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Third Party Harfbuzz
提交 c424b417 编写于 作者: B Behdad Esfahbod
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Merge pull request #129 from simoncozens/docs 

First two chapters. More to follow.
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<sect1 id="what-is-harfbuzz">
<title>What is Harfbuzz?</title>
<para>
Harfbuzz is a <emphasis>text shaping engine</emphasis>. It solves
the problem of selecting and positioning glyphs from a font given a
Unicode string.
</para>
<sect2 id="why-do-i-need-it">
<title>Why do I need it?</title>
<para>
Text shaping is an integral part of preparing text for display. It
is a fairly low level operation; Harfbuzz is used directly by
graphic rendering libraries such as Pango, and the layout engines
in Firefox, LibreOffice and Chromium. Unless you are
<emphasis>writing</emphasis> one of these layout engines yourself,
you will probably not need to use Harfbuzz - normally higher level
libraries will turn text into glyphs for you.
</para>
<para>
However, if you <emphasis>are</emphasis> writing a layout engine
or graphics library yourself, you will need to perform text
shaping, and this is where Harfbuzz can help you. Here are some
reasons why you need it:
</para>
<itemizedlist>
<listitem>
<para>
OpenType fonts contain a set of glyphs, indexed by glyph ID.
The glyph ID within the font does not necessarily relate to a
Unicode codepoint. For instance, some fonts have the letter
&quot;a&quot; as glyph ID 1. To pull the right glyph out of
the font in order to display it, you need to consult a table
within the font (the &quot;cmap&quot; table) which maps
Unicode codepoints to glyph IDs. Text shaping turns codepoints
into glyph IDs.
</para>
</listitem>
<listitem>
<para>
Many OpenType fonts contain ligatures: combinations of
characters which are rendered together. For instance, it's
common for the <literal>fi</literal> combination to appear in
print as the single ligature &quot;fi&quot;. Whether you should
render text as <literal>fi</literal> or &quot;fi&quot; does not
depend on the input text, but on the capabilities of the font
and the level of ligature application you wish to perform.
Text shaping involves querying the font's ligature tables and
determining what substitutions should be made.
</para>
</listitem>
<listitem>
<para>
While ligatures like &quot;fi&quot; are typographic
refinements, some languages <emphasis>require</emphasis> such
substitutions to be made in order to display text correctly.
In Tamil, when the letter &quot;TTA&quot; (ட) letter is
followed by &quot;U&quot; (உ), the combination should appear
as the single glyph &quot;டு&quot;. The sequence of Unicode
characters &quot;டஉ&quot; needs to be rendered as a single
glyph from the font - text shaping chooses the correct glyph
from the sequence of characters provided.
</para>
</listitem>
<listitem>
<para>
Similarly, each Arabic character has four different variants:
within a font, there will be glyphs for the initial, medial,
final, and isolated forms of each letter. Unicode only encodes
one codepoint per character, and so a Unicode string will not
tell you which glyph to use. Text shaping chooses the correct
form of the letter and returns the correct glyph from the font
that you need to render.
</para>
</listitem>
<listitem>
<para>
Other languages have marks and accents which need to be
rendered in certain positions around a base character. For
instance, the Moldovan language has the Cyrillic letter
&quot;zhe&quot; (ж) with a breve accent, like so: ӂ. Some
fonts will contain this character as an individual glyph,
whereas other fonts will not contain a zhe-with-breve glyph
but expect the rendering engine to form the character by
overlaying the two glyphs ж and ˘. Where you should draw the
combining breve depends on the height of the preceding glyph.
Again, for Arabic, the correct positioning of vowel marks
depends on the height of the character on which you are
placing the mark. Text shaping tells you whether you have a
precomposed glyph within your font or if you need to compose a
glyph yourself out of combining marks, and if so, where to
position those marks.
</para>
</listitem>
</itemizedlist>
<para>
If this is something that you need to do, then you need a text
shaping engine: you could use Uniscribe if you are using Windows;
you could use CoreText on OS X; or you could use Harfbuzz. In the
rest of this manual, we are going to assume that you are the
implementor of a text layout engine.
</para>
</sect2>
<sect2 id="why-is-it-called-harfbuzz">
<title>Why is it called Harfbuzz?</title>
<para>
Harfbuzz began its life as text shaping code within the FreeType
project, (and you will see references to the FreeType authors
within the source code copyright declarations) but was then
abstracted out to its own project. This project is maintained by
Behdad Esfahbod, and named Harfbuzz. Originally, it was a shaping
engine for OpenType fonts - &quot;Harfbuzz&quot; is the Persian
for &quot;open type&quot;.
