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Font Descriptor Registry
A font descriptor is a 4-byte identifier and a 4-byte value. Descriptors
are stored in the fdsc table in the sfnt resource in a QuickDraw GX font. A font may have any number of
descriptors.
GX libraries provide a standard way to search for and iterate
through font descriptors. Future extensions to GX may use the
descriptors for services such as cross-font style matching.
Font developers should assign values to fdsc tags with user functionality in mind.
The defined tags
So far, the following five tags have been defined:
nalf Non-Alphabetic Forms
For non-alphabetic fonts, the nalf descriptor indicates what class of non-alphabetic forms is provided
by a font. The defined values are simply those for the "Ornament
Sets" feature type:
- None
- Dingbats
- Pi Characters
- Fleurons
- Decorative Borders
- International Symbols
- Math Symbols
Any additions or changes to that list should be reflected here.
A nalf which is zero or undefined means the font is alphabetic. A style-matching
scheme might look first for a matching nalf font type, and only then attempt to match the other font descriptors.
opsz Optical size
Optical size is the output size which is best for the design of
the font. For a family with multiple design sizes, the opsz descriptor could be used for style matching. In the absence of
other information, the recommended optical sizes for Roman fonts
are 12 point for text and 24 point for display.
slnt Slant
Slant is the font's angle right of vertical, expressed in degrees.
For upright Roman fonts, slnt should be 0.
wdth Width
Values for width range from 0.0 to 32767.0, centered on 1.0.
Width is a ratio relative to the center of the family. The regular
or default face has, by definition, a width of 1.0. Typical "condensed"
values are 0.7-0.85, and typical "extended" values are 1.2-1.3.
The recommended measurement method uses the total length of the
Shannon Text in mixed-case setting. That text is:
The Head And In Frontal Attack On An English Writer That The Character
Of This Point Is Therefore Another Method For The Letters In A
Time When Whom Ever Told The Problem To An Unexpected.
(All characters, including the period and space characters, are
included in the measurement.) These words represent the frequency
of letters in typical English.
wght Weight
Values for weight range from 0.0 to 32767.0, centered on 1.0.
Weight is a ratio relative to the center of the family. For Roman
faces, weight is typically measured by the thickness of vertical
strokes. The regular or default face has, by definition, a weight
of 1.0. Typical "bold" values are 1.5-2.0.
Apple does not recommend any particular method of measuring weight.
Registering your descriptor
A manufacturer may want to include other pieces of metric information
with its fonts. To accommodate these, register additional descriptor
tags with Apple via the address
fontregistry@apple.com
For instance, Adobe, ElseWare, and others use "color" metrics,
which assign an absolute "color" value to a typeface, independent
of where it lies relative to other members of its family. "Color"
in this sense combines elements of weight and width. Apple can
accommodate these "color" metrics by defining a specific descriptor
tag for each scheme, allowing the font manufacturer to include
whatever additional metric information they wish.
The Apple Fonts Group (applefonts.apple.com)
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