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+                        Virtual Fonts -- a Tutorial
+
+                              Thomas A. Schmitz
+
+                               February 9, 2005
+
+
+
+Lots of information on virtual fonts can be found on the web and in books
+(e.g., Knuth's very own ``Virtual Fonts: More Fun for Grand Wizards,''
+available on CTAN [CTAN.info.virtual-fonts.knuth]). However, there
+doesn't seem to be a step-by-step tutorial for non-wizards like myself. I
+experimented a bit with virtual fonts during the last couple of days. It took
+me a while to understand the basics, so I thought that other people might find
+it useful to hear about this and avoid some common mistakes. Most of what I
+write is fairly trivial, but maybe someone can make good use of it.
+
+
+
+1. Basic Facts about Virtual Fonts
+
+Virtual fonts can do two immensely useful things. They can remap characters
+within the same font. If you have a font foo with files foo.pfb and foo.tfm,
+you can have a virtual font foobar that will be identical to foo but print an
+``A'' whenever you have a ``B'' in your \TeX-file. This may sound absurd at
+first, but there are cases where this is useful. Some fonts offer alternative
+forms for letters. With the help of a virtual font, you can remap the
+characters and thus switch to these alternative forms without changing your
+\TeX-source.  The second use for virtual fonts, however, is much more common:
+Given a font foo, you can create a virtual font foobar that will use some
+characters from a second font, say bar. This is often used to include old-style
+numerals or additional ligatures that are not provided by the normal font.
+
+
+
+2. Copying the font-files
+
+So let's begin. We assume that you will be using two postscript-fonts, foo and
+bar. Usually, for each of these fonts, you will have two files foo.tfm
+(bar.tfm) and foo.pfb (bar.pfb) and nothing else, so we need to create a
+vf-file from scratch.  I didn't find it mentioned anywhere that this is not
+only possible, but even fairly easy. 
+
+We assume that you will be performing these operations on the command line in a
+working directory like /tmp. So the first step would be to copy foo.tfm to this
+directory:
+
+cd /tmp
+
+cp PATH_TO_foo.tfm .
+
+(Watch the trailing period, it's necessary!
+
+
+
+3. Create a Human-Oriented Property-List
+
+The file foo.tfm is a binary in a format that \TeX can read. If we want to edit
+it, we will have to convert it to a human-oriented property list (with
+extension .pl). We will be using tools that come with any complete
+\TeX-installation. On the command-line, do this:
+
+tftopl foo foo
+
+(yes, that's right: you have to type foo twice)
+
+
+
+4. Open the Property-List
+
+You now have a new file foo.pl which contains all the information about the
+font that \TeX needs. Open it in your favorite text editor. If you're editing
+it in a non-UNIX environment like Windows or Mac OS X, make sure that your
+editor is set to use UNIX line-endings (all kinds of nasty things can happen
+when UNIX apps like \TeX  encounter non-UNIX line-endings). The first couple
+of lines will read like this:
+
+(FAMILY TEX-FOO)
+(FACE F MRR)
+(CODINGSCHEME FONTSPECIFIC + TEX TEXT)
+(DESIGNSIZE R 10.0)
+(COMMENT DESIGNSIZE IS IN POINTS)
+(COMMENT OTHER SIZES ARE MULTIPLES OF DESIGNSIZE)
+(FONTDIMEN
+   (SLANT R 0.0)
+   (SPACE R 0.252)
+   (STRETCH R 0.2)
+   (SHRINK R 0.1)
+   (XHEIGHT R 0.459)
+   (QUAD R 1.0)
+   (EXTRASPACE R 0.111)
+   )
+(LIGTABLE
+
+etc.
+
+If there is a line (CHECKSUM O ...), delete it (it will be regenerated
+later).
+
+
+
+5. Editing the Property-List
+
+In order to generate a virtual font, you need to modify this file. First, you
+will have to tell \TeX which fonts this virtual font will be referring
+to. Let's say they're foo.tfm and bar.tfm (needless to say, both have to be
+installed and functional in your \TeX-installation). In a first step, we will
+create a virtual font that will remap some characters within foo. So just
+before the line starting with (LIGTABLE, add this:
+
+(MAPFONT D 0
+   (FONTNAME foo)
+   (FONTDSIZE R 10.0)
+   )
+
+The FONTDSIZE of foo could be found in the first lines (DESIGNSIZE R 10.0), so
+all you have to do is copy this information.
