step1.html

嵌入式freetype库的应用文档包含freetype的简要说明和应用example

        <td>
          <tt>num_glyphs</tt>
        </td>
        <td>
          <p>This variable gives the number of <em>glyphs</em> available in
          the font face.  A glyph is simply a character image.  It doesn't
          necessarily correspond to a <em>character code</em> though.</p>
        </td>
      </tr>
      <tr valign="top">
        <td>
          <tt>flags</tt>
        </td>
        <td>
          <p>A 32-bit integer containing bit flags used to describe some
          face properties.  For example, the flag
          <tt>FT_FACE_FLAG_SCALABLE</tt> is used to indicate that the face's
          font format is scalable and that glyph images can be rendered for
          all character pixel sizes.  For more information on face flags,
          please read the <a
          href="../reference/ft2-index.html">FreeType&nbsp;2 API
          Reference</a>.</p>
        </td>
      </tr>
      <tr valign="top">
        <td>
          <tt>units_per_EM</tt>
        </td>
        <td>
          <p>This field is only valid for scalable formats (it is set
          to&nbsp;0 otherwise).  It indicates the number of font units
          covered by the EM.</p>
        </td>
      </tr>
      <tr valign="top">
        <td>
          <tt>num_fixed_sizes</tt>
        </td>
        <td>
          <p>This field gives the number of embedded bitmap strikes
          in the current face.  A <em>strike</em> is simply a series of
          glyph images for a given character pixel size.  For example, a
          font face could include strikes for pixel sizes 10, 12
          and&nbsp;14.  Note that even scalable font formats can have
          embedded bitmap strikes!</p>
        </td>
      </tr>
      <tr valign="top">
        <td>
          <tt>fixed_sizes</tt>
        </td>
        <td>
          <p>A pointer to an array of <tt>FT_Bitmap_Size</tt>
          elements.  Each <tt>FT_Bitmap_Size</tt> indicates the horizontal
          and vertical <em>character pixel sizes</em> for each of the strikes
          that are present in the face.</p>

          <p><font color="red">Note that, generally speaking, these are
          <em>not</em> the <em>cell size</em> of the bitmap strikes.</font>
          </p>
        </td>
      </tr>
    </table>

    <hr>

    <h3>
      5.&nbsp;Setting the current pixel size
    </h3>

    <p>FreeType&nbsp;2 uses <em>size objects</em> to model all information
    related to a given character size for a given face.  For example, a size
    object will hold the value of certain metrics like the ascender or text
    height, expressed in 1/64th of a pixel, for a character size of
    12&nbsp;points.</p>

    <p>When the <tt>FT_New_Face</tt> function is called (or one of its
    cousins), it <em>automatically</em> creates a new size object for the
    returned face.  This size object is directly accessible as
    <tt>face&minus;&gt;size</tt>.</p>

    <p><em>NOTE: A single face object can deal with one or more size
    objects at a time; however, this is something that few programmers
    really need to do.  We have thus decided to simplify the API for the
    most common use (i.e., one size per face) while keeping this feature
    available through additional functions.</em></p>

    <p>When a new face object is created, its size object defaults to the
    character size of 10&nbsp;pixels (both horizontally and vertically) for
    scalable formats.  For fixed-sizes formats, the size is more or less
    undefined, which is why you must set it before trying to load a
    glyph.</p>

    <p>To do that, simply call <a
    href="../reference/ft2-base_interface.html#FT_Set_Char_Size">
    <tt>FT_Set_Char_Size</tt></a>.  Here is an example where the character
    size is set to 16pt for a 300&times;300&nbsp;dpi device:</p>

    <div class="pre">
  error = FT_Set_Char_Size(
            face,    <span class="comment">/* handle to face object           */</span>
            0,       <span class="comment">/* char_width in 1/64th of points  */</span>
            16*64,   <span class="comment">/* char_height in 1/64th of points */</span>
            300,     <span class="comment">/* horizontal device resolution    */</span>
            300 );   <span class="comment">/* vertical device resolution      */</span>
    </div>

    <p>Some notes:</p>

    <ul>
      <li>
        <p>The character widths and heights are specified in 1/64th of
        points.  A point is a <em>physical</em> distance, equaling 1/72th
        of an inch.  Normally, it is not equivalent to a pixel.<p>
      </li>

      <li>
        <p>The horizontal and vertical device resolutions are expressed in
        <em>dots-per-inch</em>, or <em>dpi</em>.  Normal values are 72 or
        96&nbsp;dpi for display devices like the screen.  The resolution
        is used to compute the character pixel size from the character
        point size.</p>
      </li>

      <li>
        <p>A value of&nbsp;0 for the character width means &lsquo;same as
        character height&rsquo;, a value of&nbsp;0 for the character height
        means &lsquo;same as character width&rsquo;.  Otherwise, it is
        possible to specify different character widths and heights.</p>
      </li>

      <li>
        <p>Using a value of&nbsp;0 for the horizontal or vertical
        resolution means 72&nbsp;dpi, which is the default.</p>
      </li>

      <li>
        <p>The first argument is a handle to a face object, not a size
        object.</p>
      </li>
    </ul>

    <p>This function computes the character pixel size that corresponds to
    the character width and height and device resolutions.  However, if you
    want to specify the pixel sizes yourself, you can simply call
    <a href="../reference/ft2-base_interface.html#FT_Set_Pixel_Sizes">
    <tt>FT_Set_Pixel_Sizes</tt></a>, as in</p>

    <div class="pre">
  error = FT_Set_Pixel_Sizes(
            face,   <span class="comment">/* handle to face object */</span>
            0,      <span class="comment">/* pixel_width           */</span>
            16 );   <span class="comment">/* pixel_height          */</span>
    </div>

