harfbuzz/docs/usermanual-getting-started.xml

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<chapter id="getting-started">
  <title>Getting started with HarfBuzz</title>
  <section id="an-overview-of-the-harfbuzz-shaping-api">
    <title>An overview of the HarfBuzz shaping API</title>
    <para>
      The core of the HarfBuzz shaping API is the function
      <function>hb_shape()</function>. This function takes a font, a
      buffer containing a string of Unicode codepoints and
      (optionally) a list of font features as its input. It replaces
      the codepoints in the buffer with the corresponding glyphs from
      the font, correctly ordered and positioned, and with any of the
      optional font features applied.
    </para>
    <para>
      In addition to holding the pre-shaping input (the Unicode
      codepoints that comprise the input string) and the post-shaping
      output (the glyphs and positions), a HarfBuzz buffer has several
      properties that affect shaping. The most important are the
      text-flow direction (e.g., left-to-right, right-to-left,
      top-to-bottom, or bottom-to-top), the script tag, and the
      language tag.
    </para>

    <para>
      For input string buffers, flags are available to denote when the
      buffer represents the beginning or end of a paragraph, to
      indicate whether or not to visibly render Unicode <literal>Default
      Ignorable</literal> codepoints, and to modify the cluster-merging
      behavior for the buffer. For shaped output buffers, the
      individual X and Y offsets and <literal>advances</literal>
      (the logical dimensions) of each glyph are 
      accessible. HarfBuzz also flags glyphs as
      <literal>UNSAFE_TO_BREAK</literal> if breaking the string at
      that glyph (e.g., in a line-breaking or hyphenation process)
      would require re-shaping the text.
    </para>
    
    <para>
      HarfBuzz also provides methods to compare the contents of
      buffers, join buffers, normalize buffer contents, and handle
      invalid codepoints, as well as to determine the state of a
      buffer (e.g., input codepoints or output glyphs). Buffer
      lifecycles are managed and all buffers are reference-counted.
    </para>

    <para>
      Although the default <function>hb_shape()</function> function is
      sufficient for most use cases, a variant is also provide that
      lets you specify which of HarfBuzz's shapers to use on a buffer. 
    </para>

    <para>
      HarfBuzz can read TrueType fonts, TrueType collections, OpenType
      fonts, and OpenType collections. Functions are provided to query
      font objects about metrics, Unicode coverage, available tables and
      features, and variation selectors. Individual glyphs can also be
      queried for metrics, variations, and glyph names. OpenType
      variable fonts are supported, and HarfBuzz allows you to set
      variation-axis coordinates on font objects.
    </para>
    
    <para>
      HarfBuzz provides glue code to integrate with various other
      libraries, including FreeType, GObject, and CoreText. Support
      for integrating with Uniscribe and DirectWrite is experimental
      at present.
    </para>
  </section>

  <section id="terminology">
    <title>Terminology</title>
    <para>
      
    </para>
      <variablelist>
	<?dbfo list-presentation="blocks"?> 
	<varlistentry>
	  <term>script</term>
	  <listitem>
	    <para>
	      In text shaping, a <emphasis>script</emphasis> is a
	      writing system: a set of symbols, rules, and conventions
	      that is used to represent a language or multiple
	      languages.
	    </para>
	    <para>
	      In general computing lingo, the word "script" can also
	      be used to mean an executable program (usually one
	      written in a human-readable programming language). For
	      the sake of clarity, HarfBuzz documents will always use
	      more specific terminology when referring to this
	      meaning, such as "Python script" or "shell script." In
	      all other instances, "script" refers to a writing system.
	    </para>
	    <para>
	      For developers using HarfBuzz, it is important to note
	      the distinction between a script and a language. Most
	      scripts are used to write a variety of different
	      languages, and many languages may be written in more
	      than one script.
	    </para>
	  </listitem>
	</varlistentry>
	
	<varlistentry>
	  <term>shaper</term>
	  <listitem>
	    <para>
	      In HarfBuzz, a <emphasis>shaper</emphasis> is a
	      handler for a specific script-shaping model. HarfBuzz
	      implements separate shapers for Indic, Arabic, Thai and
	      Lao, Khmer, Myanmar, Tibetan, Hangul, Hebrew, the
	      Universal Shaping Engine (USE), and a default shaper for
	      non-complex scripts. 
	    </para>
	  </listitem>
	</varlistentry>
	