+
+
+
+6. Remapping a Character
+
+If you scroll down in this file, the LIGTABLE (containing information
+about ligatures and kerning) will end with two lines
+
+(STOP)
+)
+
+After this, the section with information about all the defined characters in
+the font will follow, probably starting something like this:
+
+(CHARACTER O 0
+   (CHARWD R 0.674)
+   (CHARHT R 0.726)
+   )
+
+\TeX only cares about the dimensions of every character and puts an empty box
+with these dimensions into the .dvi-file. It is only when this .dvi is
+interpreted by a postscript- or pdf-driver that the actual character (the
+``glyph'') is put into this box. So let us assume you want to have a virtual
+font that will always print ``A'' when you have ``B'' in your source. Scroll
+further down until you reach the section with the capital letters:
+
+(CHARACTER C A
+   (CHARWD R 0.747)
+   (CHARHT R 0.747)
+   )
+(CHARACTER C B
+   (CHARWD R 0.739)
+   (CHARHT R 0.726)
+   )
+
+\TeX will be using the box described here, so you want the box for ``B'' to
+be of the same size as the box for ``A.'' Hence, the first thing you need to do
+is copy the dimensions of ``A'' into ``B.'' So the section should look like
+this:
+
+(CHARACTER C A
+   (CHARWD R 0.747)
+   (CHARHT R 0.747)
+   )
+(CHARACTER C B
+   (CHARWD R 0.747)
+   (CHARHT R 0.747)
+   )
+
+Next (and this is the magic of virtual fonts) you will tell \TeX that it should
+remap ``B'' to ``A.'' Just before the closing parenthesis of CHARACTER B, you
+will insert a new section. Now ``B'' will look like this:
+
+(CHARACTER C B
+   (CHARWD R 0.747)
+   (CHARHT R 0.747)
+   (MAP
+      (SETCHAR C A)
+      )
+   )
+
+One of the things that can be a bit confusing about these property lists is the
+fact that (apart from the numbers and the 26 letters of the alphabet)
+characters are referred to by ``octal numbers.'' If you want to know what
+character corresponds to what octal number, you can have a look at the tables
+created by testing the font (section 9).
+
+
+
+7. Saving the File
+
+That's it! You have modified the font description; now you need to generate the
+binary files for \TeX to use. The next step is extremely important:
+SAVE THE FILE TO A DIFFERENT NAME! So in our case, let's say we call
+the new virtual font foobar (the name doesn't matter; the extension has to be
+.vpl); so save to foobar.vpl
+
+
+
+8. Generating the Binaries
+
+Back to the command line. We now run a little program that will convert your
+foobar.vpl into two new files: foobar.tfm and foobar.vf.
+
+vptovf foobar.vpl
+
+This will not only do the conversion, it will also check whether you .vpl-file
+is in good order. It is very picky about the right indentation level and
+parentheses, and it will tell you exactly in which line there is a problem. So
+if you get errors, just go back and edit foobar.vpl again. vptovf may also tell
+you that it had to ``round some units,'' that's OK.
+
+
+
+9. Installing the New Font
+
+So now you should have foobar.vf and foobar.tfm. Copy both files into the right
+places. I would suggest you create your own texmf-branch in your home
+directory, under ~/texmf or ~/Library/texmf (depending on your local setup as
+defined in texmf.cnf). So the files should go here:
+
+cp foobar.tfm ~/texmf/fonts/tfm/
+
+cp foobar.vf ~/texmf/fonts/vf/
+
+(you will have to create these directories if they don't exist yet). Since this
+virtual font will only be seen by \TeX, you don't need to fiddle with any map
+file; for the postscript- or pdf-driver, only font foo will exist, which was
+already functional. Before you embark on a long journey with this new virtual
+font (say your 1200-page thesis that is due in two weeks), I would suggest you
+test it in a nice quiet little directory on your box. cd to this directory and
+then do this:
+
+cd ~
+
+pdfetex testfont
+
+pdfetex will respond to this command like so:
+
+This is pdfeTeX, Version 3.141592-1.21a-2.2 (Web2C 7.5.3)
+ \write18 enabled.
