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[This file is ./fontutil/README.virtual_fonts.    
It consists of excerpts from the electronic digest TeXhax (1990), 
issues 11 and 12.]

Excerpted from:
     TeXhax Digest    Sunday,  January 14, 1990  Volume 90 : Issue 11

Date: 08 Jan 90  1727 PST
From: Don Knuth <DEK@SAIL.Stanford.EDU>
Subject: Virtual fonts: More fun for Grand Wizards    
Keywords: fonts

Many writers to TeXhax during the past year or so have been struggling with
interfaces between differing font conventions. For example, there's been a
brisk correspondence about mixing oldstyle digits with a caps-and-small-caps
alphabet. Other people despair of working with fonts supplied by manufacturers
like Autologic, Compugraphic, Monotype, etc.; still others are afraid to leave
the limited accent capabilities of Computer Modern for fonts containing letters
that are individually accented as they should be, because such fonts are not
readily available in a form that existing TeX software understands.

There is a much better way to solve such problems than the remedies
that have been proposed in TeXhax. This better way was first realized
by David Fuchs in 1983, when he installed it in our DVI-to-APS
software at Stanford (which he also developed for commercial
distribution by ArborText). We used it, for example, to typeset my
article on Literate Programming for The Computer Journal, using native
Autologic fonts to match the typography of that journal.

I was expecting David's strategy to become widely known and adopted.
But alas --- and this has really been the only significant
disappointment I've had with respect to the way TeX has been
propagating around the world --- nobody else's DVI-to-X drivers have
incorporated anything resembling David's ideas, and TeXhax
contributors have spilled gallons of electronic ink searching for
answers in the wrong direction.

The right direction is obvious once you've seen it (although it wasn't
obvious in 1983): All we need is a good way to specify a mapping from
TeX's notion of a font character to a device's capabilities for
printing. Such a mapping was called a "virtual font" by the AMS
speakers at the TUG meetings this past August. At that meeting I spoke
briefly about the issue and voiced my hope that all DVI drivers be
upgraded within a year to add a virtual font capability.  Dave Rodgers
of ArborText announced that his company would make their WEB routines
for virtual font design freely available, and I promised to edit them
into a form that would match the other programs in the standard
TeXware distribution.

The preparation of TeX Version 3 and MF Version 2 has taken me much longer
than expected, but at last I've been able to look closely at the concept of
virtual fonts. (The need for such fonts is indeed much greater now than it
was before, because TeX's new multilingual capabilities are significantly
more powerful only when suitable fonts are available. Virtual fonts can
easily be created to meet these needs.)

After looking closely at David Fuchs's original design, I decided to design
a completely new file format that would carry his ideas further, making the
virtual font mechanism completely device-independent; David's original code
was very APS-specific. Furthermore I decided to extend his notions so that
arbitrary DVI commands (including rules and even specials) could be
part of a virtual font. The new file format I've just designed is called
VF; it's easy for DVI drivers to read VF files, because VF format is similar
to the PK and DVI formats they already deal with.

The result is two new system routines called VFtoVP and VPtoVF. These
routines are extensions of the old ones called TFtoPL and PLtoTF;
there's a property-list language called VPL that extends the ordinary
PL format so that virtual fonts can be created easily.

In addition to implementing these routines, I've also tested the ideas by
verifying that virtual fonts could be incorporated into Tom Rokicki's dvips
system without difficulty. I wrote a C program (available from Tom) that
converts Adobe AFM files into virtual fonts for TeX; these virtual fonts
include almost all the characteristics of Computer Modern text fonts
(lacking only the uppercase Greek and the dotless j) and they include all
the additional Adobe characters as well. These virtual fonts even include
all the "composite characters" listed in the AFM file, from `Aacute' to
`zcaron'; such characters are available as ligatures. For example, to get
`Aacute' you type first `acute' (which is character 19 = ^S in Computer Modern
font layout, it could also be character 194 = Meta-B if you're using an
8-bit keyboard with the new TeX) followed by `A'. Using such fonts, it's
now easier for me to typeset European language texts in Times-Roman and
Helvetica and Palatino than in Computer Modern! [But with less than an hour's
work I could make a virtual font for Computer Modern that would do the same
things; I just haven't gotten around to it yet.]

[A nice ligature scheme for dozens of European languages was just published
by Haralambous in the November TUGboat. He uses only ASCII characters, getting
Aacute with the combination <A. I could readily add his scheme to mine, by
adding a few lines to my VPL files. Indeed, multiple conventions can be
supported simultaneously (although I don't recommend that really).]

Virtual fonts make it easy to go from DVI files to the font layouts of
any manufacturer or font supplier. They also (I'm sorry to say) make
"track kerning" easy, for people who have to resort to that oft-abused
feature of lead-free type.

