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<h1>
FreeType Glyph Conventions</h1></center>
<center>
<h2>
version 2.0</h2></center>
<center>
<h3>
Copyright 1998-1999David Turner (<a href="mailto:david@freetype.org">david@freetype.org</a>)<br>
Copyright 1999 The FreeType Development Team (<a href="devel@freetype.org">devel@freetype.org</a>)</h3></center>
<p><br>
<hr WIDTH="100%">
<h2>
Introduction</h2>
<blockquote>This document discusses in great details the definition of
various concepts related to digital typography, as well as a few specific
to the FreeType library. It also explains the ways typographic information,
like glyph metrics, kerning distances, etc.. is to be managed and used.
It relates to the layout and display of text strings, either in a conventional
(i.e. Roman) layout, or with right-to-left or vertical ones. Some aspects
like rotation and transformation are explained too.
<p>Comments and corrections are highly welcomed, and can be sent to the
<a href="devel@freetype.org">FreeType
developers list</a>.</blockquote>
<hr WIDTH="100%">
<h2>
I. Basic typographic concepts</h2>
<blockquote>
<h3>
1. Font files, format and information</h3>
<blockquote>A font is a collection of various character images that can
be used to display or print text. The images in a single font share some
common properties, including look, style, serifs, etc.. Typographically
speaking, one has to distinguish between a <b>font family</b> and its multiple
<b>font
faces</b>, which usually differ in style though come from the same template.
For example, "<i>Palatino Regular</i>" and "<i>Palatino Italic</i>" are
two distinct <i>faces</i> from the same famous <i>family</i>, called "<i>Palatino</i>"
itself.
<p>The single term font is nearly always used in ambiguous ways to refer
to either a given family or given face, depending on the context. For example,
most users of word-processors use "font" to describe a font family (e.g.
Courier, Palatino, etc..); however most of these families are implemented
through several data files depending on the file format : for TrueType,
this is usually one per face (i.e. ARIAL.TFF for "Arial Regular", ARIALI.TTF
for "Arial Italic", etc..). The file is also called a "font" but really
contains a font face.
<p>A <i>digital font</i> is thus a data file that may contain <i>one or
more font faces</i>. For each of these, it contains character images, character
metrics, as well as other kind of information important to the layout of
text and the processing of specific character encodings. In some awkward
formats, like Adobe Type1, a single font face is described through several
files (i.e. one contains the character images, another one the character
metrics). We will ignore this implementation issue in most of this document
and consider digital fonts as single files, though FreeType 2.0 is able
to support multiple-files fonts correctly.
<p>As a convenience, a font file containing more than one face is called
a font collection. This case is rather rare but can be seen in many asian
fonts, which contain images for two or more scripts for a given language.</blockquote>
<h3>
2. Character images and mappings :</h3>
<blockquote>The character images are called <b>glyphs</b>. A single character
can have several distinct images, i.e. several glyphs, depending on script,
usage or context. Several characters can also take a single glyph (good
examples are roman ligatures like "oe" and "fi" which can be represented
by a single glyph like "œ" and "?"). The relationships between characters
and glyphs can be a very complex one but won't be detailed in this document.
Moreover, some formats use more or less awkward schemes to store and access
the glyphs. For the sake of clarity, we'll only retain the following notions
when working with FreeType :
<br>&nbsp;
<ul>
<li>
A font file contains a set of glyphs, each one can be stored as a bitmap,
a vector representation or any other scheme (e.g. most scalable formats
use a combination of math representation and control data/programs). These
glyphs can be stored in any order in the font file, and is typically accessed
through a simple glyph index.</li>
</ul>
</blockquote>
</blockquote>
<ul>
<ul>
<ul>
<li>
The font file contains one (or more) table, called a character map (or
charmap in short), which is used to convert character codes for a given
encoding (e.g. ASCII, Unicode, DBCS, Big5, etc..) into glyph indexes relative
to the font file. A single font face may contain several charmaps. For
example, most TrueType fonts contain an Apple-specific charmap as well
as a Unicode charmap, which makes them usable on both Mac and Windows platforms.</li>
</ul>
</ul>
<h3>
3. Character and font metrics :</h3>
<ul>Each glyph image is associated to various metrics which are used to
describe the way it must be placed and managed when rendering text. Though
they are described in more details in section III, they relate to glyph
placement, cursor advances as well as text layouts. They are extremely
important to compute the flow of text when rendering string of text.
<p>Each scalable format also contains some global metrics, expressed in
notional units, used to describe some properties of all glyphs in a same
face. For example : the maximum glyph bounding box, the ascender, descender
and text height for the font.
<p>Though these metrics also exist for non-scalable formats, they only
apply for a set of given character dimensions and resolutions, and they're
usually expressed in pixels then.</ul>
</ul>
<p><br>
<hr WIDTH="100%">
<h2>
II. Glyph Outlines</h2>
<blockquote>This section describes the vectorial representation of glyph
images, called outlines.
<br>&nbsp;
<h3>
1. Pixels, Points and Device Resolutions :</h3>
<blockquote>Though it is a very common assumption when dealing with computer
graphics programs, the physical dimensions of a given pixel (be it for
screens or printers) are not squared. Often, the output device, be it a
screen or printer exhibits varying resolutions in the horizontal and vertical
directions, and this must be taken care of when rendering text.
<p>It is thus common to define a device's characteristics through two numbers
expressed in <b>dpi</b> (dots per inch). For example, a printer with a
resolution of 300x600 dpi has 300 pixels per inch in the horizontal direction,
and 600 in the vertical one. The resolution of a typical computer monitor
varies with its size (a 15" and 17" monitors don't have the same pixel
sizes at 640x480), and of course the graphics mode resolution.
<p>As a consequence, the size of text is usually given in <b>points</b>,
rather than device-specific pixels. Points are a simple <i>physical</i>
unit, where 1 point = 1/72th of an inch, in digital typography. As an example,
most roman books are printed with a body text which size is chosen between
10 and 14 points.
<p>It is thus possible to compute the size of text in pixels from the size
in points through the following computation :
<center>
<p><tt>pixel_size = point_size * resolution / 72</tt></center>
<p>Where resolution is expressed in dpi. Note that because the horizontal
and vertical resolutions may differ, a single point size usually defines
different text width and height in pixels.
<br>&nbsp;
<p><b>IMPORTANT NOTE:</b>
<br><i>Unlike what is often thought, the "size of text in pixels" is not
directly related to the real dimensions of characters when they're displayed
or printed. The relationship between these two concepts is a bit more complex
and relate to some design choice made by the font designer. This is described
in more details the next sub-section (see the explanations on the EM square).</i></blockquote>
<h3>
2. Vectorial representation :</h3>
<blockquote>The source format of outlines is a collection of closed paths
called <b>contours</b>. Each contour delimits an outer or inner <i>region</i>
of the glyph, and can be made of either <b>line segments</b> or <b>bezier
arcs</b>.
<p>The arcs are defined through <b>control points</b>, and can be either
second-order (these are "conic beziers") or third-order ("cubic" beziers)
polynomials, depending on the font format. Hence, each point of the outline
has an associated <b>flag</b> indicating its type (normal or control point).
And scaling the points will scale the whole outline.
<p>Each glyph's original outline points are located on a grid of indivisible
units. The points are usually stored in a font file as 16-bit integer grid
coordinates, with the grid origin's being at (0,0); they thus range from
-16384 to 16383. (even though point coordinates can be floats in other
formats such as Type 1, we'll restrict our analysis to integer ones, driven
by the need for simplicity..).
