contrib/OWB: add correct SDL dependency, fix compilers used

[AROS-Contrib.git] / freetype1 / docs / FAQ

                         FreeType User FAQ

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                         Table of contents


1.  How do I compile the test programs?

2.  What are the test programs?

3.  How do I use them?

4.  How do I only compile the FreeType library?

5.  The library compiles fine on my system, unfortunately it seems
    the test programs won't.  What can I do?

6.  What is FreeType, and what is it not?

7.  Can FreeType be ported to other platforms?

8.  My TrueType font only works on a Mac/in Windows.  Will I be able
    to use it with FreeType?

9.  What are the most common character mappings?

10. How do I render text with FreeType?

11. How do I render rotated/slanted text with FreeType?

12. How do I compute a glyph bitmap's dimensions?

13. Why is text rendering so slow?  Does FreeType cache glyphs?

14. Do you always render the same glyphs as Windows or the Mac?

15. The program `ftlint' reports errors with one of my fonts, though
    it works perfectly under Windows.  What does this mean?

16. What does the `fttimer' test program benchmarks?

17. Is it  possible to perform styling (like  oblique, italic, bold,
    underline, etc.) with FreeType?

18. When does  glyph hinting takes place?   Is it when  the glyph is
    loaded, or when it is converted to a bitmap/pixmap?

19. Which is the best, caching outlines or bitmaps?

20. Can I open OpenType and/or TrueType GX fonts with FreeType?

21. How  can  I  access  additional TrueType  tables  that  FreeType
    doesn't support?

22. When will I be able to use FreeType to display TrueType fonts in
    X11, Wine, or OS/2, or even other systems?

23. What does the `free' in FreeType really means?

24. Does FreeType support threads?  Is it re-entrant?

25. Does FreeType support `foreign languages'?

26. I'm using fractional char sizes, but the glyphs stay at the same
    integer size.  Why?

27. Hinting doesn't work at very small sizes.  Why?

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1. How do I compile the test programs?

  Detailed compilation  steps are given  in the `freetype/HOWTO.txt'
  file,  as  well as  system  and  compiler  specific hints  in  the
  `freetype/howto'  directory.   What  follows  is a  _very_  simple
  guide:

  For Unix:

    Do the  following on the prompt  while in the  base directory of
    the FreeType package:

      % ./configure
      % make
      % make install

    This will  build and install the library  (shared, if supported)
    together with  the test  programs.  Say `./configure  --help' to
    see the available configuring options.

    This should work with any ANSI C compiler.

  For other platforms:

    Go  to the  `test'  directory.   Look at  one  of the  makefiles
    located   in  the   `arch/<system>'  directory,   and   use  the
    appropriate makefile from there.

    For example:

      make -f arch/msdos/makefile.gcc    (DJGPP or emx)
      wmake -f=arch\msdos\makefile.wat   (Watcom DOS)
      nmake -f arch\os2\makefile.icc     (Visual Age OS/2)
      make -f arch/amigaos/makefile.gcc  (GCC Amiga)

    This will compile both the library and the test programs.


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2. What are the test programs?

  The test programs are simple  sources that `show off' the FreeType
  library.  They are located in the `test' directory.  They make use
  of a  small graphics sub-system  which is able to  display bitmaps
  and  pixmaps  on  a  variety  of  platforms  through  the  use  of
  system-specific `drivers'.

  The test programs are:

    ftdump:   A  simple  TTF information  dumper.  Also  prints  the
              memory used  by each  opened font file  with FreeType.
              Doesn't use the graphics sub-system.

    ftlint:   A simple TrueType  bytecode verifier.  It simply hints
              all glyphs of  one or more font files  at a given size
              and   reports  errors.    Doesn't  use   the  graphics
              sub-system.

    ftview:   A simple  font viewer.  Displays all glyphs  in a font
              file in a window.

    ftstring: Renders a  simple string to  the screen.  Demonstrates
              how to produce text with FreeType.

    fttimer:  A  simple  benchmark, used  to  profile the  scan-line
              conversion  routines (and  only them).   Supports also
              display.

    ftstrpnm: A version of `ftstring'  which saves a bit/pixmap to a
              PNM  file  rather than  to  the  screen.  No  graphics
              sub-system needed.

    ftzoom:   A simple glyph viewer, useful to view, zoom and rotate
              individual glyphs in a font file.

    fterror:  This  program demonstrates  how  to  use the  ftxerr18
              extension.   Note that internationalized  strings will
              be available on some platforms only (e.g. Linux).

    ftmetric: A simple  metric/glyph dumper.  No graphics sub-system
              needed.

    ftsbit:   Displays  data about  embedded bitmaps  in  a TrueType
              font.

    ftstrtto: Renders  a  string  to  screen,  using  TrueType  Open
              features.
   
