Fix handling of non-square pixels, part 4.

[ttfautohint.git] / doc / ttfautohint-1.pandoc

% ttfautohint
% Werner Lemberg
%

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Introduction
============

**ttfautohint** is a library written in\ C that takes a TrueType font as
the input, removes its bytecode instructions (if any), and returns a new
font where all glyphs are bytecode hinted using the information given by
FreeType's autohinting module.  The idea is to provide the excellent quality
of the autohinter on platforms that don't use FreeType.

The library has a single API function, `TTF_autohint`, which is described
[below](#the-ttfautohint-api).

Bundled with the library there are two front-end programs, [`ttfautohint`
and `ttfautohintGUI`](#ttfautohint-and-ttfautohintgui), being a command line
and an application with a Graphics User Interface (GUI), respectively.


What exactly are hints?
-----------------------

To cite [Wikipedia](http://en.wikipedia.org/wiki/Font_hinting):

> **Font hinting** (also known as **instructing**) is the use of
> mathematical instructions to adjust the display of an outline font so that
> it lines up with a rasterized grid.  At low screen resolutions, hinting is
> critical for producing a clear, legible text.  It can be accompanied by
> antialiasing and (on liquid crystal displays) subpixel rendering for
> further clarity.

and Apple's [TrueType Reference
Manual](https://developer.apple.com/fonts/TTRefMan/RM03/Chap3.html#features):

> For optimal results, a font instructor should follow these guidelines:
>
>  - At small sizes, chance effects should not be allowed to magnify small
>    differences in the original outline design of a glyph.
>
>  - At large sizes, the subtlety of the original design should emerge.


In general, there are three possible ways to hint a glyph.

 1. The font contains hints (in the original sense of this word) to guide
    the rasterizer, telling it which shapes of the glyphs need special
    consideration.  The hinting logic is partly in the font and partly in
    the rasterizer.  More sophisticated rasterizers are able to produce
    better rendering results.

    This is how Type\ 1 and CFF hints work.

 2. The font contains exact instructions (also called *bytecode*) on how to
    move the points of its outlines, depending on the resolution of the
    output device, and which intentionally distort the (outline) shape to
    produce a well-rasterized result.  The hinting logic is in the font;
    ideally, all rasterizers simply process these instructions to get the
    same result on all platforms.

    This is how TrueType hints work.

 3. The font gets auto-hinted (at run-time).  The hinting logic is
    completely in the rasterizer.  No hints in the font are used or needed;
    instead, the rasterizer scans and analyzes the glyphs to apply
    corrections by itself.

    This is how FreeType's auto-hinter works; see
    [below](#background-and-technical-details) for more.


What problems can arise with TrueType hinting?
----------------------------------------------

While it is relatively easy to specify PostScript hints (either manually or
by an auto-hinter that works at font creation time), creating TrueType
hints is far more difficult.  There are at least two reasons:

  - TrueType instructions form a programming language, operating at a very
    low level.  They are comparable to assembler code, thus lacking all
    high-level concepts to make programming more comfortable.

    Here an example how such code looks like:

    ```
        SVTCA[0]
        PUSHB[ ]  /* 3 values pushed */
        18 1 0
        CALL[ ]
        PUSHB[ ]  /* 2 values pushed */
        15 4
        MIRP[01001]
        PUSHB[ ]  /* 3 values pushed */
        7 3 0
        CALL[ ]
    ```

    Another major obstacle is the fact that font designers usually aren't
    programmers.

  - It is very time consuming to manually hint glyphs.  Given that the
    number of specialists for TrueType hinting is very limited, hinting a
    large set of glyphs for a font or font family can become very expensive.


Why ttfautohint?
----------------

The ttfautohint library brings the excellent quality of FreeType rendering
to platforms that don't use FreeType, yet require hinting for text to look
good -- like Microsoft Windows.  Roughly speaking, it converts the glyph
analysis done by FreeType's auto-hinting module to TrueType bytecode.
Internally, the auto-hinter's algorithm resembles PostScript hinting
methods; it thus combines all three hinting methods discussed
[previously](#what-exactly-are-hints).

The simple interface of the front-ends (both on the command line and with
the GUI) allows quick hinting of a whole font with a few mouse clicks or a
single command on the prompt.  As a result, you get better rendering results
with web browsers, for example.

