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Code Editor : font-formats-1.html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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 <TITLE>Font-formats recognized by the Linux kbd package: PSF fonts</TITLE>
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<H2><A NAME="s1">1. PSF fonts</A></H2>

<P>PSF fonts come in two types: old (psf1) and new (psf2).
A PSF font consists of
(1) 
<A HREF="#psfheader">The header</A>,
(2) 
<A HREF="#psffont">The font</A>,
(3) 
<A HREF="#psfunimap">Unicode information</A>.
<P>
<H2><A NAME="psfheader"></A> <A NAME="ss1.1">1.1 The header</A>
</H2>

<P>For psf1 this is
<BLOCKQUOTE><CODE>
<PRE>
#define PSF1_MAGIC0     0x36
#define PSF1_MAGIC1     0x04

#define PSF1_MODE512    0x01
#define PSF1_MODEHASTAB 0x02
#define PSF1_MODEHASSEQ 0x04
#define PSF1_MAXMODE    0x05

#define PSF1_SEPARATOR  0xFFFF
#define PSF1_STARTSEQ   0xFFFE

struct psf1_header {
        unsigned char magic[2];     /* Magic number */
        unsigned char mode;         /* PSF font mode */
        unsigned char charsize;     /* Character size */
};
</PRE>
</CODE></BLOCKQUOTE>
<P>For psf2 this is
<BLOCKQUOTE><CODE>
<PRE>
#define PSF2_MAGIC0     0x72
#define PSF2_MAGIC1     0xb5
#define PSF2_MAGIC2     0x4a
#define PSF2_MAGIC3     0x86

/* bits used in flags */
#define PSF2_HAS_UNICODE_TABLE 0x01

/* max version recognized so far */
#define PSF2_MAXVERSION 0

/* UTF8 separators */
#define PSF2_SEPARATOR  0xFF
#define PSF2_STARTSEQ   0xFE

struct psf2_header {
        unsigned char magic[4];
        unsigned int version;
        unsigned int headersize;    /* offset of bitmaps in file */
        unsigned int flags;
        unsigned int length;        /* number of glyphs */
        unsigned int charsize;      /* number of bytes for each character */
        unsigned int height, width; /* max dimensions of glyphs */
        /* charsize = height * ((width + 7) / 8) */
};
</PRE>
</CODE></BLOCKQUOTE>
<P>The meaning is fairly clear from the field names.
The fonts here are bitmap fonts (not, for example, vector fonts),
and each glyph has a <CODE>height</CODE> and a <CODE>width</CODE>.
The bitmap for a glyph is stored as <CODE>height</CODE> consecutive
pixel rows, where each pixel row consists of <CODE>width</CODE> bits
followed by some filler bits in order to fill an integral number
of (8-bit) bytes. Altogether the bitmap of a glyph takes
<CODE>charsize</CODE> bytes.
<P>For psf1 the width is constant 8, so that the height equals the charsize.
<P>The number of glyphs in the font equals <CODE>length</CODE>.
For psf1 the length is constant 256, unless the <CODE>PSF1_MODE512</CODE> bit
is set in the <CODE>mode</CODE> field, in which case it is 512.
<P>The font is followed by a table associating Unicode values with each
glyph in case (for psf1) the <CODE>PSF1_MODEHASTAB</CODE> bit is set in
the <CODE>mode</CODE> field, or (for psf2) the <CODE>PSF2_HAS_UNICODE_TABLE</CODE>
bit is set in the <CODE>flags</CODE> field.
<P>The starting offset of the bitmaps in the font file is given by
<CODE>headersize</CODE>. (This allows the header to grow, probably
depending on <CODE>version</CODE>, without changes in the code.)
<P>The integers in the psf2 header struct are little endian 4-byte integers.
<P>
<H2><A NAME="psffont"></A> <A NAME="ss1.2">1.2 The font</A>
</H2>

<P>The actual bitmaps.
<P>
<H2><A NAME="psfunimap"></A> <A NAME="ss1.3">1.3 Unicode information</A>
</H2>

<P>The bitmaps may be followed by a Unicode description
of the glyphs. This Unicode description of a position
has grammar
<BLOCKQUOTE><CODE>
<PRE>
&lt;unicodedescription> := &lt;uc>*&lt;seq>*&lt;term>
&lt;seq> := &lt;ss>&lt;uc>&lt;uc>*
&lt;ss> := psf1 ? 0xFFFE : 0xFE
&lt;term> := psf1 ? 0xFFFF : 0xFF
</PRE>
</CODE></BLOCKQUOTE>

where <CODE>&lt;uc&gt;</CODE> is a 2-byte little endian Unicode value (psf1),
or a Unicode value coded in UTF-8 (psf2),
and <CODE>*</CODE> denotes zero or more occurrences of the preceding item.
<P>The semantics is as follows.
The leading <CODE>&lt;uc&gt;*</CODE> part gives Unicode symbols that are all
represented by this font position. The following sequences
are sequences of Unicode symbols - probably a symbol
together with combining accents - also represented by
this font position.
<P>Example:
At the font position for a capital A-ring glyph, we
may have (psf1):
<BLOCKQUOTE><CODE>
<PRE>
     00C5,212B,FFFE,0041,030A,FFFF
</PRE>
</CODE></BLOCKQUOTE>

where the Unicode values here are
LATIN CAPITAL LETTER A WITH RING ABOVE
and
ANGSTROM SIGN
and
LATIN CAPITAL LETTER A
and
COMBINING RING ABOVE.
Some font positions may be described by sequences only,
namely when there is no precomposed Unicode value for the glyph.
<P>
<H2><A NAME="ss1.4">1.4 Historical</A>
</H2>

<P>PSF stands for PC Screen Font.
The psf1 format without Unicode map was designed by
H. Peter Anvin in 1989 or so for his DOS screen font editor
<CODE>FONTEDIT.EXE</CODE>. In Oct 1994 he added the Unicode map
and the programs <CODE>psfaddtable</CODE>, <CODE>psfgettable</CODE>,
<CODE>psfstriptable</CODE> to manipulate it - see <CODE>kbd-0.90</CODE>.
Andries Brouwer added support for sequences of Unicode values
and the psf2 format in Sep 1999 in order to handle Tibetan -
see <CODE>kbd-1.00</CODE>.
<P>
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Code Editor : font-formats-1.html