One of my co-workers forwarded Markus Kuhn's note about metric font sizes.
Not being a reader of this list, I don't know what else has been covered in
this thread, but I'll presume to make a few comments in case they're useful
and not already covered.
There are three useful vertical measures for type. The DIN 16507-2 deals
with two of them: the "body size" (generally known as point size), which
corresponds to the default vertical line spacing as measured from baseline
to baseline, and the "cap height" (or sometimes ascender height, since the
two are generally not quite the same) - the average height of the taller
alphabetic glyphs. The third measurement is the x-height (height of
non-ascending lowercase glyphs), which determines the perceived size in
normal (upper- and lowercase) text settings.
Only the first of these measures has been built into most digital fonts, so
the values for cap height and x-height must be guessed (e.g. the DIN notion
that the cap height is approximately 72% of the body size) or
algorithmically measured. The first approach is error-prone, since typeface
designs vary widely in their proportions; the second requires fairly
extensive processing of the glyph information (e.g. determining the
bounding boxes for some key glyphs), which is more than most applications
are ready to take on.
We do expect to provide explicit ascender height values in the coming
OpenType fonts from Adobe. I have no illusions that these fonts will
replace all the fonts people are presently using, so this is merely a step
toward a saner future, not a real solution. Of course the values in the
font software are relative to the body size, and scaled as specified by the
type-setting application, so they can be interpreted in any measurement
system desired.
- David Lemon
Manager, Type Development
Adobe Sysems, Inc.
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