Furthermore, virtual fonts solve the problem of proofreading with screen
fonts or with lowres laserprinter fonts, because you can have several
virtual fonts sharing a common TFM file.  Suppose, for example, that you
want to typeset camera copy on an APS machine using Univers as the
ultimate font, but you want to do proofreading with a screen previewer and
with a laserprinter. Suppose further that you don't have Univers for your
laserprinter; the closest you have is Helvetica.  And suppose that you
haven't even got Helvetica for your screen, but you do have cmss10. Here's
what you can do: First make a virtual property list (VPL) file
univers-aps.vpl that describes the high-quality font of your ultimate
output. Then edit that file into univers-laser.vpl, which has identical
font metric info but maps the characters into Helvetica; similarly, make
univers-screen.vpl, which maps them into cmss10. Now run VPtoVF on each of
the three VPL files. This will produce three identical TFM files
univers.tfm, one of which you should put on the directory read by TeX.
You'll also get three distinct VF files called univers.vf, which you
should put on three different directories --- one directory for your
DVI-to-APS software, another for your DVI-to-laserwriter software, and the
third for the DVI-to-screen previewer.  Voil^^Ra.

So virtual fonts are evidently quite virtuous. But what exactly are
virtual fonts, detail-wise? Appended to this message are excerpts from
VFtoVP.WEB and VPtoVF.WEB, which give a complete definition of the
VF and VPL file formats.

I fully expect that all people who have implemented DVI drivers will
immediately see the great potential of virtual fonts, and that they will
be unable to resist installing a VF capability into their own software
during the first few months of 1990. (The idea is this: For each font
specified in a DVI file, the software looks first in a special table to
see if the font is device-resident (in which case the TFM file is loaded,
to get the character widths); failing that, it looks for a suitable GF or
PK file; failing that, it looks for a VF file, which may in turn lead to
other actual or virtual files. The latter files should not be loaded
immediately, but only on demand, because the process is recursive.
Incidentally, if no resident or GF or PK or VF file is found, a TFM file
should be loaded as a last resort, so that the characters can be left
blank with appropriate widths.)

%--- an excerpt from VFtoVP.web -----------------------------------------------

@* Virtual fonts.  The idea behind \.{VF} files is that a 
general interface mechanism is needed to switch between the myriad font 
layouts provided by different suppliers of typesetting equipment. Without such 
a mechanism, people must go to great lengths writing inscrutable macros 
whenever they want to use typesetting conventions based on one font layout in 
connection with actual fonts that have another layout. This puts an extra 
burden on the typesetting system, interfering with the other things it needs 
to do (like kerning, hyphenation, and ligature formation). 

These difficulties go away when we have a ``virtual font,''
i.e., a font that exists in a logical sense but not a physical sense.
A typesetting system like \TeX\ can do its job without knowing where the
actual characters come from; a device driver can then do its job by
letting a \.{VF} file tell what actual characters correspond to the
characters \TeX\ imagined were present. The actual characters
can be shifted and/or magnified and/or combined with other characters
from many different fonts. A virtual font can even make use of characters
from virtual fonts, including itself.

Virtual fonts also allow convenient character substitutions for proofreading
purposes, when fonts designed for one output device are unavailable on another.

@ A \.{VF} file is organized as a stream of 8-bit bytes, using conventions
borrowed from \.{DVI} and \.{PK} files. Thus, a device driver that knows
about \.{DVI} and \.{PK} format will already
contain most of the mechanisms necessary to process \.{VF} files. 
We shall assume that \.{DVI} format is understood; the conventions in the
\.{DVI} documentation (see, for example, {\sl \TeX: The Program}, part 31)
are adopted here to define \.{VF} format.

A preamble
appears at the beginning, followed by a sequence of character definitions,
followed by a postamble. More precisely, the first byte of every \.{VF} file
must be the first byte of the following ``preamble command'':

     [The rest of the excerpt is omitted.  By using the default compilation, 
     the executable program "weave" will be made for you.  Weave the file 
     vftovp.web to create a vftovp.tex file.  TeX the vftovp.tex file to 
     create a vftovp.dvi file.  Preview or print the .dvi file to learn more 
     about virtual fonts.] 

===============================================================================

Excerpted from:
     TeXhax Digest    Sunday,  January 14, 1990  Volume 90 : Issue 12
     %%Moderators' note: The following contribution is continued from
     %%TeXhax Digest Issue 11.

Virtual fonts: More fun for Grand Wizards    

%------------- and here's an extract from VPtoVF.web --------------------------

@* Property list description of font metric data.
The idea behind \.{VPL} files is that precise details about fonts, i.e., the
facts that are needed by typesetting routines like \TeX, sometimes have to
be supplied by hand. The nested property-list format provides a reasonably
convenient way to do this.

A good deal of computation is necessary to parse and process a
\.{VPL} file, so it would be inappropriate for \TeX\ itself to do this
every time it loads a font. \TeX\ deals only with the compact descriptions
of font metric data that appear in \.{TFM} files. Such data is so compact,
however, it is almost impossible for anybody but a computer to read it.

Device drivers also need a compact way to describe mappings from \TeX's idea
of a font to the actual characters a device can produce. They can do this
conveniently when given a packed sequence of bytes called a \.{VF} file.

The purpose of \.{VPtoVF} is to convert from a human-oriented file of text
to computer-oriented files of binary numbers. There's a companion program,
\.{VFtoVP}, which goes the other way.

...   ...   ...

@ A \.{VPL} file is like a \.{PL} file with a few extra features, so we
can begin to define it by reviewing the definition of \.{PL} files. The
material in the next few sections is copied from the program \.{PLtoTF}.
...   ...   ...

[The rest of the excerpt is omitted.  Weave the web file, TeX the resulting 
file, preview or print the output of TeX to learn more about these new, 
exciting programs from the inventor of TeX.] 


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