  NOTE: The file  `ftdebug.c' isn't part  of the test suite.   It is
        used only  by the developers team  to debug the  engine at a
        higher  level.   It  won't  work  with  a  standard  library
        compile.


  The tiny graphics subsystem is defined in the following files:

    gmain.h/gmain.c: The subsystem's main body/interface.

    gevents.h:       The  definition  of   events  defined  for  the
                     subsystem

    gdriver.h:       The  generic interface  of  all system-specific
                     drivers.

  System-specific   drivers    are   in   the   `test/arch/<system>'
  directories, like:

    test/arch/msdos/gfs_dos.c   Full-Screen DOS driver
    test/arch/os2/gpm_os2.c     PM (Windowed) OS/2 driver
    test/arch/unix/gwin_x11.c   X11 Windowed driver

    etc.


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3. How do I use them?

  Please read the file freetype/README for a full description.


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4. How do I only compile the FreeType library?

  For Unix:

    Do a `configure' run as  described in section 1.  Then change to
    the lib subdirectory and say `make' and `make install'.

  For other platforms:

    Go to the `lib' directory.  Look at one of the makefiles located
    in the `arch/<system>' directory, and use it from `lib'.

    For example:

      make -f arch/msdos/makefile.gcc    (DJGPP or emx)
      wmake -f=arch\msdos\makefile.wat   (Watcom DOS)
      nmake -f arch\os2\makefile.icc     (Visual Age OS/2)
      make -f arch/amigaos/makefile.gcc  (GCC Amiga)

    The  library  is   pure  ANSI  C  and  doesn't   depend  on  any
    system-specific package.  You need not gcc to compile it.


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5. The library  compiles fine on  my system, unfortunately  it seems
   the test programs won't.  What can I do?

  As  said  before, the  test  programs  rely  on a  small  graphics
  sub-system  to display  the bitmaps  and pixmaps  produced  by the
  library.  The following systems are currently supported:

    DOS      Full-Screen
    Amiga    Full-Screen    Note that the windowed graphics
    Amiga    Windowed       driver are very 'basic', they do
    OS/2     Full-Screen    not provide menus, dialog boxes,
    OS/2     Windowed       etc.  Rather, they provide one
    X11      Windowed       windowed bitmap/pixmap and translate
    Win32    Windowed       events to the ones defined in
                            `test/gevents'.

  If you want to add support  to your system, you should write a new
  graphics driver.  To do that, read the file `test/gdriver.h' which
  contains  the driver  generic interface,  then browse  the drivers
  that are already provided.

  The  graphics  drivers  are  located in  the  `test/arch/<system>'
  directories.  They're not part of the library proper.

  IMPORTANT NOTE:

    You can also  directly link the library to  your own application
    and render glyphs  directly into your own bitmaps  with very few
    efforts.


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6. What is FreeType, and what is it not?

  FreeType is a rather low-level  font engine.  It has been designed
  primarily  to be  the  basis  of several  font  servers, for  very
  different systems  which all have  their own metrics  and graphics
  models.  This means that it focuses on the following points:

  - Opening and loading font files in memory.

  - Giving  access to  most important  font properties  and TrueType
    tables.

  - Providing  a simple way  to translate  system-specific character
    codes  into  glyph indexes,  according  to  the TrueType  `cmap'
    specification.

  - Loading and rendering _individual_  glyphs as nicely as possible
    (either  in  outlines,  bitmaps,  or  pixmaps),  and  performing
    excellent hinting.

  It isn't a high-level text  rendering library, and many tasks will
  have to be performed by higher level layers like:

  - glyph caching (outlines and/or maps)
  - text rendering
  - justification, kerning
  - font mapping
  - rotated/slanted text

  However,  its   API  provides  many  functions   that  ease  these
  operations considerably:

  - Glyph outlines can be copied and transformed easily with the API
    functions, then  rendered to bitmaps or  pixmaps with FreeType's
    scan-line converter, which is very fast.