</para>
</sect2>
</sect1>
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<sect1 id="hello-harfbuzz">
<title>Hello, Harfbuzz</title>
<para>
Here's the simplest Harfbuzz that can possibly work. We will improve
it later.
</para>
<orderedlist numeration="arabic">
<listitem>
<para>
Create a buffer and put your text in it.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
#include &lt;hb.h&gt;
hb_buffer_t *buf;
buf = hb_buffer_create();
hb_buffer_add_utf8(buf, text, strlen(text), 0, strlen(text));
</programlisting>
<orderedlist numeration="arabic">
<listitem override="2">
<para>
Guess the script, language and direction of the buffer.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
hb_buffer_guess_segment_properties(buf);
</programlisting>
<orderedlist numeration="arabic">
<listitem override="3">
<para>
Create a face and a font, using FreeType for now.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
#include &lt;hb-ft.h&gt;
FT_New_Face(ft_library, font_path, index, &amp;face)
hb_font_t *font = hb_ft_font_create(face);
</programlisting>
<orderedlist numeration="arabic">
<listitem override="4">
<para>
Shape!
</para>
</listitem>
</orderedlist>
<programlisting>
hb_shape(font, buf, NULL, 0);
</programlisting>
<orderedlist numeration="arabic">
<listitem override="5">
<para>
Get the glyph and position information.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
hb_glyph_info_t *glyph_info = hb_buffer_get_glyph_infos(buf, &amp;glyph_count);
hb_glyph_position_t *glyph_pos = hb_buffer_get_glyph_positions(buf, &amp;glyph_count);
</programlisting>
<orderedlist numeration="arabic">
<listitem override="6">
<para>
Iterate over each glyph.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
for (i = 0; i &lt; glyph_count; ++i) {
glyphid = glyph_info[i].codepoint;
x_offset = glyph_pos[i].x_offset / 64.0;
y_offset = glyph_pos[i].y_offset / 64.0;
x_advance = glyph_pos[i].x_advance / 64.0;
y_advance = glyph_pos[i].y_advance / 64.0;
draw_glyph(glyphid, cursor_x + x_offset, cursor_y + y_offset);
cursor_x += x_advance;
cursor_y += y_advance;
}
</programlisting>
<orderedlist numeration="arabic">
<listitem override="7">
<para>
Tidy up.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
hb_buffer_destroy(buf);
hb_font_destroy(hb_ft_font);
</programlisting>
<sect2 id="what-harfbuzz-doesnt-do">
<title>What Harfbuzz doesn't do</title>
<para>
The code above will take a UTF8 string, shape it, and give you the
information required to lay it out correctly on a single
horizontal (or vertical) line using the font provided. That is the
extent of Harfbuzz's responsibility.
</para>
<para>
If you are implementing a text layout engine you may have other
responsibilities, that Harfbuzz will not help you with:
</para>
<itemizedlist>
<listitem>
<para>
Harfbuzz won't help you with bidirectionality. If you want to
lay out text with mixed Hebrew and English, you will need to
ensure that the buffer provided to Harfbuzz has those
characters in the correct layout order. This will be different
from the logical order in which the Unicode text is stored. In
other words, the user will hit the keys in the following
sequence:
</para>
<programlisting>
A B C [space] ג ב א [space] D E F
</programlisting>
<para>
but will expect to see in the output:
</para>
<programlisting>
ABC אבג DEF
</programlisting>
<para>
This reordering is called <emphasis>bidi processing</emphasis>
(&quot;bidi&quot; is short for bidirectional), and there's an
algorithm as an annex to the Unicode Standard which tells you how
to reorder a string from logical order into presentation order.
Before sending your string to Harfbuzz, you may need to apply the
bidi algorithm to it. Libraries such as ICU and fribidi can do
this for you.
</para>
<listitem>
<para>
Harfbuzz won't help you with text that contains different font
properties. For instance, if you have the string &quot;a
<emphasis>huge</emphasis> breakfast&quot;, and you expect
&quot;huge&quot; to be italic, you will need to send three
strings to Harfbuzz: <literal>a</literal>, in your Roman font;
<literal>huge</literal> using your italic font; and
<literal>breakfast</literal> using your Roman font again.
Similarly if you change font, font size, script, language or
direction within your string, you will need to shape each run
independently and then output them independently. Harfbuzz
expects to shape a run of characters sharing the same
properties.
</para>
</listitem>
<listitem>
<para>
Harfbuzz won't help you with line breaking, hyphenation or
justification. As mentioned above, it lays out the string
along a <emphasis>single line</emphasis> of, notionally,
infinite length. If you want to find out where the potential
word, sentence and line break points are in your text, you
could use the ICU library's break iterator functions.