<p><b>IMPORTANT NOTE:</b>
<br><i>The grid is always oriented like the traditional mathematical 2D
plane, i.e. the X axis from the left to the right, and the Y axis from
bottom to top.</i>
<p>In creating the glyph outlines, a type designer uses an imaginary square
called the "EM square". Typically, the EM square can be thought of as a
tablet on which the character are drawn. The square's size, i.e., the number
of grid units on its sides, is very important for two reasons:
<br>&nbsp;
<blockquote>
<li>
it is the reference used to scale the outlines to a given text dimension.
For example, a size of 12pt at 300x300 dpi corresponds to 12*300/72 = 50
pixels. This is the size the EM square would appear on the output device
if it was rendered directly. In other words, scaling from grid units to
pixels uses the formula:</li>
</blockquote>
<center><tt>pixel_size = point_size * resolution / 72</tt>
<br><tt>pixel_coordinate = grid_coordinate * pixel_size / EM_size</tt></center>
<blockquote>
<li>
the greater the EM size is, the larger resolution the designer can use
when digitizing outlines. For example, in the extreme example of an EM
size of 4 units, there are only 25 point positions available within the
EM square which is clearly not enough. Typical TrueType fonts use an EM
size of 2048 units (note: with Type 1 PostScript fonts, the EM size is
fixed to 1000 grid units. However, point coordinates can be expressed in
floating values).</li>
</blockquote>
Note that glyphs can freely extend beyond the EM square if the font designer
wants so. The EM is used as a convenience, and is a valuable convenience
from traditional typography.
<center>
<p><b>Note : Grid units are very often called "font units" or "EM units".</b></center>
<p><b>NOTE:</b>
<br><i>As said before, the pixel_size computed in&nbsp; the above formula
does not relate directly to the size of characters on the screen. It simply
is the size of the EM square if it was to be displayed directly. Each font
designer is free to place its glyphs as it pleases him within the square.
This explains why the letters of the following text have not the same height,
even though they're displayed at the same point size with distinct fonts
:</i>
<center>
<p><img SRC="body_comparison.gif" height=40 width=580></center>
<p>As one can see, the glyphs of the Courier family are smaller than those
of Times New Roman, which themselves are slightly smaller than those of
Arial, even though everything is displayed or printed&nbsp; at a size of
16 points. This only reflect design choices.
<br>&nbsp;</blockquote>
<h3>
3. Hinting and Bitmap rendering</h3>
<blockquote>The outline as stored in a font file is called the "master"
outline, as its points coordinates are expressed in font units. Before
it can be converted into a bitmap, it must be scaled to a given size/resolution.
This is done through a very simple transform, but always creates undesirable
artifacts, e.g. stems of different widths or heights in letters like "E"
or "H".
<p>As a consequence, proper glyph rendering needs the scaled points to
be aligned along the target device pixel grid, through an operation called
"grid-fitting", and often "hinting". One of its main purpose is to ensure
that important widths and heights are respected throughout the whole font
(for example, it is very often desirable that the "I" and the "T" have
their central vertical line of the same pixel width), as well as manage
features like stems and overshoots, which can cause problems at small pixel
sizes.
<p>There are several ways to perform grid-fitting properly, for example
most scalable formats associate some control data or programs with each
glyph outline. Here is an overview :
<br>&nbsp;
<blockquote>
<blockquote><b>explicit grid-fitting :</b>
<blockquote>The TrueType format defines a stack-based virtual machine,
for which programs can be written with the help of more than 200 opcodes
(most of these relating to geometrical operations). Each glyph is thus
made of both an outline and a control program, its purpose being to perform
the actual grid-fitting in the way defined by the font designer.</blockquote>
<p><br><b>implicit grid-fitting (also called hinting) :</b>
<blockquote>The Type 1 format takes a much simpler approach : each glyph
is made of an outline as well as several pieces called "hints" which are
used to describe some important features of the glyph, like the presence
of stems, some width regularities, and the like. There aren't a lot of
hint types, and it's up to the final renderer to interpret the hints in
order to produce a fitted outline.</blockquote>
<p><br><b>automatic grid-fitting :</b>
<blockquote>Some formats simply include no control information with each
glyph outline, apart metrics like the advance width and height. It's then
up to the renderer to "guess" the more interesting features of the outline
in order to perform some decent grid-fitting.</blockquote>
</blockquote>
</blockquote>
<center>
<p><br>The following table summarises the pros and cons of each scheme
:</center>
</blockquote>
<center><table BORDER=0 WIDTH="80%" BGCOLOR="#CCCCCC" >
<tr BGCOLOR="#999999">
<td>
<blockquote>
<center><b><font color="#000000">Grid-fitting scheme</font></b></center>
</blockquote>
</td>
<td>
<blockquote>
<center><b><font color="#000000">Pros</font></b></center>
</blockquote>
</td>
<td>
<blockquote>
<center><b><font color="#000000">Cons</font></b></center>
</blockquote>
</td>
</tr>
<tr>
<td>
<blockquote>
<center><b><font color="#000000">Explicit</font></b></center>
</blockquote>
</td>
<td>
<blockquote>
<center><b><font color="#000000">Quality</font></b>
<br><font color="#000000">excellence at small sizes is possible. This is
very important for screen display.</font>
<p><b><font color="#000000">Consistency</font></b>
<br><font color="#000000">all renderers produce the same glyph bitmaps.</font></center>
</blockquote>
</td>
<td>
<blockquote>
<center><b><font color="#000000">Speed</font></b>
<br><font color="#000000">intepreting bytecode can be slow if the glyph
programs are complex.</font>
<p><b><font color="#000000">Size</font></b>
<br><font color="#000000">glyph programs can be long</font>
<p><b><font color="#000000">Technicity</font></b>
<br><font color="#000000">it is extremely difficult to write good hinting
programs. Very few tools available.</font></center>
</blockquote>
</td>
</tr>
<tr>
<td>
<blockquote>
<center><b><font color="#000000">Implicit</font></b></center>
</blockquote>
</td>
<td>
<blockquote>
<center><b><font color="#000000">Size</font></b>
<br><font color="#000000">hints are usually much smaller than explicit
glyph programs.</font>
<p><b><font color="#000000">Speed</font></b>
<br><font color="#000000">grid-fitting is usually a fast process</font></center>
</blockquote>
</td>
<td>
<blockquote>
<center><b><font color="#000000">Quality</font></b>
<br><font color="#000000">often questionable at small sizes. Better with
anti-aliasing though.</font>
<p><b><font color="#000000">Inconsistency</font></b>
<br><font color="#000000">results can vary between different renderers,
or even distinct versions of the same engine.</font></center>
</blockquote>
</td>
</tr>
<tr>
<td>
<blockquote>
<center><b><font color="#000000">Automatic</font></b></center>
</blockquote>
</td>
<td>
<blockquote>
<center><b><font color="#000000">Size</font></b>
<br><font color="#000000">no need for control information, resulting in
smaller font files.</font>
<p><b><font color="#000000">Speed</font></b>
<br><font color="#000000">depends on the grid-fitting algo.Usually faster
than explicit grid-fitting.</font></center>
</blockquote>
</td>
<td>
<blockquote>
<center><b><font color="#000000">Quality</font></b>
<br><font color="#000000">often questionable at small sizes. Better with
anti-aliasing though</font>
<p><b><font color="#000000">Speed</font></b>
<br><font color="#000000">depends on the grid-fitting algo.</font>
<p><b><font color="#000000">Inconsistency</font></b>
<br><font color="#000000">results can vary between different renderers,
or even distinct versions of the same engine.</font></center>
</blockquote>
</td>
</tr>
</table></center>
</blockquote>
<hr WIDTH="100%">
<h2>
III. Glyph metrics</h2>
<blockquote>
<h3>
1. Baseline, Pens and Layouts</h3>
The baseline is an imaginary line that is used to "guide" glyphs when rendering
text. It can be horizontal (e.g. Roman, Cyrillic, Arabic, etc.) or vertical
(e.g. Chinese, Japanese, Korean, etc). Moreover, to render text, a virtual
point, located on the baseline, called the "pen position" or "origin",
is used to locate glyphs.
