Double mapping is bad

Ray Larabie
Ray Larabie Posts: 1,431
edited October 2015 in Technique and Theory
I know it's considered bad to assign two Unicode values one glyph, but I can't remember why. Someone recently suggested to me that it would be a good idea in the case of all-caps fonts, to double map the uppercase as to eliminate the superfluous lowercase set. It sounds like a disaster, but I couldn't explain specifically why it's so bad. I recall a discussion about this on another type design forum but I can't recall the details. It's bad, right?
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  • Chris Lozos
    Chris Lozos Posts: 1,458
    What about the duplicate Greek and math glyphs?
  • John Hudson
    John Hudson Posts: 3,186
    I know it's considered bad to assign two Unicode values one glyph


    It isn't. There are plenty of good candidates for double-mapped encodings. Adobe tends to avoid them because they want to have an unambiguous mapping from GID to Unicode for the sake of Acrobat, but even they accept double-encodings in some circumstances (e.g. legacy isolated presentation form mappings in Adobe Arabic).

    Now, that said, the specific case of mapping a single glyph to both upper- and lowercase characters might be more susceptible to problems than other kinds of double mapping if software is making any assumptions about case mapping based on glyph names. That shouldn't be the case, but it's remarkable how assumptions about glyph names linger.

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