RE: position of baseline relative to em square

The otmLineGap field contains an optional default leading, which is usually
around 20% of the em.
If the text is set tight, baseline to baseline is the em. Of course you can
add any additonial amount of leading to that, with the otmLineGap being a
scaled value usually equal to around 20%. The font designer could change
that otmLineGap value if the font needs more or less space.


-----Original Message-----
From: Nick Nussbaum []
Sent: Tuesday, November 30, 1999 9:18 PM
To: Greg Hitchcock;;
Subject: Re: position of baseline relative to em square

Shouldn't it include extra leading ?
I can't check right now because I'm home offline but is the extra leading
considered part of the
the baseline to baseline? Is baseline to baseline considered an em square in
your fonts?<>

Finally  the sort of discussion I'd hoped for from this list. :-)

----- Original Message -----
From: Greg Hitchcock <>
To: 'David Lemon' <>; <>
Cc: <>
Sent: Tuesday, November 30, 1999 2:01 PM
Subject: RE: position of baseline relative to em square

> In general David is correct here. The Windows metrics otmAscent and
> otmDescent map to the sTypoAscender and sTypoDescender in the OS/2 table
> the font. Typically otmAscent maps to the top of the l/c f (or sometimes
> and the otmDescent maps to the bottom of the l/c g (or sometimes p) This
> usually very close to the em, although not quite. I believe that this
> difference is sometimes caused by the bevel on the hot-metal slug that was
> used for strengthening. One could reasonably take the difference between
> otmAscent and otmDescent and evenly divide it above and below to determine
> the baseline in the em.
> GregH
> -----Original Message-----
> From: David Lemon []
> Sent: Tuesday, November 30, 1999 8:15 AM
> To:
> Cc:
> Subject: Re: position of baseline relative to em square
> At 11:49 PM -0800 11/29/99, Erik van der Poel wrote:
> > However, if CSS wants us to center in terms of the em square, we would
> > need to know where the baseline lies in the em square.
> >
> > As far as I can tell, neither the Windows APIs nor the TrueType file
> > give us this info. Is this true?
> Although not very clearly specified, the (absolute values of the)
> WindowsTypographicAscent and WindowsTypographicDescent in the OS/2 table
> should sum to the em square height, positioning the baseline in the em
> square. I can't guarantee the general case, but can state that Adobe's
> OpenType fonts do exactly that.
> - David Lemon

Received on Wednesday, 1 December 1999 03:17:08 UTC