<p>Each layout uses a different convention for glyph placement:
<br>&nbsp;
<blockquote>
<li>
with horizontal layout, glyphs simply "rest" on the baseline. Text is rendered
by incrementing the pen position, either to the right or to the left.</li>
</blockquote>
</blockquote>
<ul>
<ul>the distance between two successive pen positions is glyph-specific
and is called the "advance width". Note that its value is _always_ positive,
even for right-to-left oriented alphabets, like Arabic. This introduces
some differences in the way text is rendered.
<p>IMPORTANT NOTE:&nbsp; The pen position is always placed on the baseline.</ul>
<center><img SRC="Image1.gif" height=179 width=458></center>
<ul>
<li>
with a vertical layout, glyphs are centered around the baseline:</li>
</ul>
<center><img SRC="Image2.gif" height=275 width=162></center>
<p><br>
<h3>
2. Typographic metrics and bounding boxes</h3>
<ul>A various number of face metrics are defined for all glyphs in a given
font.
<p><b>the ascent</b>
<ul>this is the distance from the baseline to the highest/upper grid coordinate
used to place an outline point. It is a positive value, due to the grid's
orientation with the Y axis upwards.</ul>
<p><br><b>the descent</b>
<ul>the distance from the baseline to the lowest grid coordinate used to
place an outline point. This is a negative value, due to the grid's orientation.</ul>
<p><br><b>the linegap</b>
<ul>the distance that must be placed between two lines of text. The baseline-to-baseline
distance should be computed as:
<center>
<p><tt>ascent - descent + linegap</tt></center>
if you use the typographic values.</ul>
Other, simpler metrics are:
<p><b>the glyph's bounding box</b>, also called "<b>bbox</b>"
<ul>this is an imaginary box that encloses all glyphs from the font, as
tightly as possible. It is represented by four fields, namely <tt>xMin</tt>,
<tt>yMin</tt>,
<tt>xMax</tt>,
and <tt>yMax</tt>, that can be computed for any outline. Their values can
be in font units (if measured in the original outline) or in fractional/integer
pixel units (when measured on scaled outlines).
<p>Note that if it wasn't for grid-fitting, you wouldn't need to know a
box's complete values, but only its dimensions to know how big is a glyph
outline/bitmap. However, correct rendering of hinted glyphs needs the preservation
of important grid alignment on each glyph translation/placement on the
baseline.</ul>
<b>the internal leading</b>
<ul>this concept comes directly from the world of traditional typography.
It represents the amount of space within the "leading" which is reserved
for glyph features that lay outside of the EM square (like accentuation).
It usually can be computed as:
<center>
<p><tt>internal leading = ascent - descent - EM_size</tt></center>
</ul>
<b>the external leading</b>
<ul>this is another name for the line gap.</ul>
</ul>
<h3>
3. Bearings and Advances</h3>
<ul>Each glyph has also distances called "bearings" and "advances". Their
definition is constant, but their values depend on the layout, as the same
glyph can be used to render text either horizontally or vertically:
<p><b>the left side bearing: a.k.a. bearingX</b>
<ul>this is the horizontal distance from the current pen position to the
glyph's left bbox edge. It is positive for horizontal layouts, and most
generally negative for vertical one.</ul>
<p><br><b>the top side bearing: a.k.a. bearingY</b>
<ul>this is the vertical distance from the baseline to the top of the glyph's
bbox. It is usually positive for horizontal layouts, and negative for vertical
ones</ul>
<p><br><b>the advance width: a.k.a. advanceX</b>
<ul>is the horizontal distance the pen position must be incremented (for
left-to-right writing) or decremented (for right-to-left writing) by after
each glyph is rendered when processing text. It is always positive for
horizontal layouts, and null for vertical ones.</ul>
<p><br><b>the advance height: a.k.a. advanceY</b>
<ul>is the vertical distance the pen position must be decremented by after
each glyph is rendered. It is always null for horizontal layouts, and positive
for vertical layouts.</ul>
<p><br><b>the glyph width</b>
<ul>this is simply the glyph's horizontal extent. More simply it is (bbox.xMax-bbox.xMin)
for unscaled font coordinates. For scaled glyphs, its computation requests
specific care, described in the grid-fitting chapter below.</ul>
<p><br><b>the glyph height</b>
<ul>this is simply the glyph's vertical extent. More simply, it is (bbox.yMax-bbox.yMin)
for unscaled font coordinates. For scaled glyphs, its computation requests
specific care, described in the grid-fitting chapter below.</ul>
<p><br><b>the right side bearing</b>
<ul>is only used for horizontal layouts to describe the distance from the
bbox's right edge to the advance width. It is in most cases a non-negative
number.</ul>
<center><tt>advance_width - left_side_bearing - (xMax-xMin)</tt></center>
<p>Here is a picture giving all the details for horizontal metrics :
<center>
<p><img SRC="Image3.gif" height=253 width=388></center>
<p>And here is another one for the vertical metrics :
<center>
<p><img SRC="Image4.gif" height=278 width=294></center>
</ul>
<h3>
4. The effects of grid-fitting</h3>
<ul>Because hinting aligns the glyph's control points to the pixel grid,
this process slightly modifies the dimensions of character images in ways
that differ from simple scaling.
<p>For example, the image of the lowercase "m" letter sometimes fits a
square in the master grid. However, to make it readable at small pixel
sizes, hinting tends to enlarge its scaled outline in order to keep its
three legs distinctly visible, resulting in a larger character bitmap.
<p>The glyph metrics are also influenced by the grid-fitting process. Mainly
because :
<br>&nbsp;
<ul>
<li>
The image's width and height are altered. Even if this is only by one pixel,
it can make a big difference at small pixel sizes</li>
<li>
The image's bounding box is modified, thus modifying the bearings</li>
<li>
The advances must be updated. For example, the advance width must be incremented
when the hinted bitmap is larger than the scaled one, to reflect the augmented
glyph width.</li>
</ul>
<p><br>Note also that :
<br>&nbsp;
<ul>
<li>
Because of hinting, simply scaling the font ascent or descent might not
give correct results. A simple solution consists in keeping the ceiling
of the scaled ascent, and floor of the scaled descent.</li>
</ul>
<ul>
<li>
There is no easy way to get the hinted glyph and advance widths of a range
of glyphs, as hinting works differently on each outline. The only solution
is to hint each glyph separately and record the returned values. Some formats,
like TrueType, even include a table of pre-computed values for a small
set of common character pixel sizes.</li>
</ul>
<ul>
<li>
Hinting depends on the final character width and height in pixels, which
means that it is highly resolution-dependent. This property makes correct
WYSIWYG layouts difficult to implement.</li>
</ul>
<p><br><b>IMPORTANT NOTE:</b>
<br>Performing 2D transforms on glyph outlines is very easy with FreeType.
However, when using translation on a hinted outlines, one should aways
take care of&nbsp; <b>exclusively using integer pixel distances</b> (which
means that the parameters to the FT_Translate_Outline API should all be
multiples of 64, as the point coordinates are in 26.6 fixed float format).
<p><b>Otherwise</b>, the translation will simply <b>ruin the hinter's work</b>,
resulting in a very low quality bitmaps.
<br>&nbsp;
<br>&nbsp;</ul>
<h3>
&nbsp;5. Text widths and bounding box :</h3>
<ul>As seen before, the "origin" of a given glyph corresponds to the position
of the pen on the baseline. It is not necessarily located on one of the
glyph's bounding box corners, unlike many typical bitmapped font formats.
In some cases, the origin can be out of the bounding box, in others, it
can be within it, depending on the shape of the given glyph.
<p>Likewise, the glyph's "advance width" is the increment to apply to the
pen position during layout, and is not related to the glyph's "width",
which really is the glyph's bounding width.
