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1. Raster Font File Format

1.1 Introduction
  A raster font (RST) file contains a description of a font suitable for use on
a  specific output device.  The images of characters are described by sequences
of bits specifying which pixels in a bounding  rectangle  should  be  blackened
when  the  character is painted.  RST files are not sufficient for typesetting,
which requires information about kerning and  ligatures  (as  supplied  by  TFM
files, for example, to the TEX typesetting system).  However, RST files contain
enough information to print listings nicely, for example.



1.1.1 Fonts, Glyphs, Characters, Symbols
  The  term  font  can name potentially many concepts.  We will use it below in
its more restricted sense of a collection of pictures or glyphs in a particular
style, at a particular point size, for a particular output device.  A RST  file
thus describes a font by this definition.

  A  symbol  is  an  ideal picture with which one or more meanings are commonly
associated.  The word ``commonly'' is used because a symbol such as  the  Greek
alpha  symbol  may  have  a  meaning  arising  from  the  context  of  its use,
independent of its common meaning.  It is ``ideal'' in the sense that it is the
same symbol no matter what the style in which it is written or drawn.

  The word character is used in different ways.    A  character  should  be  an
alphabetic symbol.  But it is often used to denote a wider class of symbols, as
in  the  phrase  ``the  ascii  character set.''  Sometimes it is used to mean a
symbol written in a certain style, for example,  ``the  character  Times  Roman
A.''    We will use the phrase character number in a fairly specific sense: the
ordinal of a character in a given font, starting with zero.  For  example,  the
character number of the symbol upper-case a in the ascii character set is 65.

  A glyph is a particular instance of a symbol, a picture captured by a graphic
description.  In RST files, glyphs are represented as bit maps, and thus, for a
given  printer  with  fixed resolution and shape of dot, a glyph always has the
same size and shape.



1.1.2 Bit Map
  Glyphs are represented in RST files as bit maps, a lists of rows; each row is
a list of black and white dots.  To view the glyph, the rows should be  stacked
vertically and displayed.



1.1.3 Units of Measure in RST Files
                                  -20
  A  fix is an integer, equal to 2    points.  There are 72.27 points per inch.
To convert a fix, f, to a real number of points, r, use the formula 

             20
    r = f / 2  .




1.1.4 Strings
  A string in an RST file is represented as a sequence of  8-bit  bytes.    The
first  byte  contains  the  number  of bytes in the rest of the string.  Thus a
string can contain at most 255 characters.



1.1.5 File Addressing
  An  RST  file  is  a  sequence  of  8-bit  bytes  (often  combined  to   form
multiple-byte fields).  All pointers in the file are offsets from the beginning
of the file, starting with zero.  Thus, a pointer to the first byte in the file
would be zero.



1.1.6 Host RST File Representation
  This  document can say nothing about the actual representation of an RST file
on a given file system.  Instead, it describes a virtual  representation  which
is a randomly addressable sequence of 8-bit bytes.

  In  practice,  RST  files  have  a host file-system name descriptive of their
contents.  For example, on some systems, a Computer Modern  Roman  font,  point
size  10,  magnification  1, would have the filename CMR10.R10 (the R10 encodes
the unity magnification).  This  allows  typesetting  software  to  access  the
correct  RST file for its needs (based on name, size and magnification) without
searching  all  the  RST  files  in  a  font  database  (but   this   is   very
host-environment-dependent).

1.2 RST File Structure
  There  are  four  portions of an RST file, viz., a font marker, the preamble,
the directory and the raster section.

  At the beginning of the file there is a file mark, declaring the file  to  be
an  RST  file.   The preamble follows, giving vital information about the font,
such as how many characters are in it, as  well  as  nonessential  information,
such  as  who created the file.  Next, the glyph directory contains the size of
each glyph and a pointer into the last section, which contains the raster  data
for each glyph.



1.2.1 File Mark
  The  file  mark  is  8  bytes  long.   The first four bytes contain the ascii
letters Rast, identifying the file as a raster-format  file.    The  next  four
bytes are currently unused, and should contain zero.



1.2.2 Preamble
  The  preamble  begins  at  the  ninth byte (index 8) of the RST file with the
number of bytes occupied by the rest of the preamble, followed by  the  version
number  of the RST format.  An RST file format is version dependent.  Version 0
has 18 fields of preamble information in at least 40 bytes.

  Below are the fields of the preamble, followed by a discussion of some of the
fields.  If the type of a field is not specified, it is an integer of the given
size.