Across Windows rendering environments today, fonts processed with
ttfautohint look best with ClearType enabled.  This is the default for
Windows\ 7.  Good visual results are also seen in recent MacOS\ X versions
and GNU/Linux systems that use FreeType for rendering.

The goal of the project is to generate a 'first pass' of hinting that font
developers can refine further for ultimate quality.



`ttfautohint` and `ttfautohintGUI`
==================================

On all supported platforms (GNU/Linux, Windows, and Mac OS\ X), the GUI
looks quite similar; the used toolkit is [Qt], which in turn uses the
platform's native widgets.

![`ttfautohintGUI` on GNU/Linux running KDE](img/ttfautohintGUI.png)

Both the GUI and console version share the same features, to be discussed in
the next subsection.

**Warning: ttfautohint cannot always process a font a second time.**
If the font contains composite glyphs, and option `-c` is used,
reprocessing with ttfautohint will fail.  For this reason it is strongly
recommended to *not* delete the original, unhinted font so that you can
always rerun ttfautohint.


Calling `ttfautohint`
---------------------

```
    ttfautohint [OPTION]... [IN-FILE [OUT-FILE]]
```

The TTY binary, `ttfautohint`, works like a Unix filter, this is, it reads
data from standard input if no input file name is given, and it sends its
output to standard output if no output file name is specified.

A typical call looks like the following.

```
    ttfautohint -v -f latn foo.ttf foo-autohinted.ttf
```

For demonstration purposes, here the same using a pipe and redirection.
Note that Windows's default command line interpreter, `cmd.exe`, doesn't
support piping with binary files, unfortunately.

```
    cat foo.ttf | ttfautohint -v -f latn > foo-autohinted.ttf
```


Calling `ttfautohintGUI`
------------------------

```
    ttfautohintGUI [OPTION]...
```

`ttfautohintGUI` doesn't send any output to a console; however, it accepts
the same command line options as `ttfautohint`, setting default values for
the GUI.


Options
-------

Long options can be given with one or two dashes, and with and without an
equal sign between option and argument.  This means that the following forms
are acceptable: `-foo=`*bar*, `--foo=`*bar*, `-foo`\ *bar*, and
`--foo`\ *bar*.

Below, the section title refers to the command's label in the GUI, then
comes the name of the corresponding long command line option and its short
equivalent, followed by a description.

Background and technical details on the meaning of the various options are
given [afterwards](#background-and-technical-details).

### Hint Set Range Minimum, Hint Set Range Maximum

See ['Hint Sets'](#hint-sets) for a definition and explanation.

`--hinting-range-min=`*n*, `-l`\ *n*
:   The minimum PPEM value (in pixels) at which hint sets are created.  The
    default value for *n* is\ 8.

`--hinting-range-max=`*n*, `-r`\ *n*
:   The maximum PPEM value (in pixels) at which hint sets are created.  The
    default value for *n* is 50.

### Fallback Script

`--fallback-script=`*s*, `-f`\ *s*
:   Set fallback script to tag *s*, which is a string consisting of four
    characters like `latn` or `dflt`.  It gets used for for all glyphs that
    can't be assigned to a script automatically.  See [below](#scripts) for
    more details.

### Hinting Limit

`--hinting-limit=`*n*, `-G`\ *n*
:   The *hinting limit* is the PPEM value (in pixels) where hinting gets
    switched off (using the `INSTCTRL` bytecode instruction); it has zero
    impact on the file size.  The default value for *n* is 200, which means
    that the font is not hinted for PPEM values larger than 200.

    Note that hinting in the range 'hinting-range-max' up to 'hinting-limit'
    uses the hinting configuration for 'hinting-range-max'.

    To omit a hinting limit, use `--hinting-limit=0` (or check the 'No
    Hinting Limit' box in the GUI).  Since this will cause internal math
    overflow in the rasterizer for large pixel values (>\ 1500px approx.) it
    is strongly recommended to not use this except for testing purposes.

### x Height Increase Limit

`--increase-x-height=`*n*, `-x`\ *n*
:   Normally, ttfautohint rounds the x\ height to the pixel grid, with a
    slight preference for rounding up.  If this flag is set, values in the
    range 6\ PPEM to *n*\ PPEM are much more often rounded up.  The default
    value for *n* is 14.  Use this flag to increase the legibility of small
    sizes if necessary; you might get weird rendering results otherwise for
    glyphs like 'a' or 'e', depending on the font design.