  - The  glyph loader  places  the  outlines in  order  to ease  the
    process  of text  rendering.  See  the documentation  file named
    `glyphs.htm' (resp. `glyphs.txt') or `ftstring's source for more
    details.

  - The engine gives you access  to several TrueType tables that can
    be  processed  by  your  application to  provide  more  powerful
    rendering (e.g. kerning).

  - It  is   possible,  and  rather  simple,   to  write  separately
    compilable  extensions to  the engine  to access  other TrueType
    tables that  are not  supported by the  engine in  this release.
    This can be handy if one  wants to access data in TrueType GX or
    OpenType fonts  (as these formats comply to  the TrueType `sfnt'
    file storage format,  they can be opened by  the FreeType engine
    as a normal TrueType file).


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7. Can FreeType be ported to other platforms?

  FreeType is written in pure ANSI  C and should compile well on all
  16, 32,  and 64 bits  processors.  Note, however, that  the 16-bit
  port  requires a  large  memory  model, as  some  tables found  in
  TrueType programs  could exceed 64kByte  (this is really  true for
  CJK and  Unicode fonts.  Nearly  all others should work  fine with
  it).

  It  doesn't support  some  very exotic  platforms  though, like  a
  32-bit only  processor (where the  only word size is  32-bit, even
  for chars and  shorts).  However, nothing prevents you  to test it
  by yourself...

  The memory management and  file access routines have been gathered
  in two  components, namely `ttmemory'  and `ttfile', which  can be
  specialized for a specific  system.  Their standard version, found
  in  the `lib'  directory,  uses simply  the  ANSI libc.   However,
  examples  are given of  alternative file  components for  OS/2 and
  Unix in:

    lib/arch/os2/ttfile.c 
    (accessing the low-level OS/2 file API directly)

    lib/arch/unix/ttmmap.c
    (using memory-mapped files for improved access)

  You would certainly need to rewrite these components if you intend
  to use FreeType in an embedded system.


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8. My TrueType font only works on  a Mac/in Windows.  Will I be able
   to use it with FreeType?

  Short Answer: YES, but be cautious!

  If you have read section 9 or  25, you know that a font file might
  have different charMaps entries, used to translate character codes
  to glyph indexes.  The  problem of most `system-specific' TrueType
  fonts (sigh)  is that they  only contain one single  mapping, like
  Apple Roman, or  Windows Glyph List, making it  usable only on the
  platform it was `designed' for.

  The test  program `ftdump'  can be used  to display  the character
  encodings supported  in a font  file.  Most fonts come  with Apple
  Roman and Windows Unicode.

  FreeType can use  every charmap found in a font file,  so it is up
  to your application to choose the one that fits its task best.  If
  you use a font which only provides an Apple Roman charcode, you'll
  probably  have a hard  time rendering  Unicode strings  without an
  additional translation.

  Note that the tool `ttf_edit'  can actually add missing cmaps to a
  TrueType font file.  More info on this can be found at:

    http://www.truetex.com

  Another  possibility is  to  use Just  van  Rossum's TTX  compiler
  (still  beta)  which can  convert  a  TrueType  font into  an  XML
  description and vice versa; it can be found at

    http://www.letterror.com/ttx/

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9. What are the most common character mappings?

  If you don't want to read the TrueType specification, here is some
  information about the  most used char maps.  Each  map is designed
  by a `platform ID', followed by a platform-specific `encoding ID':

  Examples:

    0, 0 : Apple Unicode
    1, 0 : Apple Roman
    3, 0 : Windows Symbol
    3, 1 : Windows Unicode

  Windows and Apple Unicode charmaps differ only in internal storage
  layout.  Both can be used transparently with FreeType.

  Many fonts come also with both Apple Roman and Windows Unicode.


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10. How do I render text with FreeType?

  This is  explained with great  detail in the  glyphs documentation
  file  available in  both text  (glyphs.txt) and  HTML (glyphs.htm,
  including schematics).

  Text rendering isn't difficult.  One can also look at the code for
  `ftstring' or `ftstrtto' to see how it is done.


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11. How do I render rotated/slanted text with FreeType?

  It is  possible to  transform the outlines  returned by  the glyph
  loader, hence producing rotated  or slanted text.  Please read the
  `glyphs' documentation file, which  explains this in great detail,
  as well as some other important things.


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12. How do I compute a glyph bitmap's dimensions?

