Re: ascender, descender, cap-height and x-height

I wrote:
> ... I can say that's what Adobe is doing in its OpenType fonts, and
> recommends to other developers. We consider it important, because without
> this one can't know the position of the baseline within the em height. And
> in the (unusual but real) case of fonts with a non-square em, there's no
> other way to even define the em height.

and John Hudson asked:
> Since the baseline position varies within the em height depending on the
> design, how is this information useful?

There are at least two uses. At the very least, knowing the relationship
between the em height and the primary baseline offers a logical method for
positioning the baseline relative to the top of a text block. Further, as I
stated above, some fonts have non-square ems (e.g. some Japanese newspaper
text fonts are vertically compressed). It seems important to have a method
for defining their height.

> Will having (em = ascender + descender) create cliping problems in
> TrueType fonts for stacked diacritics?

Some Windows GDI-based applications make assumptions about the font
bounding box (not the em) which sometimes lead to clipping problems, but
this is simply bad app design - not a function of TrueType.

- David Lemon