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

Sorry for the late contribution; I was out sick for a while there. In case
this is still of any use, I'll chip it in now.

At 1:19 PM -0800 1/24/00, Erik van der Poel wrote:
> ... in a message previously sent to this mailing list, David Lemon
> said that a future version of the OpenType spec may specify that
> typoAscender and typoDescender must sum to the em square height:
>
>  http://lists.w3.org/Archives/Public/www-font/1999OctDec/0010.html
>
> David, does this basically mean that OpenType would mandate that the
> em square be from the top of the "nominal" ascender to the bottom of
> the nominal descender?

Since this change to the OpenType spec is not yet approved by both
development partners (Adobe and Microsoft), I can't promise that's what it
will say. 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.

> It would be nice if the next version of the OpenType spec would mandate
> that cap-height and x-height be accurate.

All values should be accurate, within the definition of the value; there's
no point in putting them into a font if they aren't. When Greg Hitchcock
wrote about unreliability, I believe he was referring to the fact that
these values are recent additions, not found in most shipping TrueType
fonts. We added them to OpenType so that it could be possible for future
users to specify type in terms of x-height or cap-height size (as was
becoming popular in Germany in the 1970s). I wouldn't be surprised if there
are other uses for this information we didn't even anticipate.

On the question of the ration to be used for size adjustment, it seems to
me that the most interesting ratio would be that of x-height to em height.
For normal upper- and lowercase text, this would most clearly describe the
apparent size, presuming that em height is a determinant in linespacing. Of
course, this "size" is vertical only; as Todd Fahrner pointed out,
horizontal characteristics are also significant. Apparent size will vary
not only with H&J behaviors, but things like the internal space in the
glyphs.

- David Lemon

Received on Sunday, 30 January 2000 23:42:25 UTC