Metafont is a programming language used to typeset outline fonts.
This programming language was devised by Donald Knuth as part of his famous TeX typesetting system. One of the characteristics of Metafont is that all the outlines of the glyphs are defined with powerful geometrical equations, eg. you can define a given point to be the intersection of a segment of line and a Bézier curve.
You can use Metafont to render any kind of graphical output you wish, not just glyphs. However, MetaPost with its PostScript output is preferred for advanced illustrations.
It is also important to note that, in normal TeX and dvi files usage, a "link" to the font will be used in the dvi file and the application used to view this file will then generate a bitmap version of the font (to the desired dpi resolution) and store it on disk for later usage. This is contrary to the traditional usage of the PostScript fonts where no such storage is done.
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