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Subject: What does a .afm file do, anyway?
From: James Souttar <[log in to unmask]>
Reply-To:TYPO-L Discussion of Type and Typographic Design <[log in to unmask]>
Date:Tue, 13 Aug 1996 18:41:09 +0100

text/plain (43 lines)


>This may be an insanely obvious question, but what function do .afm
>files perform?  I have loaded many Adobe fonts without doing
>anything with these files and haven't noticed any problems...  Can
>anyone explain for me?

Adobe Font Metrics files define a font's character metrics, composite
characters, kerning pairs and ligatures in human readable, 7-bit
ASCII form.

On a Macintosh they are all but totally redundant (these functions
are all carried out by the screen fonts). I believe there are/were
some applications that used them, but I've never been able to verify

On a PostScript friendly UNIX system (Solaris 2+ running Open Windows
or NeXTSTEP) they are used to provide generic font data as well as
font metrics information to the system - rather like screen fonts on
a Macintosh. They are paired with .pfa files - basically identical to
a Macintosh printer font but in an ASCII (data) file. For those of us
who use these things, it is an absolute godsend that .afm files are
bundled with Macintosh fonts.

The idea behind having fonts defined as a pair of plain text files
was a really smart one. They could be hand edited, easily used as
input/output, sent by email etc. Even now, type designers can import
.afm files into a spreadsheet and manipulate the kerning pairs
(fantastic for interpolating, BTW) - before importing them back into
Fontographer (or whatever). Furthermore, .afm files defined character
metrics much more precisely than Macintosh screen fonts - solving the
problem of WYS not being WYG (but only on a Display PostScript
system...). Finally, they had built in support for glyph substitution
(but there's only one program I've ever seen use this - Glenn Reid's
Adobe TouchType for NeXTSTEP).

Adobe also applied this idea to bitmaps - developing an ASCII file
format variously called Bitmap Distribution Format (BDF) or Adobe
Bitmap Format (ABF). Some UNIX systems use a compiled (binary) form
of this.


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