<br>&nbsp;
<p>The same conventions apply to strings of text. This means that :
<br>&nbsp;
<ul>
<ul>
<li>
The bounding box of a given string of text doesn't necessarily contain
the text cursor, nor is the latter located on one of its corners.</li>
</ul>
<ul>
<li>
The string's advance width isn't related to its bounding box's dimensions.
Especially if it contains beginning and terminal spaces or tabs.</li>
</ul>
<ul>
<li>
Finally, additional processing like kerning creates strings of text whose
dimensions are not directly related to the simple juxtaposition of individual
glyph metrics. For example, the advance width of "VA" isn't the sum of
the advances of "V" and "A" taken separately.</li>
</ul>
</ul>
</ul>
</ul>
<hr WIDTH="100%">
<h2>
&nbsp;IV. Kerning</h2>
<blockquote>The term 'kerning' refers to specific information used to adjust
the relative positions of coincident glyphs in a string of text. This section
describes several types of kerning information, as well as the way to process
them when performing text layout.
<br>&nbsp;
<h3>
1. Kerning pairs</h3>
<blockquote>Kerning consists in modifying the spacing between two successive
glyphs according to their outlines. For example, a "T" and a "y" can be
easily moved closer, as the top of the "y" fits nicely under the "T"'s
upper right bar.
<p>When laying out text with only their standard widths, some consecutive
glyphs sometimes seem a bit too close or too distant. For example, the
space between the 'A' and the 'V' in the following word seems a little
wider than needed.
<center>
<p><img SRC="bravo_unkerned.gif" height=37 width=116></center>
<p>Compare this to the same word, when the distance between these two letters
has been slightly reduced :
<center>
<p><img SRC="bravo_kerned.gif" height=37 width=107></center>
<p>As you can see, this adjustment can make a great difference. Some font
faces thus include a table containing kerning distances for a set of given
glyph pairs, used during text layout. Note that :
<br>&nbsp;
<blockquote>
<ul>
<li>
The pairs are ordered, i.e. the space for pair (A,V) isn't necessarily
the space for pair (V,A). They also index glyphs, and not characters.</li>
</ul>
<ul>
<li>
Kerning distances can be expressed in horizontal or vertical directions,
depending on layout and/or script. For example, some horizontal layouts
like arabic can make use of vertical kerning adjustments between successive
glyphs. A vertical script can have vertical kerning distances.</li>
</ul>
<ul>
<li>
Kerning distances are expressed in grid units. They are usually oriented
in the X axis, which means that a negative value indicates that two glyphs
must be set closer in a horizontal layout.</li>
</ul>
</blockquote>
</blockquote>
<h3>
2. Applying kerning</h3>
<blockquote>Applying kerning when rendering text is a rather easy process.
It merely consists in adding the scaled kern distance to the pen position
before writing each next glyph. However, the typographically correct renderer
must take a few more details in consideration.
<p>The "sliding dot" problem is a good example : many font faces include
a kerning distance between capital letters like "T" or "F" and a following
dot ("."), in order to slide the latter glyph just right to their main
leg. I.e.
<center>
<p><img SRC="twlewis1.gif" height=38 width=314></center>
<p>However, this sometimes requires additional adjustments between the
dot and the letter following it, depending on the shapes of the enclosing
letters. When applying "standard" kerning adjustments, the previous sentence
would become :
<center>
<p><img SRC="twlewis2.gif" height=36 width=115></center>
<p>Which clearly is too contracted. The solution here, as exhibited in
the first example is to only slide the dots when possible. Of course, this
requires a certain knowledge of the text's meaning. The above adjustments
would not necessarily be welcomed if we were rendering the final dot of
a given paragraph.
<p>This is only one example, and there are many others showing that a real
typographer is needed to layout text properly. If not available, some kind
of user interaction or tagging of the text could be used to specify some
adjustments, but in all cases, this requires some support in applications
and text libraries.
<p>For more mundane and common uses, however, we can have a very simple
algorithm, which&nbsp; avoids the sliding dot problem, and others, though
not producing optimal results. It can be seen as :
<br>&nbsp;
<blockquote>
<ol>
<li>
place the first glyph on the baseline</li>
<li>
save the location of the pen position/origin in pen1</li>
<li>
adjust the pen position with the kerning distance between the first and
second glyph</li>
<li>
place the second glyph and compute the next pen position/origin in pen2.</li>
<li>
use pen1 as the next pen position if it is beyond pen2, use pen2 otherwise.</li>
</ol>
</blockquote>
</blockquote>
</blockquote>
<h2>
<hr WIDTH="100%"></h2>
<h2>
V. Text processing</h2>
<blockquote>This section demonstrates how to use the concepts previously
defined to render text, whatever the layout you use.
<br>&nbsp;
<h3>
1. Writing simple text strings :</h3>
<blockquote>In this first example, we'll generate a simple string of Roman
text, i.e. with a horizontal left-to-right layout. Using exclusively pixel
metrics, the process looks like :
<blockquote><tt>1) convert the character string into a series of glyph
indexes.</tt>
<br><tt>2) place the pen to the cursor position.</tt>
<br><tt>3) get or load the glyph image.</tt>
<br><tt>4) translate the glyph so that its 'origin' matches the pen position</tt>
<br><tt>5) render the glyph to the target device</tt>
<br><tt>6) increment the pen position by the glyph's advance width in pixels</tt>
<br><tt>7) start over at step 3 for each of the remaining glyphs</tt>
<br><tt>8) when all glyphs are done, set the text cursor to the new pen
position</tt></blockquote>
Note that kerning isn't part of this algorithm.</blockquote>
<h3>
2. Sub-pixel positioning :</h3>
<blockquote>It is somewhat useful to use sub-pixel positioning when rendering
text. This is crucial, for example, to provide semi-WYSIWYG text layouts.
Text rendering is very similar to the algorithm described in sub-section
1, with the following few differences :
<ul>
<li>
The pen position is expressed in fractional pixels.</li>
<li>
Because translating a hinted outline by a non-integer distance will ruin
its grid-fitting, the position of the glyph origin must be rounded before
rendering the character image.</li>
<li>
The advance width is expressed in fractional pixels, and isn't necessarily
an integer.</li>
</ul>
<p><br>Which finally looks like :
<blockquote><tt>1. convert the character string into a series of glyph
indexes.</tt>
<br><tt>2. place the pen to the cursor position. This can be a non-integer
point.</tt>
<br><tt>3. get or load the glyph image.</tt>
<br><tt>4. translate the glyph so that its 'origin' matches the rounded
pen position.</tt>
<br><tt>5. render the glyph to the target device</tt>
<br><tt>6. increment the pen position by the glyph's advance width in fractional
pixels.</tt>
<br><tt>7. start over at step 3 for each of the remaining glyphs</tt>
<br><tt>8. when all glyphs are done, set the text cursor to the new pen
position</tt></blockquote>
Note that with fractional pixel positioning, the space between two given
letters isn't fixed, but determined by the accumulation of previous rounding
errors in glyph positioning.</blockquote>
<h3>
3.&nbsp; Simple kerning :</h3>
<blockquote>Adding kerning to the basic text rendering algorithm is easy
: when a kerning pair is found, simply add the scaled kerning distance
to the pen position before step 4. Of course, the distance should be rounded
in the case of algorithm 1, though it doesn't need to for algorithm 2.