  You should grid-fit  its bounding box, then compute  its width and
  height.   This  is explained  in  the `bitmaps.txt'  documentation
  file.


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13. Why is text rendering so slow?
    Does FreeType cache glyphs?

  The  FreeType engine  doesn't cache  anything, be  it  outlines or
  bitmaps.  Hence, a program that  renders text by calling the glyph
  loader on each letter is slow.

  Because  caching  cannot  be  performed  in  both  an  _easy_  and
  _portable_  way it  is left  to the  application.   Moreover, some
  graphics systems already  provide some sort of caching,  and it is
  better to take advantage of it rather than re-implementing it.

  The `how's  and `when's of  caching are explained in  the `glyphs'
  documentation file.   The `bitmaps'  documentation file is  also a
  good  source of  information if  you intend  to  render individual
  glyph bitmaps.


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14. Do you always render the same glyphs as Windows or the Mac?

  Short answer: No for rare cases.

  There are a lot of  technical details, too numerous and lengthy to
  be put here, that prevents the FreeType library from matching 100%
  the glyphs produced by the Windows or Macintosh rasterizers.

  This  engine  is a  clean-room  implementation,  and  most of  the
  reasons  why its  development took  so  much time  is the  largely
  deficient TrueType specification published by Apple and Microsoft.
  A number of  key technical aspects are missing,  and we found many
  `undocumented'  features in the  TrueType bytecode,  after several
  months of testing and trying.   We are now very satisfied with the
  current quality, though still try to improve it.

  The glyph metrics (bearings and advances) also match significantly
  those  computed by  Windows,  or found  in  the TrueType  pre-calc
  tables.

  As hinting  glyphs also makes use of  several rounding operations,
  we will be  unable to provide a perfect  clone unless we implement
  the  exact same  computations  _and_ rounding  errors  -- this  is
  very unlikely...


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15. The program `ftlint' reports errors with one of my fonts, though
    it works perfectly under Windows.  What does this mean?

  Associated  to each  glyph  is  a small  `program',  written in  a
  specific  bytecode language,  which is  in charge  of  hinting the
  glyph's outline to make it perfect on screen as on paper.  Some of
  these  programs can be  broken (e.g.,  accessing invalid  areas of
  memory,  performing a  divide by  zero, etc.),  and these  are the
  errors that are reported by `ftlint'.

  They can also mean a  bug in the TrueType bytecode interpreter (or
  more likely an `undocumented'  feature we haven't discovered yet),
  but should now be extremely rare.

  Surprisingly,  even largely distributed  fonts can  contain broken
  glyph programs.  For example:

  - antqua.ttf, glyph 163 (MS Office 4.2):

    The program  tries to  access point number  0x127, which  is too
    much.

  - timesbs, arialbs, courbs (MS Office International):

    The  back-slanted versions of  the MS  core fonts  produce stack
    overflows in  many glyphs, and  other oddities.  It  seems their
    `maximum profile' table is invalid.

  - a ton of `free' fonts, apparently designed with mediocre tools.

  It seems  the Windows TrueType engine doesn't  check its arguments
  often,  and let all  overflows run,  possibly writing  to critical
  portions of  memory.  Please, don't ask  us what this  could do to
  NT 4 :-)

  The FreeType  engine performs checks  on every opcode.   We cannot
  guarantee that it is bullet proof, of course, but it seems to work
  well and catch font bugs accordingly.

  We also have artificially `enlarged' some allocated tables to make
  the engine  work with the back-slanted  fonts without compromising
  security,  but it's  clearly a  hack  we would  have preferred  to
  avoid!


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16. What does the `fttimer' test program benchmarks?

  This  test  program  is  only dedicated  to  profiling  FreeType's
  scan-line converter,  a component also  called `rasterizer', which
  is in charge of converting a vectorial outline into a bitmap, or a
  pixmap.

  It simply  loads all glyphs  of a font  file (by slices of  512 at
  once),  then  converts  them.   Only the  conversion  is  actually
  benchmarked.

  The glyphs  are rendered  at size 400pt  at a resolution  of 96dpi
  (this  is about  500  pixels  high!).  As  you'll  see by  running
  `fttimer', the rasterizer is very fast.


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17. Is it  possible to perform styling (like  oblique, italic, bold,
    underline, etc.) with FreeType?