+entering extended mode
+(/usr/local/teTeX/share/texmf.tetex/tex/plain/base/testfont.tex
+
+Name of the font to test = 
+
+You type 
+
+foobar
+
+pdfetex will respond:
+
+Now type a test command (\help for help):)
+
+*
+
+you type:
+
+\table
+
+pdfetex will open a new line with another asterisk, and you type
+
+\end
+
+If all goes well, a file testfont.pdf will be created, with a table showing
+that font foobar does not have a letter ``B,'' but twice the letter
+``A''---which is just what we wanted.
+
+If your \TeX-installation includes \CONTEXT, a macro package for \TeX (see
+http://tug.org/pracjourn/2005-1/asknelly/), you can also create a very
+nice colorized table. Create a file test.tex with this content:
+
+\starttext
+
+\showfont[foobar]
+
+\stoptext
+
+Then, run this file through \CONTEXT: 
+
+texexec --nonstopmode --pdf test.tex
+
+You'll get a table with all the glyphs; every cell will indicate the decimal,
+hexadecimal, and octal value of the glyph (very handy for editing property
+lists).
+
+
+
+10. Adding a Second Font
+
+Now let's assume you want to include further stuff, like old-style numerals or
+ligatures from font bar. We go back to our working directory and delete the old
+file foobar.vpl. Don't worry, we'll create it again:
+
+rm foobar.vpl
+
+vftovp foobar foobar foobar
+
+The reason we do this is that vftovp will automagically include one important
+piece of information: every character description will now look like this:
+
+(CHARACTER C A
+   (CHARWD R 0.747)
+   (CHARHT R 0.747)
+   (MAP
+      (SETCHAR C A)
+      )
+   )
+
+That's already not bad, but if we want to mix glyphs from two fonts, we will
+have to say which font to use at every instance. So in your editor, you should
+perform a ``find and replace'' that will find every instance of (MAP
+and replace it with
+
+(MAP
+   (SELECTFONT D 0)
+
+In emacs, it is possible to include the line break in the replace pattern, I
+don't know if (and how) other editors can handle it.  
+
+Now add the information about the second font, just as described in
+section 5: 
+
+(MAPFONT D 1
+  (FONTNAME bar)
+  (FONTDSIZE R 10.0)
+  )
+
+If you want to know the DSIZE of bar, just convert bar.tfm into bar.pl, open
+this file and look into the first lines (we'll need bar.pl anyway). Or, if you
+want to be fancy, you could even do this: tftopl `kpsewhich bar.tfm` | grep
+DESIGNSIZE (watch the ``backticks,'' those are single opening quotes!).
+
+(CHARACTER C 0
+   (CHARWD R 0.514)
+   (CHARHT R 0.628)
+   (CHARDP R 0.1)
+   (MAP
+      (SELECTFONT D 0) 
+      (SETCHAR C 0)
+      )
+   ) 
+
+
+
+11. Including Glyphs from a Second Font
+
+Now look for the section containing the numerals. It should start like this:
+
+(CHARACTER C 0
+   (CHARWD R 0.514)
+   (CHARHT R 0.628)
+   (CHARDP R 0.1)
+   (MAP
+      (SELECTFONT D 0) 
+      (SETCHAR C 0)
+      )
+   ) 
+
+Again, the first thing you should do is copy the dimensions of CHARACTER C 0
+from bar.pl to foo.pl. Then replace (SELECTFONT D 0) with (SELECTFONT D 1), and
+everything should work: After you have done this for all the numerals,
+generated the tfm/vf pair (section 8) and copied these files to the right
+directories (section 9), \TeX will look at the tfm/vf pair, and will take the
+numerals from font bar, everything else from font foo. Again, test to see if
+this works as expected.
+
+
+
+12. Post-Install
+
+After you have edited your virtual font, you can discard the .vpl-file. If you
+ever want to edit your font again, you can recreate it by copying both the .vf
+and the .tfm into the same directory and running vftovp foobar foobar foobar
+
+Have great fun and feel like a grand wizard!
+
+
+
+13. Further Reading
+
+http://www.cl.cam.ac.uk/users/rf/pstex/index.htm
+
+http://homepage.mac.com/bkerstetter/tex/fonttutorial-current.html
+
+http://zoonek.free.fr/LaTeX/Fontes/fontes.html (in French)