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These days, fonts to be used on computers are represented in one of two ways: as a bitmap font, which specifies each individual pixel in the image of a character; and/or as an outline font, which specifies the image as a collection of mathematically-specified curves. Each method has its own advantages and disadvantages; typesetting programs, page description languages, and output devices can generally deal with both.
Limn converts a font from a bitmap to an outline by fitting curves to the pixels. Non-shape-related information in the bitmap font, such as that for the side bearings, is preserved in the outline output.
Specifically, the input is a bitmap (GF or PK) font. The output is a BZR outline font (see section 11.6 BZR files), which can then be converted to (for example) Metafont or PostScript with BZRto (see section 11. BZRto).
There is a fair amount of literature on converting bitmaps to outlines. We found three particularly helpful: Philip Schneider's Master's thesis on his system Phoenix; Michael Plass and Maureen Stone's article `Curve-fitting with piecewise parametric cubics' published in SIGGRAPH; and Jakob Gonczarowski's article `A fast approach to auto-tracing (with parametric cubics)' in the RIDT 91 conference proceedings. See the file `limn/README' for the full citations.
10.1 Limn algorithm How Limn fits outlines to bitmaps. 10.2 Invoking Limn Command-line options.