  Actually, these refer to very different things:

  - Italic  and Bold styles  usually mean  many variations  from the
    `Regular' font.   This is  why you normally  need a  proper font
    file for  each of  these.  For example,  the MS core  font Times
    comes in the following TrueType files:

      TIMES.TTF    Times New Roman Regular
      TIMESI.TTF   Times New Roman Italic
      TIMESB.TTF   Times New Roman Bold
      TIMESBI.TTF  Times New Roman Bold Italic
                   (sometimes named TIMESZ.TTF)

    With FreeType, you simply need the required font file to use it.

  - Oblique style  refers to a  transformation that is applied  to a
    regular font  in order to  make it `slanted', likes  italics do.
    However,  an  italic font  very  frequently  contains small  but
    important variations that cannot  be produced by this method and
    make the font more appealing.

    Slanting  can  easily  be   done  with  a  transformation  under
    FreeType, with the exact same process as rendering rotated text.
    Please  read  the  `glyphs'   documentation  file  where  it  is
    explained in details.

    Usually, Windows or the  Macintosh produce oblique versions of a
    regular  font if  the corresponding  italic TrueType  file isn't
    available.  They also stretch  regular fonts horizontally if the
    bold one isn't available.  All  of this can be done with trivial
    transformations.

  - Underlining  and stroking  are not  really part  of  the glyphs.
    They're simply lines that are  printed on the glyph after it has
    been rendered.   Each TrueType file provides, in  its OS/2 table
    (which  is  accessible through  the  face  object properties  in
    FreeType), several values that  define the position and width of
    those lines, in notional font units.

    If you  want to use them,  you'll have to scale  these values to
    your current instance/point size, then draw the lines yourself.


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18. When does  glyph hinting takes place?   Is it when  the glyph is
    loaded, or when it is converted to a bitmap/pixmap?

  The glyph  loader returns  fitted outlines by  default (it  can be
  asked to return  a non-fitted one, or simply  the original outline
  in notional coordinates too).
     
  This is important to have a glyph's correct metrics, even if it is
  not  to be rendered  immediately, like  when caching  outlines for
  rotated text.


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19. Which is the best, caching outlines or bitmaps?

  It depends on your application, and what it does with text.

  Usually, if  all you need is  render some simple  text at specific
  point sizes, then simply cache the bitmaps or pixmaps.

  However,  if you  want to  do more  advanced things,  like rotated
  text, which  require sub-pixel placement to look  good, you should
  then only  cache the outlines, and transform/place  them as needed
  before sending them to the scan-line converter.

  It's always possible to produce  a bitmap from an outline, and the
  scan-converter is very fast.  It's up to you then...


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20. Can I open OpenType and/or TrueType GX fonts with FreeType?

  TrueType GX  fonts are normal TrueType fonts  with enhanced tables
  and capabilities.  They can always  be opened by a normal TrueType
  engine (like  Windows, the Mac,  or FreeType), but  their improved
  features won't be available.

  On the contrary, OpenType fonts  may vary.  While some may contain
  legal  TrueType glyphs,  an  `otf' file  may  only contain  glyphs
  encoded in  the Type 2 format.   You won't be able  to produce any
  glyphs from such a font file without a dedicated font engine (like
  the ones promised in NT 5 and Java 2D).

  FreeType is  a TrueType  glyph engine and  doesn't support  Type 2
  fonts.  Supporting them would  require a very different philosophy
  and a different core engine (even though they could share an API).

  Note  that  you can  write  extensions  to  the engine  to  access
  supplemental tables defined in these formats (see next question).

  Note that FreeType  version 2 (which is in  alpha stage currently)
  already includes a PostScript Type 1 interpreter...


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21. How  can  I  access  additional TrueType  tables  that  FreeType
    doesn't support?

  You  can  write  an  `engine  extension'.  This  is  a  separately
  compilable  component   which  can  enhance   the  base  library's
  functionalities,   without   recompiling   it.    Some   important
  extensions are provided in the `lib/extend' directory.

  You'll  need   of  course  to  know   the  TrueType  specification
  precisely,  as well  as some  of  the conventions  used to  access
  tables  and to manage  memory.  Read  the documentation  files for
  more details, or contact the developers at:

    devel@freetype.org


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22. When will I be able to use FreeType to display TrueType fonts in
    X11, Wine or OS/2, or even other systems?

  Actually, an  OS/2 replacement for TRUETYPE.DLL  based on FreeType
  is part of this package (to be found in contrib/ftos2).  Thanks go
  to Michal Mecasek (mike@mendelu.cz) for his excellent work!