    To switch off this feature, use `--increase-x-height=0` (or check the
    'No x\ Height Increase' box in the GUI).  To switch off rounding the
    x\ height to the pixel grid in general, either partially or completely,
    see ['x Height Snapping Exceptions'](#x-height-snapping-exceptions).

    The following images again use the font 'Mertz Bold'.

    ![At 17px, without option `-x` and '`-w ""`', the hole in glyph 'e'
      looks very grey in the FontForge snapshot, and the GDI ClearType
      rendering (which is the default on older Windows versions) fills it
      completely with black because it uses B/W rendering along the y\ axis.
      FreeType's 'light' autohint mode (which corresponds to ttfautohint's
      'smooth' stem width algorithm) intentionally aligns horizontal lines
      to non-integer (but still discrete) values to avoid large glyph shape
      distortions.](img/e-17px-x14.png)

    ![The same, this time with option `-x 17` (and
      '`-w ""`').](img/e-17px-x17.png)

### x Height Snapping Exceptions

`--x-height-snapping-exceptions=`*string*, `-X`\ *string*
:   A list of comma separated PPEM values or value ranges at which no
    x-height snapping shall be applied.  A value range has the form
    *value1*`-`*value2*, meaning *value1*\ <= PPEM <=\ *value2*.  *value1*
    or *value2* (or both) can be missing; a missing value is replaced by the
    beginning or end of the whole interval of valid PPEM values,
    respectively (6\ to 32767).  Whitespace is not significant; superfluous
    commas are ignored, and ranges must be specified in increasing order.
    For example, the string `"7-9, 11, 13-"` means the values 7, 8, 9, 11,
    13, 14, 15, etc.  Consequently, if the supplied argument is `"-"`, no
    x-height snapping takes place at all.  The default is the empty string
    (`""`), meaning no snapping exceptions.

    Normally, x-height snapping means a slight increase in the overall
    vertical glyph size so that the height of lowercase glyphs gets aligned
    to the pixel grid (this is a global feature, affecting *all* glyphs of a
    font).  However, having larger vertical glyph sizes is not always
    desired, especially if it is not possible to adjust the `usWinAscent`
    and `usWinDescent` values from the font's `OS/2` table so that they are
    not too tight.  See ['Windows Compatibility'](#windows-compatibility)
    for more details.

### Windows Compatibility

`--windows-compatibility`, `-W`
:   This option makes ttfautohint add two artificial blue zones, positioned
    at the `usWinAscent` and `usWinDescent` values (from the font's `OS/2`
    table).  The idea is to help ttfautohint so that the hinted glyphs stay
    within this horizontal stripe since Windows clips everything falling
    outside.

    There is a general problem with tight values for `usWinAscent` and
    `usWinDescent`; a good description is given in the [Vertical Metrics
    How-To](http://typophile.com/node/13081).  Additionally, there is a
    special problem with tight values if used in combination with
    ttfautohint because the auto-hinter tends to slightly increase the
    vertical glyph dimensions at smaller sizes to improve legibility.  This
    enlargement can make the heights and depths of glyphs exceed the range
    given by `usWinAscent` and `usWinDescent`.

    If ttfautohint is part of the font creation tool chain, and the font
    designer can adjust those two values, a better solution instead of using
    option `-W` is to reserve some vertical space for 'padding': For the
    auto-hinter, the difference between a top or bottom outline point before
    and after hinting is less than 1px, thus a vertical padding of 2px is
    sufficient.  Assuming a minimum hinting size of 6ppem, adding two pixels
    gives an increase factor of 8÷6 = 1.33.  This is near to the default
    baseline-to-baseline distance used by TeX and other sophisticated text
    processing applications, namely 1.2×designsize, which gives satisfying
    results in most cases.  It is also near to the factor 1.25 recommended
    in the abovementioned how-to.  For example, if the vertical extension of
    the largest glyph is 2000 units (assuming that it approximately
    represents the designsize), the sum of `usWinAscent` and `usWinDescent`
    could be 1.25×2000 = 2500.

    In case ttfautohint is used as an auto-hinting tool for fonts that can
    be no longer modified to change the metrics, option `-W` in combination
    with '`-X "-"`' to suppress any vertical enlargement should prevent
    almost all clipping.