This gives us :
<p>Algorithm 1 with kerning:
<blockquote><tt>3) get or load the glyph image.</tt>
<br><tt>4) Add the rounded scaled kerning distance, if any, to the pen
position</tt>
<br><tt>5) translate the glyph so that its 'origin' matches the pen position</tt>
<br><tt>6) render the glyph to the target device</tt>
<br><tt>7) increment the pen position by the glyph's advance width in pixels</tt>
<br><tt>8) start over at step 3 for each of the remaining glyphs</tt></blockquote>
<p><br>Algorithm 2 with kerning:
<blockquote><tt>3) get or load the glyph image.</tt>
<br><tt>4) Add the scaled unrounded kerning distance, if any, to the pen
position.</tt>
<br><tt>5) translate the glyph so that its 'origin' matches the rounded
pen position.</tt>
<br><tt>6) render the glyph to the target device</tt>
<br><tt>7) increment the pen position by the glyph's advance width in fractional
pixels.</tt>
<br><tt>8) start over at step 3 for each of the remaining glyphs</tt></blockquote>
Of course, the algorithm described in section IV can also be applied to
prevent the sliding dot problem if one wants to..</blockquote>
<h3>
4. Right-To-Left Layout :</h3>
<blockquote>The process of laying out arabic or hebrew text is extremely
similar. The only difference is that the pen position must be decremented
before the glyph rendering (remember : the advance width is always positive,
even for arabic glyphs). Thus, algorithm 1 becomes :
<p>Right-to-left Algorithm 1:
<blockquote><tt>3) get or load the glyph image.</tt>
<br><tt>4) Decrement the pen position by the glyph's advance width in pixels</tt>
<br><tt>5) translate the glyph so that its 'origin' matches the pen position</tt>
<br><tt>6) render the glyph to the target device</tt>
<br><tt>7) start over at step 3 for each of the remaining glyphs</tt></blockquote>
<p><br>The changes to Algorithm 2, as well as the inclusion of kerning
are left as an exercise to the reader.
<br>&nbsp;
<br>&nbsp;</blockquote>
<h3>
5. Vertical layouts :</h3>
<blockquote>Laying out vertical text uses exactly the same processes, with
the following significant differences :
<br>&nbsp;
<blockquote>
<li>
The baseline is vertical, and the vertical metrics must be used instead
of the horizontal one.</li>
<li>
The left bearing is usually negative, but this doesn't change the fact
that the glyph origin must be located on the baseline.</li>
<li>
The advance height is always positive, so the pen position must be decremented
if one wants to write top to bottom (assuming the Y axis is oriented upwards).</li>
</blockquote>
Through the following algorithm :
<blockquote><tt>1) convert the character string into a series of glyph
indexes.</tt>
<br><tt>2) place the pen to the cursor position.</tt>
<br><tt>3) get or load the glyph image.</tt>
<br><tt>4) translate the glyph so that its 'origin' matches the pen position</tt>
<br><tt>5) render the glyph to the target device</tt>
<br><tt>6) decrement the vertical pen position by the glyph's advance height
in pixels</tt>
<br><tt>7) start over at step 3 for each of the remaining glyphs</tt>
<br><tt>8) when all glyphs are done, set the text cursor to the new pen
position</tt></blockquote>
</blockquote>
<h3>
6. WYSIWYG text layouts :</h3>
<blockquote>As you probably know, the acronym WYSIWYG stands for '<i>What
You See Is What You Get</i>'. Basically, this means that the output of
a document on the screen should match "perfectly" its printed version.
A <b><i>true</i></b> wysiwyg system requires two things :
<p><b>device-independent text layout</b>
<blockquote>Which means that the document's formatting is the same on the
screen than on any printed output, including line breaks, justification,
ligatures, fonts, position of inline images, etc..</blockquote>
<p><br><b>matching display and print character sizes</b>
<blockquote>Which means that the displayed size of a given character should
match its dimensions when printed. For example, a text string which is
exactly 1 inch tall when printed should also appear 1 inch tall on the
screen (when using a scale of 100%).</blockquote>
<p><br>It is clear that matching sizes cannot be possible if the computer
has no knowledge of the physical resolutions of the display device(s) it
is using. And of course, this is the most common case ! That's not too
unfortunate, however&nbsp; because most users really don't care about this
feature. Legibility is much more important.
<p>When the Mac appeared, Apple decided to choose a resolution of 72 dpi
to describe the Macintosh screen to the font sub-system (whatever the monitor
used). This choice was most probably driven by the fact that, at this resolution,
1 point = 1 pixel. However; it neglected one crucial fact : as most users
tend to choose a document character size between 10 and 14 points, the
resultant displayed text was rather small and not too legible without scaling.
Microsoft engineers took notice of this problem and chose a resolution
of 96 dpi on Windows, which resulted in slightly larger, and more legible,
displayed characters (for the same printed text size).
<p>These distinct resolutions explain some differences when displaying
text at the same character size on a Mac and a Windows machine. Moreover,
it is not unusual to find some TrueType fonts with enhanced hinting (tech
note: through delta-hinting) for the sizes of 10, 12, 14 and 16 points
at 96 dpi.
<br>&nbsp;
<p>As for device-independent text, it is a notion that is, unfortunately,
often abused. For example, many word processors, including MS Word, do
not really use device-independent glyph positioning algorithms when laying
out text. Rather, they use the target printer's resolution to compute <i>hinted</i>
glyph metrics for the layout. Though it guarantees that the printed version
is always the "nicest" it can be, especially for very low resolution printers
(like dot-matrix), it has a very sad effect : changing the printer can
have dramatic effects on the <i>whole</i> document layout, especially if
it makes strong use of justification, uses few page breaks, etc..
<p>Because the glyph metrics vary slightly when the resolution changes
(due to hinting), line breaks can change enormously, when these differences
accumulate over long runs of text. Try for example printing a very long
document (with no page breaks) on a 300 dpi ink-jet printer, then the same
one on a 3000 dpi laser printer : you'll be extremely lucky if your final
page count didn't change between the prints ! Of course, we can still call
this WYSIWYG, as long as the printer resolution is fixed !!
<p>Some applications, like Adobe Acrobat, which targeted device-independent
placement from the start, do not suffer from this problem. There are two
ways to achieve this : either use the scaled and unhinted glyph metrics
when laying out text both in the rendering and printing processes, or simply
use wathever metrics you want and store them with the text in order to
get sure they're printed the same on all devices (the latter being probably
the best solution, as it also enables font substitution without breaking
text layouts).
<p>Just like matching sizes, device-independent placement isn't necessarily
a feature that most users want. However, it is pretty clear that for any
kind of professional document processing work, it <b><i>is</i></b> a requirement.</blockquote>
</blockquote>
<h2>
<hr WIDTH="100%"></h2>
<h2>
VI. FreeType outlines :</h2>
<blockquote>The purpose of this section is to present the way FreeType
manages vectorial outlines, as well as the most common operations that
can be applied on them.
<br>&nbsp;
<h3>
1. FreeType outline description and structure :</h3>
<blockquote>
<h4>
a. Outline curve decomposition :</h4>
<blockquote>An outline is described as a series of closed contours in the
2D plane. Each contour is made of a series of line segments and bezier
arcs. Depending on the file format, these can be second-order or third-order
polynomials. The former are also called quadratic or conic arcs, and they
come from the TrueType format. The latter are called cubic arcs and mostly
come from the Type1 format.
<p>Each arc is described through a series of start, end and control points.
