harfbuzz/docs/usermanual-ch02.xml

<chapter 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>
  <section 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>
      <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>
  </section>
</chapter>