Why are GSUB type 5 format 1 and type 6 format 1 lookups so rare?

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Peter Constable
Peter Constable Posts: 256
in Font Technology
I've looked at over 1000 font files and haven't found any instances of GSUB type 5 format 1 and type 6 format 1 lookup subtables. Any idea why these are scarce?
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  • Grzegorz Luk (gluk)
    Grzegorz Luk (gluk) Posts: 161
    edited April 2020
    from my point of view:
    GSUB types 5.1 and 6.1 (Simple Glyph Contexts) are too simple :smile: in most cases, when I used contextual alternates, I needed classes - 5.2 and 6.2 types (Class-based Glyph Contexts) even for diacritic glyphs. In short:
    not
    AB'C
    but
    [A Aacute Aogonek][B]'[C Cacute]
  • Simon Cozens
    Simon Cozens Posts: 793
    Most people who aren’t VOLT, FontDame or HighLogic users go through the AFDKO font syntax and tools to get layout information into their fonts. This means that they’re at the mercy of what AFDKO syntax supports, and what the tools generate - neither of which is coterminous with what the spec supports. I’ve heard that AFDKO was never intended to be a general-purpose OT Layout tool, but contained just enough functionality to convert Adobe’s font library to OpenType. Those lookup types aren’t there because Adobe never needed them.
  • John Hudson
    John Hudson Posts: 3,434
    I don't know about FontDame or HighLogic, but VOLT also compiles lookups to whichever type it thinks is appropriate, without giving the user explicit control over the type used. I suspect this also contributes to some of the lookups being less common than others.

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