Each point of the outline has a specific tag which indicates wether it
is used to describe a line segment or an arc. The tags can take the following
values :
<br>&nbsp;
<br>&nbsp;</blockquote>
<center><table CELLSPACING=5 CELLPADDING=5 WIDTH="60%" >
<tr VALIGN=TOP>
<td>
<blockquote><b>FT_Curve_Tag_On&nbsp;</b></blockquote>
</td>
<td VALIGN=TOP>
<blockquote>Used when the point is "on" the curve. This corresponds to
start and end points of segments and arcs. The other tags specify what
is called an "off" point, i.e. one which isn't located on the contour itself,
but serves as a control point for a bezier arc.</blockquote>
</td>
</tr>
<tr>
<td>
<blockquote><b>FT_Curve_Tag_Conic</b></blockquote>
</td>
<td>
<blockquote>Used for an "off" point used to control a conic bezier arc.</blockquote>
</td>
</tr>
<tr>
<td>
<blockquote><b>FT_Curve_Tag_Cubic</b></blockquote>
</td>
<td>
<blockquote>Used for an "off" point used to control a cubic bezier arc.</blockquote>
</td>
</tr>
</table></center>
<blockquote>&nbsp;
<p>The following rules are applied to decompose the contour's points into
segments and arcs :
<blockquote>
<li>
two successive "on" points indicate a line segment joining them.</li>
</blockquote>
</blockquote>
<ul>
<ul>
<li>
one conic "off" point amidst two "on" points indicates a conic bezier arc,
the "off" point being the control point, and the "on" ones the start and
end points.</li>
</ul>
</ul>
<ul>
<ul>
<li>
Two successive cubic "off" points amidst two "on" points indicate a cubic
bezier arc. There must be exactly two cubic control points and two on points
for each cubic arc (using a single cubic "off" point between two "on" points
is forbidden, for example).</li>
</ul>
</ul>
<ul>
<ul>
<li>
finally, two successive conic "off" points forces the rasterizer to create
(during the scan-line conversion process exclusively) a virtual "on" point
amidst them, at their exact middle. This greatly facilitates the definition
of successive conic bezier arcs. Moreover, it's the way outlines are described
in the TrueType specification.</li>
</ul>
<p><br>Note that it is possible to mix conic and cubic arcs in a single
contour, even though no current font driver produces such outlines.
<br>&nbsp;</ul>
<center><table>
<tr>
<td>
<blockquote><img SRC="points_segment.gif" height=166 width=221></blockquote>
</td>
<td>
<blockquote><img SRC="points_conic.gif" height=183 width=236></blockquote>
</td>
</tr>
<tr>
<td>
<blockquote><img SRC="points_cubic.gif" height=162 width=214></blockquote>
</td>
<td>
<blockquote><img SRC="points_conic2.gif" height=204 width=225></blockquote>
</td>
</tr>
</table></center>
<h4>
b. Outline descriptor :</h4>
<blockquote>A FreeType outline is described through a simple structure,
called <tt>FT_Outline</tt>, which fields are :
<br>&nbsp;
<br>&nbsp;
<center><table CELLSPACING=3 CELLPADDING=3 BGCOLOR="#CCCCCC" >
<tr>
<td><b><tt>n_points</tt></b></td>
<td>the number of points in the outline</td>
</tr>
<tr>
<td><b><tt>n_contours</tt></b></td>
<td>the number of contours in the outline</td>
</tr>
<tr>
<td><b><tt>points</tt></b></td>
<td>array of point coordinates</td>
</tr>
<tr>
<td><b><tt>contours</tt></b></td>
<td>array of contour end indices</td>
</tr>
<tr>
<td><b><tt>flags</tt></b></td>
<td>array of point flags</td>
</tr>
</table></center>
<p>Here, <b><tt>points</tt></b> is a pointer to an array of <tt>FT_Vector</tt>
records, used to store the vectorial coordinates of each outline point.
These are expressed in 1/64th of a pixel, which is also known as the <i>26.6
fixed float format</i>.
<p><b><tt>contours</tt></b> is an array of point indices used to delimit
contours in the outline. For example, the first contour always starts at
point 0, and ends a point <b><tt>contours[0]</tt></b>. The second contour
starts at point "<b><tt>contours[0]+1</tt></b>" and ends at <b><tt>contours[1]</tt></b>,
etc..
<p>Note that each contour is closed, and that <b><tt>n_points</tt></b>
should be equal to "<b><tt>contours[n_contours-1]+1</tt></b>" for a valid
outline.
<p>Finally, <b><tt>flags</tt></b> is an array of bytes, used to store each
outline point's tag.
<br>&nbsp;
<br>&nbsp;</blockquote>
</blockquote>
<h3>
2. Bounding and control box computations :</h3>
<blockquote>A <b>bounding box</b> (also called "<b>bbox</b>") is simply
the smallest possible rectangle that encloses the shape of a given outline.
Because of the way arcs are defined, bezier control points are not necessarily
contained within an outline's bounding box.
<p>This situation happens when one bezier arc is, for example, the upper
edge of an outline and an off point happens to be above the bbox. However,
it is very rare in the case of character outlines because most font designers
and creation tools always place on points at the extrema of each curved
edges, as it makes hinting much easier.
<p>We thus define the <b>control box</b> (a.k.a. the "<b>cbox</b>") as
the smallest possible rectangle that encloses all points of a given outline
(including its off points). Clearly, it always includes the bbox, and equates
it in most cases.
<p>Unlike the bbox, the cbox is also much faster to compute.
<br>&nbsp;
<center><table>
<tr>
<td><img SRC="bbox1.gif" height=264 width=228></td>
<td><img SRC="bbox2.gif" height=229 width=217></td>
</tr>
</table></center>
<p>Control and bounding boxes can be computed automatically through the
functions <b><tt>FT_Get_Outline_CBox</tt></b> and <b><tt>FT_Get_Outline_BBox</tt></b>.
The former function is always very fast, while the latter <i>may</i> be
slow in the case of "outside" control points (as it needs to find the extreme
of conic and cubic arcs for "perfect" computations). If this isn't the
case, it's as fast as computing the control box.
<p>Note also that even though most glyph outlines have equal cbox and bbox
to ease hinting, this is not necessary the case anymore when a
<br>transform like rotation is applied to them.
<br>&nbsp;</blockquote>
<h3>
&nbsp;3. Coordinates, scaling and grid-fitting :</h3>
<blockquote>An outline point's vectorial coordinates are expressed in the
26.6 format, i.e. in 1/64th of a pixel, hence coordinates (1.0, -2.5) is
stored as the integer pair ( x:64, y: -192 ).
<p>After a master glyph outline is scaled from the EM grid to the current
character dimensions, the hinter or grid-fitter is in charge of aligning
important outline points (mainly edge delimiters) to the pixel grid. Even
though this process is much too complex to be described in a few lines,
its purpose is mainly to round point positions, while trying to preserve
important properties like widths, stems, etc..
<p>The following operations can be used to round vectorial distances in
the 26.6 format to the grid :
<center>
<p><tt>round(x)&nbsp;&nbsp; ==&nbsp; (x+32) &amp; -64</tt>
<br><tt>floor(x)&nbsp;&nbsp; ==&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; x &amp;
-64</tt>
<br><tt>ceiling(x) ==&nbsp; (x+63) &amp; -64</tt></center>
<p>Once a glyph outline is grid-fitted or transformed, it often is interesting
to compute the glyph image's pixel dimensions before rendering it. To do
so, one has to consider the following :
<p>The scan-line converter draws all the pixels whose <i>centers</i> fall
inside the glyph shape. It can also detect "<b><i>drop-outs</i></b>", i.e.
discontinuities coming from extremely thin shape fragments, in order to
draw the "missing" pixels. These new pixels are always located at a distance
less than half of a pixel but one cannot predict easily where they'll appear
before rendering.
<p>This leads to the following computations :
<br>&nbsp;
<ul>
<li>
compute the bbox</li>
</ul>
<ul>
<li>
grid-fit the bounding box with the following :</li>
</ul>
<ul>
<ul><tt>xmin = floor( bbox.xMin )</tt>
<br><tt>xmax = ceiling( bbox.xMax )</tt>
<br><tt>ymin = floor( bbox.yMin )</tt>
<br><tt>ymax = ceiling( bbox.yMax )</tt></ul>
<li>
return pixel dimensions, i.e. <tt>width = (xmax - xmin)/64</tt> and <tt>height
= (ymax - ymin)/64</tt></li>
</ul>
<p><br>By grid-fitting the bounding box, one guarantees that all the pixel
centers that are to be drawn, <b><i>including those coming from drop-out
control</i></b>, will be <b><i>within</i></b> the adjusted box. Then the
box's dimensions in pixels can be computed.
