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Guide to LaTeX (4th Edition) (Tools and Techniques

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498 Appendix G. T E X Fonts<br />

The printer driver programs have the task of converting the DVI output<br />

from T E X in<strong>to</strong> instruction code for a particular printer. To do this, they<br />

need <strong>to</strong> know how <strong>to</strong> print each of the symbols used in the document.<br />

These symbols are represented as a set of black <strong>and</strong> white dots (pixels)<br />

that is adjusted <strong>to</strong> the output resolution. Clearly <strong>to</strong> produce the letter A<br />

in a given size at 600 dpi (dots per inch) requires many more dots than at<br />

150 dpi (16 times). This information is contained in the corresponding<br />

.pk files, in compressed format.<br />

However, designing fonts as a set of dots, <strong>and</strong> that for any number of<br />

resolutions, <strong>and</strong> taking in<strong>to</strong> account idiosyncrasies of individual printers,<br />

is an impossible task. Rather, the fonts are defined with.mf files, containing<br />

instructions on how <strong>to</strong> draw the symbols with a pen of a given shape.<br />

This is the most general definition, describing the ideal form. METAFONT<br />

then translates this ideal in<strong>to</strong> the practical realization, the pixels, for any<br />

specified resolution. Aspects of the various printers, such as relative pixel<br />

size <strong>and</strong> shape, may also be included, so that the resulting patterns are<br />

both resolution <strong>and</strong> printer dependent. The font metric.tfm files are also<br />

produced in this process.<br />

Today it is hardly necessary for most users <strong>to</strong> know anything more<br />

about METAFONT other than that it exists. The drivers <strong>and</strong> previewers<br />

are now so constructed that when they notice that the required .pk file<br />

is missing, they invoke METAFONT themselves <strong>to</strong> generate it. In this way<br />

the collection of pixel files grows as needed <strong>and</strong> includes only those that<br />

are actually used. (Originally huge sets of pixel files had <strong>to</strong> be supplied<br />

<strong>to</strong> cover every remote possibility.)<br />

Another reason for the decline in the awareness of METAFONT is<br />

the increasing use of the type 1 fonts for PostScript <strong>and</strong> PDF output<br />

(Chapter 10).<br />

Donald Knuth has announced that he wishes <strong>to</strong> withdraw from any<br />

further development of either of these programs, being prepared merely <strong>to</strong><br />

correct any definite errors they may contain. To emphasize this decision,<br />

he will from now on give version numbers <strong>to</strong> T E X that converge <strong>to</strong> π<br />

(3.14159. . . ) <strong>and</strong> <strong>to</strong> METAFONT that approach the value of e (2.71828. . . ).<br />

At present, T E X is at version 3.14159 <strong>and</strong> METAFONT at 2.718. As a<br />

consequence of this decision, any further major developments <strong>to</strong> these<br />

two programs, such as undertaken by user groups, will be under new<br />

names, since Knuth has kept the copyright <strong>to</strong> the existing names.<br />

G.4 Extended Computer fonts<br />

The Computer Modern fonts were developed in the early days of T E X when<br />

it could only h<strong>and</strong>le 128 characters per font. The modern T E X program<br />

can deal with 256 characters in each font, requiring a new set of st<strong>and</strong>ard<br />

fonts <strong>to</strong> exploit this feature.

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