</para>
<para>
Harfbuzz can tell you how wide a shaped piece of text is, which is
useful input to a justification algorithm, but it knows nothing
about paragraphs, lines or line lengths. Nor will it adjust the
space between words to fit them proportionally into a line. If you
want to layout text in paragraphs, you will probably want to send
each word of your text to Harfbuzz to determine its shaped width
after glyph substitutions, then work out how many words will fit
on a line, and then finally output each word of the line separated
by a space of the correct size to fully justify the paragraph.
</para>
</listitem>
</itemizedlist>
<para>
As a layout engine implementor, Harfbuzz will help you with the
interface between your text and your font, and that's something
that you'll need - what you then do with the glyphs that your font
returns is up to you. The example we saw above enough to get us
started using Harfbuzz. Now we are going to use the remainder of
Harfbuzz's API to refine that example and improve our text shaping
capabilities.
</para>
</sect2>
</sect1>
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<sect1 id="buffers-language-script-and-direction">
<title>Buffers, language, script and direction</title>
<para>
The input to Harfbuzz is a series of Unicode characters, stored in a
buffer. In this chapter, we'll look at how to set up a buffer with
the text that we want and then customize the properties of the
buffer.
</para>
<sect2 id="creating-and-destroying-buffers">
<title>Creating and destroying buffers</title>
<para>
As we saw in our initial example, a buffer is created and
initialized with <literal>hb_buffer_create()</literal>. This
produces a new, empty buffer object, instantiated with some
default values and ready to accept your Unicode strings.
</para>
<para>
Harfbuzz manages the memory of objects that it creates (such as
buffers), so you don't have to. When you have finished working on
a buffer, you can call <literal>hb_buffer_destroy()</literal>:
</para>
<programlisting language="C">
hb_buffer_t *buffer = hb_buffer_create();
...
hb_buffer_destroy(buffer);
</programlisting>
<para>
This will destroy the object and free its associated memory -
unless some other part of the program holds a reference to this
buffer. If you acquire a Harfbuzz buffer from another subsystem
and want to ensure that it is not garbage collected by someone
else destroying it, you should increase its reference count:
</para>
<programlisting language="C">
void somefunc(hb_buffer_t *buffer) {
buffer = hb_buffer_reference(buffer);
...
</programlisting>
<para>
And then decrease it once you're done with it:
</para>
<programlisting language="C">
hb_buffer_destroy(buffer);
}
</programlisting>
<para>
To throw away all the data in your buffer and start from scratch,
call <literal>hb_buffer_reset(buffer)</literal>. If you want to
throw away the string in the buffer but keep the options, you can
instead call <literal>hb_buffer_clear_contents(buffer)</literal>.
</para>
</sect2>
<sect2 id="adding-text-to-the-buffer">
<title>Adding text to the buffer</title>
<para>
Now we have a brand new Harfbuzz buffer. Let's start filling it
with text! From Harfbuzz's perspective, a buffer is just a stream
of Unicode codepoints, but your input string is probably in one of
the standard Unicode character encodings (UTF-8, UTF-16, UTF-3 )
</para>
</sect2>
<sect2 id="setting-buffer-properties">
<title>Setting buffer properties</title>
<para>
</para>
</sect2>
<sect2 id="what-about-the-other-scripts">
<title>What about the other scripts?</title>
<para>
</para>
</sect2>
<sect2 id="customizing-unicode-functions">
<title>Customizing Unicode functions</title>
<para>
</para>
</sect2>
</sect1>
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<sect1 id="fonts-and-faces">
<title>Fonts and faces</title>
<sect2 id="using-freetype">
<title>Using FreeType</title>
<para>
</para>
</sect2>
<sect2 id="using-harfbuzzs-native-opentype-implementation">
<title>Using Harfbuzz's native OpenType implementation</title>
<para>
</para>
</sect2>
<sect2 id="using-your-own-font-functions">
<title>Using your own font functions</title>
<para>
</para>
</sect2>
</sect1>
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<sect1 id="shaping-and-shape-plans">
<title>Shaping and shape plans</title>
<sect2 id="opentype-features">
<title>OpenType features</title>
<para>
</para>
</sect2>
<sect2 id="plans-and-caching">
<title>Plans and caching</title>
<para>
</para>
</sect2>
</sect1>
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<sect1 id="glyph-information">
<title>Glyph information</title>
<sect2 id="names-and-numbers">
<title>Names and numbers</title>
<para>
</para>
</sect2>
</sect1>
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