<p>Note also that, when <i>translating</i> a <i>grid-fitted outline</i>,
one should <b><i>always</i></b> use <b><i>integer distances</i></b> to
move an outline in the 2D plane. Otherwise, glyph edges won't be aligned
on the pixel grid anymore, and the hinter's work will be lost, producing
<b><i>very
low quality </i></b>bitmaps and pixmaps..</blockquote>
</blockquote>
<hr WIDTH="100%">
<h2>
VII. FreeType bitmaps :</h2>
<blockquote>The purpose of this section is to present the way FreeType
manages bitmaps and pixmaps, and how they relate to the concepts previously
defined. The relationships between vectorial and pixel coordinates is explained.
<br>&nbsp;
<h3>
1. FreeType bitmap and pixmap descriptor :</h3>
<blockquote>A bitmap or pixmap is described through a single structure,
called <tt>FT_Raster_Map</tt>. It is a simple descriptor whose fields are
:
<br>&nbsp;
<br>&nbsp;
<center><table CELLSPACING=3 CELLPADDING=5 BGCOLOR="#CCCCCC" >
<caption><tt>FT_Raster_Map</tt></caption>
<tr>
<td><b>rows</b></td>
<td>the number of rows, i.e. lines, in the bitmap</td>
</tr>
<tr>
<td><b>width</b></td>
<td>the number of horizontal pixels in the bitmap</td>
</tr>
<tr>
<td><b>cols</b></td>
<td>the number of "columns", i.e. bytes per line, in the bitmap</td>
</tr>
<tr>
<td><b>flow</b></td>
<td>the bitmap's flow, i.e. orientation of rows (see below)</td>
</tr>
<tr>
<td><b>pix_bits</b></td>
<td>the number of bits per pixels. valid values are 1, 4, 8 and 16</td>
</tr>
<tr>
<td><b>buffer</b></td>
<td>a typeless pointer to the bitmap pixel bufer</td>
</tr>
</table></center>
<p>The bitmap's <b><tt>flow</tt></b> determines wether the rows in the
pixel buffer are stored in ascending or descending order. Possible values
are <b><tt>FT_Flow_Up</tt></b> (value 1) and <b><tt>FT_Flow_Down</tt></b>
(value -1).
<p>Remember that FreeType uses the <i>Y upwards</i> convention in the 2D
plane. Which means that a coordinate of (0,0) always refer to the <i>lower-left
corner</i> of a bitmap.
<p>In the case of an '<i>up</i>' flow, the rows are stored in increasing
vertical position, which means that the first bytes of the pixel buffer
are part of the <i>lower</i> bitmap row. On the opposite, a '<i>down</i>'
flow means that the first buffer bytes are part of the <i>upper</i> bitmap
row, i.e. the last one in ascending order.
<p>As a hint, consider that when rendering an outline into a Windows or
X11 bitmap buffer, one should always use a down flow in the bitmap descriptor.
<br>&nbsp;
<center><table>
<tr>
<td><img SRC="up_flow.gif" height=298 width=291></td>
<td><img SRC="down_flow.gif" height=298 width=313></td>
</tr>
<tr>
<td></td>
<td></td>
</tr>
</table></center>
</blockquote>
<h3>
2. Vectorial versus pixel coordinates :</h3>
<blockquote>This sub-section explains the differences between vectorial
and pixel coordinates. To make things clear, brackets will be used to describe
pixel coordinates, e.g. [3,5], while parentheses will be used for vectorial
ones, e.g. (-2,3.5).
<p>In the pixel case, as we use the <i>Y upwards</i> convention, the coordinate
[0,0] always refers to the <i>lower left pixel</i> of a bitmap, while coordinate
[width-1, rows-1] to its <i>upper right pixel</i>.
<p>In the vectorial case, point coordinates are expressed in floating units,
like (1.25, -2.3). Such a position doesn't refer to a given pixel, but
simply to an immaterial point in the 2D plane
<p>The pixels themselves are indeed <i>square boxes</i> of the 2D plane,
which centers lie in half pixel coordinates. For example, the <i>lower
left pixel</i> of a bitmap is delimited by the <i>square</i> (0,0)-(1,1),
its center being at location (0.5,0.5).
<p>This introduces some differences when computing distances. For example,
the "<i>length</i>" in pixels of the line [0,0]-[10,0] is 11. However,
the vectorial distance between (0,0)-(10,0) covers exactly 10 pixel centers,
hence its length if 10.
<center><img SRC="grid_1.gif" height=390 width=402></center>
</blockquote>
<h3>
3. Converting outlines into bitmaps and pixmaps :</h3>
<blockquote>Generating a bitmap or pixmap image from a vectorial image
is easy with FreeType. However, one must understand a few points regarding
the positioning of the outline in the 2D plane before calling the function
<b><tt>FT_Get_Outline_Bitmap</tt></b>.
These are :
<br>&nbsp;
<ul>
<li>
The glyph loader and hinter always places the outline in the 2D plane so
that (0,0) matches its character origin. This means that the glyph’s outline,
and corresponding bounding box, can be placed anywhere in the 2D plane
(see the graphics in section III).</li>
</ul>
<ul>
<li>
The target bitmap’s area is mapped to the 2D plane, with its lower left
corner at (0,0). This means that a bitmap or pixmap of dimensions [<tt>w,h</tt>]
will be mapped to a 2D rectangle window delimited by (0,0)-(<tt>w,h</tt>).</li>
</ul>
<ul>
<li>
When calling <b><tt>FT_Get_Outline_Bitmap</tt></b>, everything that falls
within the bitmap window is rendered, the rest is ignored.</li>
</ul>
<p><br>A common mistake made by many developers when they begin using FreeType
is believing that a loaded outline can be directly rendered in a bitmap
of adequate dimensions. The following images illustrate why this is a problem
:
<ul>
<ul>
<li>
the first image shows a loaded outline in the 2D plane.</li>
<li>
the second one shows the target window for a bitmap of arbitrary dimensions
[w,h]</li>
<li>
the third one shows the juxtaposition of the outline and window in the
2D plane</li>
<li>
the last image shows what will really be rendered in the bitmap.</li>
</ul>
</ul>
<center><img SRC="clipping.gif" height=151 width=539></center>
<p><br>
<br>
<br>
<br>
<br>
<p>Indeed, in nearly all cases, the loaded or transformed outline must
be translated before it is rendered into a target bitmap, in order to adjust
its position relative to the target window.
<p>For example, the correct way of creating a <i>standalone</i> glyph bitmap
is thus to :
<br>&nbsp;
<ul>
<li>
Compute the size of the glyph bitmap. It can be computed directly from
the glyph metrics, or by computing its bounding box (this is useful when
a transform has been applied to the outline after the load, as the glyph
metrics are not valid anymore).</li>
</ul>
<ul>
<li>
Create the bitmap with the computed dimensions. Don’t forget to fill the
pixel buffer with the background color.</li>
</ul>
<ul>
<li>
Translate the outline so that its lower left corner matches (0,0). Don’t
forget that in order to preserve hinting, one should use integer, i.e.
rounded distances (of course, this isn’t required if preserving hinting
information doesn’t matter, like with rotated text). Usually, this means
translating with a vector <tt>( -ROUND(xMin), -ROUND(yMin) )</tt>.</li>
</ul>
<ul>
<li>
Call the function <b><tt>FT_Get_Outline_Bitmap</tt></b>.</li>
</ul>
<p><br>In the case where one wants to write glyph images directly into
a large bitmap, the outlines must be translated so that their vectorial
position correspond to the current text cursor/character origin.</blockquote>
</blockquote>
<h2>
<hr WIDTH="100%"></h2>
<h2>
VII. FreeType anti-aliasing :</h2>
<b><i>IMPORTANT NOTE :</i></b>
<br>This section is still in progress, as the way FreeType 2 handles anti-aliased
rendering hasn't been definitely set yet. The main reason being that a
flexible way of doing things is needed in order to allow further improvements
in the raster (i.e. number of gray levels > 100, etc..).