   - the number of bytes in the preamble, not including the two  used  by  this
     field (2 bytes, offset 8)

   - format version number (currently zero) (1 byte, offset 10)

   - pointer to the glyph directory (>= 46) (3 bytes, offset 11)

   - character  number  of  the  first  glyph  in the font, usually 0 (2 bytes,
     offset 14)

   - character number of the last glyph in the  font,  usually  127  (2  bytes,
     offset 16)

   - font  magnification,  in  units of 1/1000 (dimensionless); for example, an
     unmagnified font will have magnification 1000, and a font that is twice as
     big as its designsize will have  magnification  2000  (a  0  magnification
     should be interpreted as 1000) (4 bytes, offset 18)

   - the  designsize  of  the font, in fix units; if the font is not magnified,
     this will be the intended size of this font (4 bytes, offset 22)

   - the interline spacing for the font, in fix units; if this field  is  zero,
     try designsize * 1.2 (4 bytes, offset 26)

   - the  width of a good looking interword space, in fixes.  If this parameter
     is zero, try designsize/1.2 (4 bytes, offset 30)

   - rotation of the font in counter-clockwise positive degrees;  normal  fonts
     have rotation 0; fonts read while standing the page on its right edge have
     rotation 270 (2 bytes, offset 34)

   - character  advance  direction  relative to the font's rotation; this tells
     where to place a ``next character'' on a ``line of  characters;''  0  mean
     advance  to  the  right,  1 downward, 2 to the left, 3 upward (the English
     fonts have character advance direction 0, Chinese 1, Hebrew  2)  (1  byte,
     offset 36)

   - line  advance  direction relative to the font's rotation; this tells where
     to place a ``next line of characters'' on the page, relative to  the  last
     line:   0 to the right, 1 downwards, 2 to the left, 3 upwards (the English
     fonts and Hebrew have line advance direction 1, Chinese 0) (1 byte, offset
     37)

   - check identifier: this is used with metafont files  to  associate  an  RST
     file  with  a  specific  TFM file, ensuring that the TFM and RST files are
     describing the same font; if this field is zero, no check is to be done (4
     bytes, offset 38)

   - font resolution in pixels per inch:  240  for  the  IMPRINT-10  (2  bytes,
     offset 42)

   - font identifier string (variable size, offset 44)

   - string describing the face-type encoding (variable size, offset >= 45)

   - string naming the intended output device (variable size, offset >= 46)

   - string naming the creator of this file (variable size, offset >=47)

  The  first  five  fields  (size,  version,  pointer  to  the glyph directory,
character numbers of the first and last glyphs in the font) must be correct for
any RST file to be useable.

  Though the magnification can presumably be obtained from  the  file  name  in
some  host  environments,  it  is  included  for  verification  and  to allow a
magnification of greater precision than the file name might allow.  The  widths
of  the  characters  at their designsize are stored in the glyph directory.  If
the font is at magnification 2.0, the characters are actually twice as  big  as
their  designsize.    The  correct  printing width of a character, then, is its
given width multiplied by the magnification.  (The individual character  widths
are  given  without the magnification figured in so that a printing program can
substitute a font at one magnification for one at another  magnification.    If
TEX  says  an RST font file at magnification 2.4 is called for, but such a font
does not exist and one at 2.5 does exist, the latter can  be  used.    The  2.5
magnification   characters  will  be  printed,  but  the  2.4  widths  used  by
multiplying by 2.4 instead of 2.5.  This will make words with these  characters
slightly  cramped,  but  won't  effect the spacing of the rest of the document.
Alternatively, the characters could be spaced correctly if  the  space  between
words is shortened.)

  The  designsize, interline spacing and space width are included to enable use
of the font in simpler applications, such as  listing  generation  or  document
production with software not concerned with kerns or ligatures.

  The  designsize  is the generic size of the font, and also tells the distance
between baselines.  An n point font normally has n/72 inches between baselines.

  The interline spacing distance field may differ  from  the  designsize.    In
proper  fonts  they  should  be  identical,  but  often fonts must be tuned for
different devices.  The interline spacing is a subjective spacing based on  the
look  of  text.  (Metafont, for example, does not know about interline spacing,
but in the RST files it generates, this field is set to 1.2  times  the  design
size.)

  The width of a space determines the size of spaces and tabs.

  The  rest of the fields are present to encourage correctness.  A verification
program can check that a font's file name  corresponds  to  its  contents.    A
device  dependent  program assembling text can ensure the font was made for it,
or at least for a printer of its resolution.  A user can find out  who  made  a
font in case it needs to be improved.



1.2.3 Glyph Directory
  Each glyph in the font has an entry in the glyph directory.  Each entry takes
up  15 bytes, thus the directory size in bytes is 15 times the number of glyphs
in the font.  A glyph is defined in the font if its character number is between
the first and last character numbers (inclusive) as given in the  preamble  and
if it has has a non-zero directory entry (described below).

  Let fg be the character number of the first glyph in the font file, let lg be
that  of the last glyph, and let DirPtr be the pointer to the first byte of the
directory as given in the preamble.  Then the address (offset in the RST  file)
of the directory entry for character n, assuming fg <= n <= lg, is 

    DirPtr + ((n - fg) * 15).