	<varlistentry>
	  <term>cluster</term>
	  <listitem>
	    <para>
	      In text shaping, a <emphasis>cluster</emphasis> is a
	      sequence of codepoints that must be treated as an
	      indivisible unit. Clusters can include code-point
	      sequences that form a ligature or base-and-mark
	      sequences. Tracking and preserving clusters is important
	      when shaping operations might separate or reorder
	      code points.
	    </para>
	    <para>
	      HarfBuzz provides three cluster
	      <emphasis>levels</emphasis> that implement different
	      approaches to the problem of preserving clusters during
	      shaping operations.
	    </para>
	  </listitem>
	</varlistentry>
	
	<varlistentry>
	  <term>grapheme</term>
	  <listitem>
	    <para>
	      In linguistics, a <emphasis>grapheme</emphasis> is one
	      of the indivisible units that make up a writing system or
	      script. Often, graphemes are individual symbols (letters,
	      numbers, punctuation marks, logograms, etc.) but,
	      depending on the writing system, a particular grapheme
	      might correspond to a sequence of several Unicode code
	      points.
	    </para>
	    <para>
	      In practice, HarfBuzz and other text-shaping engines
	      are not generally concerned with graphemes. However, it
	      is important for developers using HarfBuzz to recognize
	      that there is a difference between graphemes and shaping
	      clusters (see above). The two concepts may overlap
	      frequently, but there is no guarantee that they will be
	      identical.
	    </para>
	  </listitem>
	</varlistentry>
	
	<varlistentry>
	  <term>syllable</term>
	  <listitem>
	    <para>
	      In linguistics, a <emphasis>syllable</emphasis> is an 
	      a sequence of sounds that makes up a building block of a
	      particular language. Every language has its own set of
	      rules describing what constitutes a valid syllable.
	    </para>
	    <para>
	      For text-shaping purposes, the various definitions of
	      "syllable" are important because script-specific shaping
	      operations may be applied at the syllable level. For
	      example, a reordering rule might specify that a vowel
	      mark be reordered to the beginning of the syllable.
	    </para>
	    <para>
	      Syllables will consist of one or more Unicode code
	      points. The definition of a syllable for a particular
	      writing system might correspond to how HarfBuzz
	      identifies clusters (see above) for the same writing
	      system. However, it is important for developers using
	      HarfBuzz to recognize that there is a difference between
	      syllables and shaping clusters. The two concepts may
	      overlap frequently, but there is no guarantee that they
	      will be identical.
	    </para>
	  </listitem>
	</varlistentry>
      </variablelist>
    
  </section>


  <section id="a-simple-shaping-example">
    <title>A simple shaping example</title>

    <para>
      Below is the simplest HarfBuzz shaping example possible.
    </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, -1, 0, -1);
    </programlisting>
    <orderedlist numeration="arabic">
      <listitem override="2">
	<para>
          Set the script, language and direction of the buffer.
	</para>
      </listitem>
    </orderedlist>
    <programlisting language="C">
      hb_buffer_set_direction(buf, HB_DIRECTION_LTR);
      hb_buffer_set_script(buf, HB_SCRIPT_LATIN);
      hb_buffer_set_language(buf, hb_language_from_string("en", -1));
    </programlisting>
    <orderedlist numeration="arabic">
      <listitem override="3">
	<para>
          Create a face and a font from a font file.
	</para>
      </listitem>
    </orderedlist>
    <programlisting language="C">
      hb_blob_t *blob = hb_blob_create_from_file(filename);
      hb_face_t *face = hb_face_create(blob, 0);
      hb_font_t *font = hb_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">
      unsigned int glyph_count;
      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">
      hb_position_t cursor_x = 0;
      hb_position_t cursor_y = 0;
      for (unsigned int i = 0; i &lt; glyph_count; i++) {
          hb_codepoint_t glyphid  = glyph_info[i].codepoint;
          hb_position_t x_offset  = glyph_pos[i].x_offset;
          hb_position_t y_offset  = glyph_pos[i].y_offset;
          hb_position_t x_advance = glyph_pos[i].x_advance;
          hb_position_t y_advance = glyph_pos[i].y_advance;
       /* 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(font);
      hb_face_destroy(face);
      hb_blob_destroy(blob);
    </programlisting>
    
    <para>
      This example shows enough to get us started using HarfBuzz. In
      the sections that follow, we will use the remainder of
      HarfBuzz's API to refine and extend the example and improve its
      text-shaping capabilities.
    </para>
  </section>
</chapter>