<blockquote>
<h3>
1. What is anti-aliasing :</h3>
<blockquote>Anti-aliasing works by using various levels of grays to reduce
the "staircase" artefacts visible on the diagonals and curves of glyph
bitmaps. It is a way to artificially enhance the display resolution of
the target device. It can smooth out considerably displayed or printed
text.</blockquote>
<h3>
2. How does it work with FreeType :</h3>
<blockquote>FreeType's scan-line converter is able to produce anti-aliased
output directly. It is however limited to 8-bit pixmaps and 5 levels of
grays (or 17 levels, depending on a build configuration option). Here's
how one should use it :
<h4>
a. Set the gray-level palette :</h4>
<blockquote>The scan-line converter uses 5 levels for anti-aliased output.
Level 0 corresponds to the text background color (e.g. white), and level
5 to the text foreground color. Intermediate levels are used for intermediate
shades of grays.
<p>You must set the raster's palette when you want to use different colors,
use the function <b><tt>FT_Raster_Set_Palette</tt></b> as in :
<p><tt>{</tt>
<br><tt>&nbsp; static const char&nbsp; gray_palette[5] = { 0, 7, 15, 31,
63 };</tt>
<br><tt>&nbsp; …</tt>
<br><tt>&nbsp; error = FT_Set_Raster_Palette( library, 5, palette );</tt>
<br><tt>}</tt>
<br>&nbsp;
<ul>
<li>
The first parameter is a handle to a FreeType library object. See the user
guide for more details (the library contains a scan-line converter object).</li>
</ul>
<ul>
<li>
The second parameter is the number of entries in the gray-level palette.
Valid values are 5 and 17 for now, but this may change in later implementations.</li>
</ul>
<ul>
<li>
The last parameter is a pointer to a char table containing the pixel value
for each of the gray-levels. In this example, we use a background color
of 0, a foreground color of 63, and intermediate values in-between.</li>
</ul>
<p><br>The palette is copied in the raster object, as well as processed
to build several lookup-tables necessary for the internal anti-aliasing
algorithm.
<br>&nbsp;</blockquote>
<h4>
b. Render the pixmap :</h4>
<blockquote>The scan-line converter doesn't create bitmaps or pixmaps,
it simply renders into those that are passed as parameters to the function
<b><tt>FT_Get_Outline_Bitmap</tt></b>.
To render an anti-aliased pixmap, simply set the target bitmap’s depth
to 8. Note however that this target 8-bit pixmap must always have a '<b><tt>cols</tt></b>'
field padded to 32-bits, which means that the number of bytes per lines
of the pixmap must be a multiple of 4 !
<p>Once the palette has been set, and the pixmap buffer has been created
to receive the glyph image, simply call <b><tt>FT_Get_Outline_Bitmap</tt></b>.
Take care of clearing the target pixmap with the background color before
calling this function. For the sake of simplicity and efficiency, the raster
is not able to compose anti-aliased glyph images on a pre-existing images.
<p>Here's some code demonstrating how to load and render a single glyph
pixmap :
<p><tt>{</tt>
<br><tt>&nbsp; FT_Outline&nbsp;&nbsp;&nbsp;&nbsp; outline;</tt>
<br><tt>&nbsp; FT_Raster_Map&nbsp; pixmap;</tt>
<br><tt>&nbsp; FT_BBox&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; cbox;</tt>
<br><tt>&nbsp; …</tt>
<p><i><tt>&nbsp; // load the outline</tt></i>
<br><tt>&nbsp; …</tt>
<p><i><tt>&nbsp; // compute glyph dimensions (grid-fit cbox, etc..)</tt></i>
<br><tt>&nbsp; FT_Get_Outline_CBox( &amp;outline, &amp;cbox );</tt>
<p><tt>&nbsp; cbox.xMin = cbox.xMin &amp; -64;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
// floor(xMin)</tt>
<br><tt>&nbsp; cbox.yMin = cbox.yMin &amp; -64;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
// floor(yMin)</tt>
<br><tt>&nbsp; cbox.xMax = (cbox.xMax+32) &amp; -64;&nbsp; // ceiling(xMax)</tt>
<br><tt>&nbsp; cbox.yMax = (cbox.yMax+32) &amp; -64;&nbsp; // ceiling(yMax)</tt>
<p><tt>&nbsp; pixmap.width = (cbox.xMax - cbox.xMin)/64;</tt>
<br><tt>&nbsp; pixmap.rows&nbsp; = (cbox.yMax - cbox.yMin)/64;</tt>
<p><i><tt>&nbsp; // fill the pixmap descriptor and create the pixmap buffer</tt></i>
<br><i><tt>&nbsp; // don't forget to pad the 'cols' field to 32 bits</tt></i>
<br><tt>&nbsp; pixmap.pix_bits = 8;</tt>
<br><tt>&nbsp; pixmap.flow&nbsp;&nbsp;&nbsp;&nbsp; = FT_Flow_Down;</tt>
<br><tt>&nbsp; pixmap.cols&nbsp;&nbsp;&nbsp;&nbsp; = (pixmap.width+3) &amp;
-4;&nbsp; // pad 'cols' to 32 bits</tt>
<br><tt>&nbsp; pixmap.buffer&nbsp;&nbsp; = malloc( pixmap.cols * pixmap.rows
);</tt>
<p><i><tt>&nbsp; // fill the pixmap buffer with the background color</tt></i>
<br><i><tt>&nbsp; //</tt></i>
<br><tt>&nbsp; memset( pixmap.buffer, 0, pixmap.cols*pixmap.rows );</tt>
<p><i><tt>&nbsp; // translate the outline to match (0,0) with the glyph's</tt></i>
<br><i><tt>&nbsp; // lower left corner (ignore the bearings)</tt></i>
<br><i><tt>&nbsp; // the cbox is grid-fitted, we won't ruin the hinting.</tt></i>
<br><i><tt>&nbsp; //</tt></i>
<br><tt>&nbsp; FT_Translate_Outline( &amp;outline, -cbox.xMin, -cbox.yMin
);</tt>
<p><i><tt>&nbsp; // render the anti-aliased glyph pixmap</tt></i>
<br><tt>&nbsp; error = FT_Get_Outline_Bitmap( library, &amp;outline, &amp;pixmap
);</tt>
<p><tt>&nbsp; // save the bearings for later use..</tt>
<br><tt>&nbsp; corner_x = cbox.xMin / 64;</tt>
<br><tt>&nbsp; corner_y = cbox.yMin / 64;</tt>
<br><tt>}</tt>
<p>The resulting pixmap is always anti-aliased.</blockquote>
</blockquote>
<h3>
3. Possible enhancements :</h3>
<blockquote>FreeType's raster (i.e. its scan-line converter) is currently
limited to producing either 1-bit bitmaps or anti-aliased 8-bit pixmaps.
It is not possible, for example, to draw directly a bitmapped glyph image
into a 4, 8 or 16-bit pixmap through a call to FT_Get_Outline_Bitmap.
<p>Moreover, the anti-aliasing filter is limited to use 5 or 17 levels
of grays (through 2x2 and 4x4 sub-sampling). There are cases where this
could seem insufficient for optimal results and where a higher number of
levels like 64 or 128 would be a good thing.
<p>These enhancements are all possible but not planned for an immediate
future of the FreeType engine.</blockquote>
</blockquote>
</body>
</html>