### Pre-Hinting

`--pre-hinting`, `-p`
:   *Pre-hinting* means that a font's original bytecode is applied to all
    glyphs before it is replaced with bytecode created by ttfautohint.
    This makes only sense if your font already has some hints in it that
    modify the shape even at EM size (normally 2048px); for example, some
    CJK fonts need this because the bytecode is used to scale and shift
    subglyphs.  For most fonts, however, this is not the case.

### Hint Composites

`--composites`, `-c`
:   By default, the components of a composite glyph get hinted separately.
    If this flag is set, the composite glyph itself gets hinted (and the
    hints of the components are ignored).  Using this flag increases the
    bytecode size a lot, however, it might yield better hinting results.

    If this option is used (and a font actually contains composite glyphs),
    ttfautohint currently cannot reprocess its own output for technical
    reasons, see [below](#the-.ttfautohint-glyph).

### Symbol Font

`--symbol`, `-s`
:   Apply default values for standard (horizontal) stem width and height
    instead of deriving them from a script-specific, hard-coded default
    character (which usually resembles the shape of a lowercase 'o').  Use
    this option (usually in combination with option `--fallback-script`) to
    hint symbol or dingbat fonts or math glyphs, for example, that lack a
    default character, at the expense of possibly poor hinting results at
    small sizes.

### Dehint

`--dehint`, `-d`
:   Strip off all hints without generating new hints.  Consequently, all
    other hinting options are ignored.  This option is intended for testing
    purposes.

### Add ttfautohint Info

`--no-info`, `-n`
:   Don't add ttfautohint version and command line information to the
    version string or strings (with name ID\ 5) in the font's `name` table.
    In the GUI it is similar: If you uncheck the 'Add ttfautohint info' box,
    information is not added to the `name` table.  Except for testing and
    development purposes it is strongly recommended to not use this option.

### Strong Stem Width and Positioning

`--strong-stem-width=`*string*, `-w`\ *string*
:   ttfautohint offers two different routines to handle (horizontal) stem
    widths and stem positions: 'smooth' and 'strong'.  The former uses
    discrete values that slightly increase the stem contrast with almost no
    distortion of the outlines, while the latter snaps both stem widths and
    stem positions to integer pixel values as much as possible, yielding a
    crisper appearance at the cost of much more distortion.

    These two routines are mapped onto three possible rendering targets:

    - grayscale rendering, with or without optimization for subpixel
      positioning (e.g. Mac OS\ X)

    - 'GDI ClearType' rendering: the rasterizer version, as returned by the
      GETINFO bytecode instruction, is in the range 36\ <= version <\ 38 and
      ClearType is enabled (e.g. Windows XP)

    - 'DirectWrite ClearType' rendering: the rasterizer version, as returned
      by the GETINFO bytecode instruction, is >=\ 38, ClearType is enabled,
      and subpixel positioning is enabled also (e.g. Internet Explorer\ 9
      running on Windows\ 7)

    GDI ClearType uses a mode similar to B/W rendering along the vertical
    axis, while DW ClearType applies grayscale rendering.  Additionally,
    only DW ClearType provides subpixel positioning along the x\ axis.  For
    what it's worth, the rasterizers version\ 36 and version\ 38 in
    Microsoft Windows are two completely different rendering engines.

    The command line option expects *string* to contain up to three letters
    with possible values '`g`' for grayscale, '`G`' for GDI ClearType, and
    '`D`' for DW ClearType.  If a letter is found in *string*, the strong
    stem width routine is used for the corresponding rendering target (and
    smooth stem width handling otherwise).  The default value is '`G`', which
    means that strong stem width handling is activated for GDI ClearType
    only.  To use smooth stem width handling for all three rendering
    targets, use the empty string as an argument, usually connoted with
    '`""`'.

    In the GUI, simply set the corresponding check box to select the strong
    width routine for a given rendering target.  If you unset the check box,
    the smooth width routine gets used.

    The following FontForge snapshot images use the font ['Mertz
    Bold'](http://code.newtypography.co.uk/mertz-sans/) (still under
    development) from [Vernon Adams].

    ![The left part shows the glyph 'g' unhinted at 26px, the right part
     with hints, using the 'smooth' stem algorithm.](img/ff-g-26px.png)

    ![The same, but this time using the 'strong'
     algorithm.  Note how the stems are aligned to the pixel
     grid.](img/ff-g-26px-wD.png)

### Font License Restrictions

`--ignore-restrictions`, `-i`
:   By default, fonts that have bit\ 1 set in the 'fsType' field of the
    `OS/2` table are rejected.  If you have a permission of the font's legal
    owner to modify the font, specify this command line option.

