place. The following extract is from the OpenType spec:

    Reverse Chaining contextual single substitution, allows one glyph to be  substituted with another by
    chaining input glyph to a 'backtrack' and/or  'lookahead' sequence. The difference between this and
    other lookup types is  that processing of input glyph sequence goes from end to start.

In many contexts, a simple substitution may not be enough to carry out the necessary changes. Moreover, the normal (i.e., forward-scanning) chaining contextual lookup permits a broader choice of actions when a pattern is matched, including the direct invocation of another lookup.

I propose the addition of a new reverse-chaining, contextual-substitution lookup type which is identical in functionality to the ordinary contextual-substitution lookup except for directionality.

I look forward to your comments.

Kamal Mansour
Monotype Imaging
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