Postscript fonts and LineGap

[Bill Troop billtroop@xxxxxxxx]

On the way towards a solution for finding a Postscript equivalent for
the TrueType LineGap setting, I have this information courtesy of
Adam Twardoch at Fontlab which I am trying to digest. No doubt it
will all make sense to Robert (my comments in brackets):
Type 1 font [i.e., PostScript] don't have this setting. I've noticed
that ATM on Windows [probably including the form of ATM built in to
XP and Vista] is automatically adding a 15% linegap (15% of the sum
of the absolute values of Ascender and Descender). In other words, to
get a Type 1 and an OpenType font to behave the same way
linespacing-wise, and the OpenType font perform the same way on all
platforms, one had to set it up so that:

1] hhea.LineGap = 0
2] Type1.Ascender + Type1.Descender = OS/2.TypoAscender +
OS/2.TypoDescender = UPM size
3] (Type1.Ascender + Type1.Descender) * 115% = OS/2.TypoAscender +
OS/2.TypoDescender + OS/2.TypoLineGap = hhea.Ascender +
hhea.Descender = OS/2.WinAscent + OpenTypeOS/2.WinDescent
At least that's what I remember finding out a long time ago. I hope
that Yuri and I can make this more clear in the next version of FontLab Studio.