    If this option is not set, `ttfautohintGUI` shows a dialogue to handle
    such fonts if necessary.

### Miscellaneous

`--help`, `-h`
:   On the console, print a brief documentation on standard output and exit.
    This doesn't work with `ttfautohintGUI` on MS Windows.

`--version`, `-v`
:   On the console, print version information on standard output and exit.
    This doesn't work with `ttfautohintGUI` on MS Windows.

`--debug`
:   Print *a lot* of debugging information on standard error while
    processing a font (you should redirect stderr to a file).  This
    doesn't work with `ttfautohintGUI` on MS Windows.



Background and Technical Details
================================

[Real-Time Grid Fitting of Typographic
Outlines](http://www.tug.org/TUGboat/tb24-3/lemberg.pdf) is a scholarly
paper that describes FreeType's auto-hinter in some detail.  Regarding the
described data structures it is slightly out of date, but the algorithm
itself hasn't changed.

The next few subsections are mainly based on this article, introducing some
important concepts.  Note that ttfautohint only does hinting along the
vertical direction (this is, modifying y\ coordinates).


Segments and Edges
------------------

A glyph consists of one or more *contours* (this is, closed curves).  For
example, glyph 'O' consists of two contours, while glyph 'I' has only one.

![The letter 'O' has two contours, an inner and an outer one, while letter
  'I' has only an outer contour.](img/o-and-i)

A *segment* is a series of consecutive points of a contour (including its
Bézier control points) that are approximately aligned along a coordinate
axis.

![A serif.  Contour and control points are represented by squares and
  circles, respectively.  The bottom 'line' DE is approximately aligned
  along the horizontal axis, thus it forms a segment of 7\ points.  Together
  with the two other horizontal segments, BC and FG, they form two edges
  (BC+FG, DE).](img/segment-edge)

An *edge* corresponds to a single coordinate value on the main dimension
that collects one or more segments (allowing for a small threshold).  While
finding segments is done on the unscaled outline, finding edges is bound to
the device resolution.  See [below](#hint-sets) for an example.

The analysis to find segments and edges is specific to a script.


Feature Analysis
----------------

The auto-hinter analyzes a font in two steps.  Right now, everything
described below happens for the horizontal axis only, providing vertical
hinting.

  * Global Analysis

    This affects the hinting of all glyphs, trying to give them a uniform
    appearance.

      + Compute standard stem widths and heights of the font.  The values
        are normally taken from a glyph that resembles letter 'o'.

      + Compute blue zones, see [below](#blue-zones).

    If stem widths and heights of single glyphs differ by a large value, or
    if ttfautohint fails to find proper blue zones, hinting becomes quite
    poor, leading even to severe shape distortions.


Table: script-specific standard characters of the 'latin' module

  script    standard character
  --------  --------------------
  `cyrl`    'о', U+043E, CYRILLIC SMALL LETTER O
  `grek`    'ο', U+03BF, GREEK SMALL LETTER OMICRON
  `hebr`    'ם', U+05DD, HEBREW LETTER FINAL MEM
  `latn`    'o', U+006F, LATIN SMALL LETTER O


  * Glyph Analysis

    This is a per-glyph operation.

      + Find segments and edges.

      + Link edges together to find stems and serifs.  The abovementioned
        paper gives more details on what exactly constitutes a stem or a
        serif and how the algorithm works.


Blue Zones
----------

![Two blue zones relevant to the glyph 'a'.  Vertical point coordinates of
  *all* glyphs within these zones are aligned, provided the blue zone is
  active (this is, its vertical size is smaller than
  3/4\ pixels).](img/blue-zones)

Outlines of certain characters are used to determine *blue zones*.  This
concept is the same as with Type\ 1 fonts: All glyph points that lie in
certain small horizontal zones get aligned vertically.

Here a series of tables that show the blue zone characters of the latin
module's available scripts; the values are hard-coded in the source code.


