Use of Postscript Private Dictionary

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AbrahamLee
AbrahamLee Posts: 263
in Font Technology
I’ve not been very successful in discovering the usefulness (much less, the meaning) of the PS Private Dictionary options. For example, the BlueValues, FamilyBlues, BlueFuzz, among others. Does anyone know, or at least know of a resource that describes,  what they mean, how they are used, how pertinent they are with today’s font technology?
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  • Thomas Phinney
    Thomas Phinney Posts: 3,013
    The PostScript Type 1 font spec has all of this, I think. There are some more recent additions in the supplement, as well.
    http://www.adobe.com/content/dam/Adobe/en/devnet/font/pdfs/T1_SPEC.pdf
    http://www.adobe.com/content/dam/Adobe/en/devnet/font/pdfs/5015.Type1_Supp.pdf
  • AbrahamLee
    AbrahamLee Posts: 263
    Thanks, @Thomas Phinney! That's exactly what I was looking for. Much appreciated.

    So, does everyone here provide numbers to these? Or are there other properties where a similar influence is taken into account?
  • Thomas Phinney
    Thomas Phinney Posts: 3,013
    I have always provided numbers to these in fonts based on PostScript outlines (Type 1 and OpenType CFF a.k.a. OTF).

    Blue zones are what is more generically referred to as overshoot zones. FamilyBlues are blue zones held in common across members of a family. For example, perhaps your x-height varies but your cap height is constant for all family members. Virtually all alphabetic fonts need these.

    BlueFuzz is a kind of goofy thing, a margin of error to catch points that are, say, just n units outside the blue zone. It is my understanding that in a really carefully produced and well-crafted font, not made from interpolating between masters, you ought to be able to set it to zero.

    BlueScale tells the rasterizer at what size to turn off overshoot suppression, expressed in some funky decimal units.

    FontLab VI still supports all these as well as BlueShift... and I don't even remember exactly what BlueShift is right off, except that it is also involved in overshoot suppression—check the spec! I expect most serious font editors allow you to set any of these manually that you wish.
  • AbrahamLee
    AbrahamLee Posts: 263
    Indeed they do. Hence, my questions. Thanks for the explanation!

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