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The suggested usage for sTypoDescender is that it be used<br />
in conjunction with unitsPerEm to compute a typographically<br />
correct default line spacing. The goal is to free applications<br />
from Macintosh or Windows-specific metrics which are<br />
constrained by backward compatability requirements. These<br />
new metrics, when combined with the character design<br />
widths, will allow applications to lay out documents in a<br />
typographically correct and portable fashion. These metrics<br />
will be exposed through Windows APIs. Macintosh applications<br />
will need to access the sfnt resource and parse it to<br />
extract this data from the “OS/2” table (unless Apple exposes<br />
the “OS/2” table through a new API).<br />
For CJK (Chinese, Japanese, and Korean) fonts that are<br />
intended to be used for vertical writing (in addition to horizontal<br />
writing), the required value for sTypoDescender is that<br />
which describes the bottom of the of the ideographic em-box.<br />
For example, if the ideographic em-box of the font extends<br />
<br />
set 120 design units below the Latin baseline), then the value<br />
of sTypoDescender must be set to -120. Failing to adhere to<br />
these requirements will result in incorrect vertical layout.’<br />
typoLineGap integer Line gap value. Corresponds to the OpenType<br />
OS/2 table sTypoLineGap field. ‘The typographic line gap for<br />
this font. Remember that this is not the same as the LineGap<br />
value in the hhea table, which Apple defines in a far dierent<br />
manner.<br />
The suggested usage for usTypoLineGap is that it be used in<br />
conjunction with unitsPerEm to compute a typographically<br />
correct default line spacing. Typical values average 7–10% of<br />
units per em. The goal is to free applications from Macintosh<br />
or Windows-specific metrics which are constrained by backward<br />
compatability requirements. These new metrics, when<br />
combined with the character design widths, will allow applications<br />
to lay out documents in a typographically correct and<br />
portable fashion.’<br />
underlinePosition integer or float Underline position value.<br />
Corresponds to the Type 1/CFF/post table UnderlinePosition<br />
field. Default is –100.<br />
underlineThickness integer or float Underline thickness value.<br />
Corresponds to the Type 1/CFF/post table UnderlineThickness<br />
field. Default is 50.<br />
<strong>Glyphs</strong> <strong>Handbook</strong>, May <strong>2013</strong> 95
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