[Tex/LaTex] What are .gf and .pk font files

fontsmetafont

I've recently read this very interesting thread: How (La)TeX makes use of font related files, i.e., .fd, .map, .enc, .def,etc when selecting fonts?.

In the meantime, I've been playing with compiling (for fun) MetaFont .mf files found in my MiKTeX distribution.

For example, I've compiled the file cmr10.mf and it has produced the file cmr10.2602gf, which seems to have one of the filetypes mentioned in the above thread (.gf, or something similar).

Now, given that .gf files are the output of MetaFont (or not?),

  1. what are they used for? are they only useful for being converted to .pk files through gftopk?
  2. what are exactly .pk files and what are they used for? (egreg's answer doesn't mention this filetype)
  3. how do I obtain .tfm or .vf font metric files from MetaFont output, since I seem to understand from that thread that these are the "real" font files searched by LaTeX?

Best Answer

If you are using a metafont font (which is increasingly rare) then the original source is written in MetaFont. Processing that source with metafont produces two files: the tfm file which encodes the font metrics (that is essentially just the size of each character, and information about ligatures and kerns with no information about shape) and a gf file which contains a sequence of bitmaps at a specified resolution (that needed to be specified to metafont, usually as part of a printer configuration). (The resolution by default got recorded in the file extension hence the example in your question)

tfm files are read directly by TeX but few programs could read gf fonts directly so they were almost always converted immediately to pk (packed) format which contained the same information in a more compressed form and could be read by dvi drivers such as dvips or xdvi.

So the end result was you could generate bitmap pk fonts exactly tuned to the printer you were using, not just the basic resolution but "blackness" control, ie how well the printer could cope with thin lines, or lines close together, metafont would if needed produce bitmaps with thicker lines etc. For printing on paper this produced very high quality output and for viewing on screen it could produce good results if you had high resolution and sensibly sampled and anti-aliased the bitmaps to the screen resolution, as dviview or xdvi did, but acrobat in its earlier versions made bitmap fonts more or less unreadable, which increased the urgency to develop type1 and other scalable format versions of the TeX fonts, which are the forms most commonly used today.

To see how the bitmaps were tuned to each printer look at modes.mf

/usr/local/texlive/2013/texmf-dist/metafont/misc/modes.mf

on my system

It's a very long file with information such as

% From {\tt ..........compulink.co.uk}, 9 February 1994.
mode_def bjtenex =                  %\[ Canon BubbleJet 10ex (360dpi)
  mode_param (pixels_per_inch, 360);
  mode_param (blacker, .6);
  mode_param (fillin, 0);
  mode_param (o_correction, .6);
  mode_common_setup_;
enddef;

which tells metafont how to make bitmaps tuned to that particular printer model. (You can see this isn't ideal for putting pdf on the web to be printed anywhere)

pixels_per_inch is the resolution

blacker is a measure of how much blacker (ie thicker) thin lines need to be before they disappear when printed on this device.

fillin controls diagonals so that the angles in the corners would remain sharp and not be "filled in" as an artefact of the resolution.

o_correction is overshoot correction to control whether curves should overshoot the bounding box (which works better at higher resolutions than lower)

These mode corrections have no effect on the font metrics in the tfm file, so tex would produce the same dvi file in all cases but dvips would use them to pull in printer-specific bitmaps depending which printer was being targeted with the PostScript.

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