Table: `latn` blue zones

  ID    Blue zone                              Characters
  ----  -----------                            ------------
  1     top of capital letters                 THEZOCQS
  2     bottom of capital letters              HEZLOCUS
  3     top of 'small f' like letters          fijkdbh
  4     top of small letters                   xzroesc
  5     bottom of small letters                xzroesc
  6     bottom of descenders of small letters  pqgjy


The 'round' characters (e.g. 'OCQS') from Zones 1, 2, and 5 are also used to
control the overshoot handling; to improve rendering at small sizes, zone\ 4
gets adjusted to be on the pixel grid; cf. the [`--increase-x-height`
option](#x-height-increase-limit).


Table: `grek` blue zones

  ID    Blue zone                              Characters
  ----  -----------                            ------------
  1     top of capital letters                 ΓΒΕΖΘΟΩ
  2     bottom of capital letters              ΒΔΖΞΘΟ
  3     top of 'small beta' like letters       βθδζλξ
  4     top of small letters                   αειοπστω
  5     bottom of small letters                αειοπστω
  6     bottom of descenders of small letters  βγημρφχψ


Table: `cyrl` blue zones

  ID    Blue zone                              Characters
  ----  -----------                            ------------
  1     top of capital letters                 БВЕПЗОСЭ
  2     bottom of capital letters              БВЕШЗОСЭ
  3     top of small letters                   хпншезос
  4     bottom of small letters                хпншезос
  5     bottom of descenders of small letters  руф


Table: `hebr` blue zones

  ID    Blue zone                              Characters
  ----  -----------                            ------------
  1     top of letters                         בדהחךכםס
  2     bottom of letters                      בטכםסצ
  3     bottom of descenders of letters        קךןףץ


![This image shows the relevant glyph terms for vertical blue zone
  positions.](img/glyph-terms)


Grid Fitting
------------

Aligning outlines along the grid lines is called *grid fitting*.  It doesn't
necessarily mean that the outlines are positioned *exactly* on the grid,
however, especially if you want a smooth appearance at different sizes. 
This is the central routine of the auto-hinter; its actions are highly
dependent on the used script.  Currently, only support for scripts that work
similarly to Latin (e.g. Greek or Cyrillic) is available.

  * Align edges linked to blue zones.

  * Fit edges to the pixel grid.

  * Align serif edges.

  * Handle remaining 'strong' points.  Such points are not part of an edge
    but are still important for defining the shape.  This roughly
    corresponds to the `IP` TrueType instruction.

  * Everything else (the 'weak' points) is handled with an `IUP`
    instruction.

The following images illustrate the hinting process, using glyph 'a' from
the freely available font ['Ubuntu Book'](http://font.ubuntu.com).  The
manual hints were added by [Dalton Maag Ltd], the used application to create
the hinting debug snapshots was [FontForge].

![Before hinting.](img/a-before-hinting.png)

![After hinting, using manual hints.](img/a-after-hinting.png)

![After hinting, using ttfautohint.  Note that the hinting process
  doesn't change horizontal positions.](img/a-after-autohinting.png)


Hint Sets
---------

In ttfautohint terminology, a *hint set* is the *optimal* configuration for
a given PPEM (pixel per EM) value.

In the range given by the `--hinting-range-min` and `--hinting-range-max`
options, ttfautohint creates hint sets for every PPEM value.  For each
glyph, ttfautohint automatically determines if a new set should be emitted
for a PPEM value if it finds that it differs from a previous one.  For some
glyphs it is possible that one set covers, say, the range 8px-1000px, while
other glyphs need 10 or more such sets.

In the PPEM range below `--hinting-range-min`, ttfautohint always uses just
one set, in the PPEM range between `--hinting-range-max` and
`--hinting-limit`, it also uses just one set.

One of the hinting configuration parameters is the decision which segments
form an edge.  For example, let us assume that two segments get aligned on a
single horizontal edge at 11px, while two edges are used at 12px.
This change makes ttfautohint emit a new hint set to accomodate this
situation.

The next images illustrate this, using a Cyrillic letter (glyph 'afii10108')
from the 'Ubuntu book' font, processed with ttfautohint.