  At least  three X11  TrueType font servers  are available;  two of
  them  (xfsft and  xtt) are  based on  FreeType, whereas  the third
  (xfstt) uses a separately developed TrueType engine.

  More information about these  servers is available at the FreeType
  homepage (http://www.freetype.org).

  Don't hesitate to contact us if you plan to use or port the engine
  to exotic platforms, we're always interested in helping out.


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23. What does the `free' in FreeType really means?

  The previous  releases of FreeType  (alphas and beta)  were placed
  under the LGPL.  FreeType 1.0, and later releases, come with an an
  alternate  license, inspired  from  the BSD,  Artistic, and  IJG's
  (Independent JPEG Group) ones.  In short:

  - You are  encouraged to  use and distribute  this program  in all
    kinds of products, including commercial ones.

  - You don't have to pay us anything for it (royalty-free).

  - You may not pretend you wrote it, or part of it that you may use
    in one of  your product, and you have to  state clearly that you
    use the FreeType code if you distribute products based on it, or
    parts of it (credits clause).

  - This source code is provided AS IS, with no warranty whatsoever,
    and we cannot promise support for it.

  The exact and legal terms are in the file `license.txt'.

  Enjoy ;-)
     

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24. Does FreeType support threads?  Is it re-entrant?

  Short answer: Basically yes, but not fully tested.

  We have changed  some code since FreeType 1.0  in order to support
  multi-threaded environments.  However,  these have not been tested
  yet.  Apparently,  the thread-safe  and reentrant builds  now work
  well  perfectly  once compiled;  however,  no serious  concurrency
  testing has been performed (of course, a serious lock analysis was
  done in order to modify the source).

  Right now, the mutex management  functions are all gathered in the
  component  `ttmutex', which  has to  be specialized  for  your own
  system if a thread build will be necessary.


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25. Does FreeType support `foreign languages'?

  Short Answer: YES, it does!

  From a TrueType  font file point of view,  there are several parts
  to  the  file, one  of  them  being  the `glyphs',  i.e.,  picture
  representations of the symbols.

  Another part is the mapping table, also called `charMap'.

  For example, glyph  #1 could be letter `A', and  glyph #2 could be
  letter `Z'.  Glyphs can be stored in any order in a font file.

  The  mapping tables  contain  at least  one  char-map entry.   For
  example, you could have an ASCII-map that maps character code 0x41
  to glyph #1, and code 0x5A  to glyph #2, etc.  FreeType provides a
  `charMap' object class to access and use this information easily.

  There are  several character  encodings recognized and  defined by
  the TrueType specification,  like Latin-1, Unicode, Apple Scripts,
  WGL, etc., but a font file might only contain one or two of them.

  When using a more `exotic' character encoding like EBCDIC (this is
  IBM mainframe  stuff!), you would need  to translate it  to one of
  the available  formats (or  to add a  charmap table to  the font).
  Cf. section 8.


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26. I'm using fractional char sizes, but the glyphs stay at the same
    integer size.  Why?

  Because hinting  only works well with integer  pixel sizes.  There
  is a flag that can be set in a TrueType font file to force integer
  pixel size scaling,  even if you ask for  fractional sizes with an
  API  function like  TT_Set_Instance_CharSize().  Needless  to say,
  nearly all fonts have the flag set.  So this is normal.

  Also,  notice that when  giving a  charsize in  fractional points,
  this will be converted to integer pixels with the formula:

              pixel_size = char_size * y_resolution / 72

  For example, using a resolution of 96 dpi, we would have:

    9.5 pts => 12.666 pixels   => 13 pixels
   10.0 pts => 13.333 pixels   => 13 pixels, i.e. same size
   10.5 pts => 14     pixels   => 14 pixels
   11.0 pts => 14.666 pixels   => 15 pixels
   11.5 pts => 15.333 pixels   => 15 pixels, i.e. same size

  If you  want to control the  exact size in pixels  of your glyphs,
  simply use a resolution of 72dpi, where char_size = pixel_size.


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27. Hinting doesn't work at very small sizes.  Why?

  This is normal.  There are very good technical reasons why hinting
  doesn't  work well  at sizes  under  7 ppem,  usually meaning  the
  appearance of ugly spikes and  glyph distortions.  This is why the
  engine disables hinting by default at such low sizes.


--- end of FAQ ---