NAME

       Font::TTF::Manual - Information regarding the whole module set

INTRODUCTION

       This document looks at the whole issue of how the various modules in the TrueType Font work together. As
       such it is partly information on this font system and partly information on TrueType fonts in general.

       Due to the inter-relation between so many tables in a TrueType font, different tables will make
       expectations as to which other tables exist. At the very least a font should consist of a "head" table
       and a "maxp" table. The system has been designed around the expectation that the necessary tables for
       font rendering in the Windows environment exist. But inter table dependencies have been kept to what are
       considered necessary.

       This module set is not meant as a simple to use, mindless, font editing suite, but as a low-level, get
       your hands dirty, know what you are doing, set of classes for those who understand the intricacies (and
       there are many) of TrueType fonts. To this end, if you get something wrong in the data structures, etc.
       then this module set won't tell you and will happily create fonts which don't work.

       At the time of writing, not every TrueType table in existence has been implemented! Only the core basic
       tables of TrueType 1.0 (i.e. no embedded bitmap tables, no postscript type tables, no OpenType tables and
       no GX tables) have been implemented. If you want to help by implementing another table or two, then
       please go ahead and send me your code. For a full list of tables, see Font::TTF::Font.

   Design Principles
       PERL is not C++. C++ encourages methods to be written for changing and reading each instance variable in
       a class. If we did this in this PERL program the results would be rather large and slow. Instead, since
       most access will be read access, we expose as much of the inner storage of an object to user access
       directly via hash lookup. The advantage this gives are great. For example, by following an instance
       variable chain, looking up the "yMax" parameter for a particular glyph becomes:

           $f->{'loca'}{'glyphs'}[$glyph]{'yMax'}

       Or, if we are feeling very lazy and don't mind waiting:

           $f->{'loca'}{'glyphs'}[$f->{'cmap'}->ms_lookup(0x41)]{'yMax'}

       The disadvantage of this method is that it behoves module users to behave themselves. Thus it does not
       hold your hand and ensure that if you make a change to a table, that the table is marked as dirty, or
       that other tables are updated accordingly.

       It is up to the application developer to understand the implications of the changes they make to a font,
       and to take the necessary action to ensure that the data they get out is what they want. Thus, you could
       go and change the "yMax" value on a glyph and output a new font with this change, but it is up to you to
       ensure that the font's bounding box details in the "head" table are correct, and even that your changing
       "yMax" is well motivated.

       To help with using the system, each module (or table) will not only describe the methods it supports,
       which are relatively few, but also the instance variables it supports, which are many. Most of the
       variables directly reflect table attributes as specified in the OpenType specification, available from
       Microsoft (<http://www.microsoft.com/typography>), Adobe and Apple. A list of the names used is also
       given in each module, but not necessarily with any further description. After all, this code is not a
       TrueType manual as well!

   Conventions
       There are various conventions used in this system.

       Firstly we consider the documentation conventions regarding instance variables.  Each instance variable
       is marked indicating whether it is a (P)rivate variable which users of the module are not expected to
       read and certainly not write to or a (R)ead only variable which users may well want to read but not write
       to.

METHODS

       This section examines various methods and how the various modules work with these methods.

   read and read_dat
       Before the data structures for a table can be accessed, they need to be filled in from somewhere. The
       usual way to do this is to read an existing TrueType font. This may be achieved by:

           $f = Font::TTF::Font->open($filename) || die "Unable to read $filename";

       This will open an existing font and read its directory header. Notice that at this point, none of the
       tables in the font have been read. (Actually, the "head" and "maxp" tables are read at this point too
       since they contain the commonly required parameters of):

           $f->{'head'}{'unitsPerEm'}
           $f->{'maxp'}{'numGlyphs'}

       In order to be able to access information from a table, it is first necessary to "read" it. Consider
       trying to find the advance width of a space character (U+0020). The following code should do it:

           $f = Font::TTF::Font->open($ARGV[0]);
           $snum = $f->{'cmap'}->ms_lookup(0x0020);
           $sadv = $f->{'hmtx'}{'advance'}[$snum];
           print $sadv;

       This would result in the value zero being printed, which is far from correct.  But why? The first line
       would correctly read the font directory. The second line would, incidently, correctly locate the space
       character in the Windows cmap (assuming a non symbol encoded font). The third line would not succeed in
       its task since the "hmtx" table has not been filled in from the font file. To achieve what we want we
       would first need to cause it to be read:

           $f->{'hmtx'}->read;
           $sadv = $f->{'hmtx'}{'advance'}[$snum];

       Or for those who are too lazy to write multiple lines, "read" returns the object it reads. Thus we could
       write:

           $sadv = $f->{'hmtx'}->read->{'advance'}[$snum];

       Why, if we always have to read tables before accessing information from them, did we not have to do this
       for the "cmap" table? The answer lies in the method call. It senses that the table hasn't been read and
       reads it for us. This will generally happen with all method calls, it is only when we do direct data
       access that we have to take the responsibility to read the table first.

       Reading a table does not necessarily result in all the data being placed into internal data structures.
       In the case of a simple table "read" is sufficient.  In fact, the normal case is that "read_dat" reads
       the data from the file into an instance variable called ' dat' (including the space) and not into the
       data structures.

       This is true except for the "glyph" class which represents a single glyph. Here the process is reversed.
       Reading a "glyph" reads the data for the glyph into the ' dat' instance variable and sets various header
       attributes for the glyph ("xMin", "numContours", etc.). The data is converted out of the variable into
       data structures via the "read_dat" method.

       The aim, therefore, is that "read" should do the natural thing (read into data structures for those
       tables and elements for which it is helpful -- all except "glyph" at present) and "read_dat" should do
       the unnatural thing: read just the binary data for normal tables and convert binary data to data
       structures for "glyph"s.

       In summary, therefore, use "read" unless you want to hack around with the internals of glyphs in which
       case see Font::TTF::Glyph for more details.

   update
       The aim of this method is to allow the various data elements in a "read" font to update themselves. All
       tables know how to update themselves. All tables also contain information which cannot be updated but is
       new knowledge in the font.  As a result, certain tables do nothing when they are updated. We can,
       therefore, build an update hierarchy of tables, with the independent tables at the bottom and "Font" at
       the top:

              +--loca
              |
        glyf--+--maxp
              |
              +---+--head
                  |
        hmtx------+--hhea

        cmap-----OS/2


        name--
        post--
       There is an important universal dependency which it is up to the user to
       keep up to date. This is C<maxp/numOfGlyphs> which is used to iterate over all
       the glyphs. Note that the glyphs themselves are not held in the C<glyph> table
       but in the C<loca> table, so adding glyphs, etc. automatically involves keeping
       the C<loca> table up to date.

   Creating fonts
       Suppose we were creating a font from scratch. How much information do we need to supply and how much will
       "update" do for us?

       The following information is required:

           $f->{'loca'}{'glyphs'}
           $f->{'head'}{'upem'}
           $f->{'maxp'}{'numGlyphs'}   (doesn't come from $f->{'loca'}{'glyphs'})
           $f->{'hmtx'}{'advance'}
           $f->{'post'}['format'}
           $f->{'post'}{'VAL'}
           $f->{'cmap'}
           $f->{'name'}

       Pretty much everything else is calculated for you. Details of what is needed for a glyph may be found in
       Font::TTF::Glyph. Once we have all the information we need (and there is lots more that you could add)
       then we simply

           $f->dirty;          # mark all tables dirty
           $f->update;         # update the font

AUTHOR

       Martin Hosken <http://scripts.sil.org/FontUtils>.  (see CONTRIBUTORS for other authors).

LICENSING

       Copyright (c) 1998-2014, SIL International (http://www.sil.org)

       This module is released under the terms of the Artistic License 2.0.  For details, see the full text of
       the license in the file LICENSE.