![Before hinting, size 11px.](img/afii10108-11px-before-hinting.png)

![After hinting, size 11px.  Segments 43-27-28 and 14-15 are aligned on a
  single edge, as are segments 26-0-1 and
  20-21.](img/afii10108-11px-after-hinting.png)

![Before hinting, size 12px.](img/afii10108-12px-before-hinting.png)

![After hinting, size 12px.  The segments are not aligned.  While
  segments 43-27-28 and 20-21 now have almost the same horizontal position,
  they don't form an edge because the outlines passing through the segments
  point into different directions.](img/afii10108-12px-after-hinting.png)

Obviously, the more hint sets get emitted, the larger the bytecode
ttfautohint adds to the output font.  To find a good value\ *n* for
`--hinting-range-max`, some experimentation is necessary since *n* depends
on the glyph shapes in the input font.  If the value is too low, the hint
set created for the PPEM value\ *n* (this hint set gets used for all larger
PPEM values) might distort the outlines too much in the PPEM range given
by\ *n* and the value set by `--hinting-limit` (at which hinting gets
switched off).  If the value is too high, the font size increases due to
more hint sets without any noticeable hinting effects.

Similar arguments hold for `--hinting-range-min` except that there is no
lower limit at which hinting is switched off.

An example.  Let's assume that we have a hinting range 10\ <= ppem <=\ 100,
and the hinting limit is set to 250.  For a given glyph, ttfautohint finds
out that four hint sets must be computed to exactly cover this hinting
range: 10-15, 16-40, 41-80, and 81-100.  For ppem values below 10ppem, the
hint set covering 10-15ppem is used, for ppem values larger than 100 the
hint set covering 81-100ppem is used.  For ppem values larger than 250, no
hinting gets applied.


The '\.ttfautohint' Glyph
-------------------------

If [option `--composites`](#hint-composites) is used, ttfautohint doesn't
hint subglyphs of composite glyphs separately.  Instead, it hints the whole
glyph, this is, composites get recursively expanded internally so that they
form simple glyphs, then hints are applied -- this is the normal working
mode of FreeType's auto-hinter.

One problem, however, must be solved: Hinting for subglyphs (which usually
are used as normal glyphs also) must be deactivated so that nothing but the
final bytecode of the composite gets executed.

The trick used by ttfautohint is to prepend a composite element called
'\.ttfautohint', a dummy glyph with a single point, and which has a single
job: Its bytecode increases a variable (to be more precise, it is a CVT
register called `cvtl_is_subglyph` in the source code), indicating that we
are within a composite glyph.  The final bytecode of the composite glyph
eventually decrements this variable again.

As an example, let's consider composite glyph 'Agrave' ('À'), which has the
subglyph 'A' as the base and 'grave' as its accent.  After processing with
ttfautohint it consists of three components: '\.ttfautohint', 'A', and
'grave' (in this order).

  Bytecode of    Action
  -------------  --------
  .ttfautohint   increase `cvtl_is_subglyph` (now: 1)
  A              do nothing because `cvtl_is_subglyph` > 0
  grave          do nothing because `cvtl_is_subglyph` > 0
  Agrave         decrease `cvtl_is_subglyph` (now: 0)\
                 apply hints because `cvtl_is_subglyph` == 0

Some technical details (which you might skip): All glyph point indices get
adjusted since each '\.ttfautohint' subglyph shifts all following indices by
one.  This must be done for both the bytecode and one subformat of
OpenType's `GPOS` anchor tables.

While this approach works fine on all tested platforms, there is one single
drawback: Direct rendering of the '\.ttfautohint' subglyph (this is,
rendering as a stand-alone glyph) disables proper hinting of all glyphs in
the font!  Under normal circumstances this never happens because
'\.ttfautohint' doesn't have an entry in the font's `cmap` table.  (However,
some test and demo programs like FreeType's `ftview` application or other
glyph viewers that are able to bypass the `cmap` table might be affected.)


Scripts
-------

ttfautohint checks which auto-hinting module should be used to hint a
specific glyph.  To do so, it checks a glyph's Unicode character code
whether it belongs to a given script.  Currently, only FreeType's 'latin'
autohinting module is implemented, but more are expected to come.  Note,
however, that this module is capable to hint other scripts too.

Here is the hardcoded list of character ranges that are hinted by this
'latin' module.  As you can see, this also covers some non-latin scripts (in
the Unicode sense) that have similar typographical properties.

In ttfautohint, scripts are identified by four-character tags.  The value
`dflt` indicates 'no script', which gets hinted by 'dummy' auto-hinting
module.