  The format of a directory entry is:

   - h, height (in pixels) of raster picture (2 bytes, offset 0)

   - w, width (in pixels) of raster picture (2 bytes, offset 2)

   - y,  distance  (in pixels) from the top of the raster array to the glyphs's
     reference point (2 bytes, offset 4)

   - x, distance (in pixels) from the left of the raster array to  the  glyph's
     reference point (2 bytes, offset 6)

   - fw,  advance  width  of  the  character, in fix units; note that this is a
     signed quantity (4 bytes, offset 8) also  called  the  printing  width  or
     nominal width) (4 bytes)

   - p, pointer to raster data (3 bytes, offset 12)

  H and w are the height and width in pixels of the raster array containing the
glyph.  The glyph is stored as a sequence of h rows, each row being w % 8 bytes
long.    Again,  the pixel height and width have no connection with the nominal
height and width of the glyph as far as  a  typesetting  system  is  concerned;
rather,  they denote the size of the smallest bounding box that fits around the
black pixels comprising the character's raster representation.

  A character has a reference point often near, or just left of, its center.  Y
and x are the distances from the top left corner  to  this  point  measured  in
pixels.     Both  of  these  numbers  may  be  negative,  in  two's  complement
representation.  For  the  y-coordinate,  positive  is  downward;  for  the  x,
positive is rightward.

  Fw  is  the  advance width of the character in fix units (a signed quantity).
Remember to multiply this width by the  font  magnification  to  get  the  true
physical  width.   This is also called the printing width or nominal width (and
is, for example, the same as the width of the glyph in a TEX TFM file).

  The final value contains a pointer to the character's raster picture  in  the
raster  data section.  This pointer is an absolute byte address (the first byte
of the file has address 0).



1.2.4 Raster Data
  The last section  of  the  RST  file  is  the  raster  data;  each  glyph  is
represented  by a bit map in this section, but in no particular order (i.e., if
you want to know where a particular glyph's raster  data  lives,  look  in  the
glyph's directory entry).

  Let  bw  be  the  width of the character in bytes, or (w % 8), where w is the
pixel width of the raster data for the glyph as given in the  directory  entry.
All  rows start on byte boundaries and are packed to the left, so the last byte
in a row will not be fully used unless the width of the character is a multiple
of 8.  Thus a glyph's ``picture'' (raster data, bit map) takes up  bw*h  bytes,
where h is the pixel height of the glyph, also from the directory entry.

  Here is a letter ``Q'' as represented in the raster section of some RST file:
Row
 0  00001111 11100000 0.......
 1  00011111 11110000 0.......
 2  00111100 01111000 0.......
 3  01110000 00011100 0.......
 4  11110000 00011110 0.......
 5  11100000 00001110 0.......
 6  11100000 00001110 0.......
 7  11100111 11001110 0.......
 8  11111111 11011110 0.......
 9  01111100 11111100 0.......
10  00111100 01111000 0.......
11  00011111 11110000 0.......
12  00X01111 11100011 1.......
13  00000000 11100111 0.......
14  00000000 01111100 0.......
15  00000000 00111000 0.......

Col 76543210 76543210 76543210
    (Bit number within a byte,
     high order bit is number 7)
Here,  the  rows  and  columns  are  numbered,  and  the reference point of the
character is marked with an X, but only the 0's and 1's are  actually  part  of
the  character;  periods  represent  padding zero bits that are not part of the
character.  H is 16 pixels, w 17 pixels, y 12 pixels, x  2  pixels,  and  bw  3
bytes.  Note that the pixel width is just large enough to span the leftmost and
rightmost  black  pixels  in the character.  Likewise, the pixel height is just
large enough to span the topmost and bottommost black pixels.

  If this ``Q'' had a TEX TFM width of 5620393 fixes, it  would  print  on  the
IMPRINT-10 at a width of 

                      20
    5620393 fixes / (2   fixes/points) / (72.27 points/inch)
      * 240 pixels/inch  =  17.8 pixels

which would be rounded up to 18.

  Suppose this were a font 5.4 points high.  If this font were made from a font
10.8  points  high  and  thus  at magnification .5 (one half), then the nominal
width would  have  been  11367088,  twice  as  much  as  before,  and  only  by
multiplying  it  by  the  magnification  (.5) would one get the actual printing
width.



                               Table of Contents

1. Raster Font File Format                                                    0

1.1 Introduction                                                              0
     1.1.1 Fonts, Glyphs, Characters, Symbols                                 0
     1.1.2 Bit Map                                                            0
     1.1.3 Units of Measure in RST Files                                      0
     1.1.4 Strings                                                            0
     1.1.5 File Addressing                                                    0
     1.1.6 Host RST File Representation                                       0
1.2 RST File Structure                                                        0
     1.2.1 File Mark                                                          0
     1.2.2 Preamble                                                           0
     1.2.3 Glyph Directory                                                    1
     1.2.4 Raster Data                                                        1
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