Table: `latn` character ranges

     Character range     Description
  ---------------------  -------------
  `0x0020` - `0x007F`    Basic Latin (no control characters)
  `0x00A0` - `0x00FF`    Latin-1 Supplement (no control characters)
  `0x0100` - `0x017F`    Latin Extended-A
  `0x0180` - `0x024F`    Latin Extended-B
  `0x0250` - `0x02AF`    IPA Extensions
  `0x02B0` - `0x02FF`    Spacing Modifier Letters
  `0x0300` - `0x036F`    Combining Diacritical Marks
  `0x1D00` - `0x1D7F`    Phonetic Extensions
  `0x1D80` - `0x1DBF`    Phonetic Extensions Supplement
  `0x1DC0` - `0x1DFF`    Combining Diacritical Marks Supplement
  `0x1E00` - `0x1EFF`    Latin Extended Additional
  `0x2000` - `0x206F`    General Punctuation
  `0x2070` - `0x209F`    Superscripts and Subscripts
  `0x20A0` - `0x20CF`    Currency Symbols
  `0x2150` - `0x218F`    Number Forms
  `0x2460` - `0x24FF`    Enclosed Alphanumerics
  `0x2C60` - `0x2C7F`    Latin Extended-C
  `0x2E00` - `0x2E7F`    Supplemental Punctuation
  `0xA720` - `0xA7FF`    Latin Extended-D
  `0xFB00` - `0xFB06`    Alphabetical Presentation Forms (Latin Ligatures)
  `0x1D400` - `0x1D7FF`  Mathematical Alphanumeric Symbols
  `0x1F100` - `0x1F1FF`  Enclosed Alphanumeric Supplement


Table: `grek` character ranges

     Character range     Description
  ---------------------  -------------
  `0x0370` - `0x03FF`    Greek and Coptic
  `0x1F00` - `0x1FFF`    Greek Extended


Table: `cyrl` character ranges

     Character range     Description
  ---------------------  -------------
  `0x0400` - `0x04FF`    Cyrillic
  `0x0500` - `0x052F`    Cyrillic Supplement
  `0x2DE0` - `0x2DFF`    Cyrillic Extended-A
  `0xA640` - `0xA69F`    Cyrillic Extended-B


Table: `hebr` character ranges

     Character range     Description
  ---------------------  -------------
  `0x0590` - `0x05FF`    Hebrew
  `0xFB1D` - `0xFB4F`    Alphabetic Presentation Forms (Hebrew)


If a glyph's character code is not covered by a script range, it is not
hinted (or rather, it gets hinted by the 'dummy' auto-hinting module that
essentially does nothing).  This can be changed by specifying a *fallback
script* with [option `--fallback-script`](#fallback-script).

It is planned to extend ttfautohint so that the `GSUB` OpenType table gets
analyzed, mapping character codes to all glyph indices that can be reached
by switching on or off various OpenType features.


SFNT Tables
-----------

ttfautohint touches almost all SFNT tables within a TrueType or OpenType
font.  Note that only OpenType fonts with TrueType outlines are supported.
OpenType fonts with a `CFF` table (this is, with PostScript outlines) won't
work.

  * `glyf`: All hints in the table are replaced with new ones.  If option
    [`--composites`](#hint-composites) is used, one glyph gets added (namely
    the '\.ttfautohint' glyph) and all composites get an additional
    component.

  * `cvt`, `prep`, and `fpgm`: These tables get replaced with data
    necessary for the new hinting bytecode.

  * `gasp`: Set up to always use grayscale rendering with grid-fitting
    for standard hinting, and symmetric grid-fitting and symmetric
    smoothing for horizontal subpixel hinting (ClearType).

  * `DSIG`: If it exists, it gets replaced with a dummy version.
    ttfautohint can't digitally sign a font; you have to do that afterwards.

  * `name`: The 'version' entries are modified to add information about the
    parameters that have been used for calling ttfautohint.  This can be
    controlled with the [`--no-info`](#add-ttfautohint-info) option.

  * `GPOS`, `hmtx`, `loca`, `head`, `maxp`, `post`: Updated to fit the
    additional '\.ttfautohint' glyph, the additional subglyphs in
    composites, and the new hinting bytecode.

  * `LTSH`, `hdmx`: Since ttfautohint doesn't do any horizontal hinting,
    those tables are superfluous and thus removed.

  * `VDMX`: Removed, since it depends on the original bytecode, which
    ttfautohint removes.  A font editor might recompute the necessary data
    later on.


Problems
--------

Diagonals.

TODO