    <p>This example will set the character pixel sizes to
    16&times;16&nbsp;pixels.  As previously, a value of&nbsp;0 for one of
    the dimensions means &lsquo;same as the other&rsquo;.</p>

    <p>Note that both functions return an error code.  Usually, an error
    occurs with a fixed-size font format (like FNT or PCF) when trying to
    set the pixel size to a value that is not listed in the
    <tt>face-&gt;fixed_sizes</tt> array.</p>

    <hr>

    <h3>
      6.&nbsp;Loading a glyph image
    </h3>

      <h4>
        a.&nbsp;Converting a character code into a glyph index
      </h4>

      <p>Usually, an application wants to load a glyph image based on its
      <em>character code</em>, which is a unique value that defines the
      character for a given <em>encoding</em>.  For example, the character
      code&nbsp;65 represents the &lsquo;A&rsquo; in ASCII encoding.</p>

      <p>A face object contains one or more tables, called
      <em>charmaps</em>, that are used to convert character codes to glyph
      indices.  For example, most TrueType fonts contain two charmaps.  One
      is used to convert Unicode character codes to glyph indices, the other
      is used to convert Apple Roman encoding into glyph indices.  Such
      fonts can then be used either on Windows (which uses Unicode) and
      Macintosh (which uses Apple Roman).  Note also that a given
      charmap might not map to all the glyphs present in the font.</p>

      <p>By default, when a new face object is created, it selects a
      Unicode charmap.  FreeType tries to emulate a Unicode charmap if the
      font doesn't contain such a charmap, based on glyph names.  Note that
      it is possible that the emulation misses glyphs if glyph names are
      non-standard.  For some fonts, including symbol fonts and (older)
      fonts for Asian scripts, no Unicode emulation is possible at all.</p>

      <p>We will describe later how to look for specific charmaps in a face.
      For now, we will assume that the face contains at least a Unicode
      charmap that was selected during a call to <tt>FT_New_Face</tt>.  To
      convert a Unicode character code to a font glyph index, we use
      <tt>FT_Get_Char_Index</tt>, as in</p>

      <div class="pre">
  glyph_index = FT_Get_Char_Index( face, charcode );
      </div>

      <p>This will look the glyph index corresponding to the given
      <tt>charcode</tt> in the charmap that is currently selected for the
      face.  If no charmap was selected, the function simply returns the
      charcode.</p>

      <p>Note that this is one of the rare FreeType functions that do not
      return an error code.  However, when a given character code has no
      glyph image in the face, the value&nbsp;0 is returned.  By convention,
      it always correspond to a special glyph image called the <em>missing
      glyph</em>, which is commonly displayed as a box or a space.</p>

      <h4>
        b.&nbsp;Loading a glyph from the face
      </h4>

      <p>Once you have a glyph index, you can load the corresponding glyph
      image.  The latter can be stored in various formats within the font
      file.  For fixed-size formats like FNT or PCF, each image is a bitmap.
      Scalable formats like TrueType or Type&nbsp;1 use vectorial shapes,
      named <em>outlines</em> to describe each glyph.  Some formats may have
      even more exotic ways of representing glyphs (e.g., MetaFont &mdash;
      but this format is not supported).  Fortunately, FreeType&nbsp;2 is
      flexible enough to support any kind of glyph format through a simple
      API.</p>

      <p>The glyph image is always stored in a special object called a
      <em>glyph slot</em>.  As its name suggests, a glyph slot is simply a
      container that is able to hold one glyph image at a time, be it a
      bitmap, an outline, or something else.  Each face object has a single
      glyph slot object that can be accessed as <tt>face-&gt;glyph</tt>.
      Its fields are explained by the <a
      href="../reference/ft2-base_interface.html#FT_GlyphSlotRec">
      <tt>FT_GlyphSlotRec</tt></a> structure documentation.</p>

      <p>Loading a glyph image into the slot is performed by calling <a
      href="../reference/ft2-base_interface.html#FT_Load_Glyph">
      <tt>FT_Load_Glyph</tt></a> as in</p>

      <div class="pre">
  error = FT_Load_Glyph(
            face,          <span class="comment">/* handle to face object */</span>
            glyph_index,   <span class="comment">/* glyph index           */</span>
            load_flags );  <span class="comment">/* load flags, see below */</span>
      </div>

      <p>The <tt>load_flags</tt> value is a set of bit flags used to
      indicate some special operations.  The default value
      <tt>FT_LOAD_DEFAULT</tt> is&nbsp;0.</p>

      <p>This function will try to load the corresponding glyph image
      from the face:</p>

      <ul>
        <li>
          <p>If a bitmap is found for the corresponding glyph and pixel
          size, it will be loaded into the slot.  Embedded bitmaps are
          always favored over native image formats, because we assume that
          they are higher-quality versions of the same glyph.  This can be
          changed by using the <tt>FT_LOAD_NO_BITMAP</tt> flag.</p>
        </li>

        <li>
          <p>Otherwise, a native image for the glyph will be loaded.  It
          will also be scaled to the current pixel size, as well as hinted
          for certain formats like TrueType and Type&nbsp;1.</p>
        </li>
      </ul>

      <p>The field <tt>face&minus;&gt;glyph&minus;&gt;format</tt> describes
      the format used to store the glyph image in the slot.  If it is not
      <tt>FT_GLYPH_FORMAT_BITMAP</tt>, one can immediately convert it to a
      bitmap through <a
      href="../reference/ft2-base_interface.html#FT_Render_Glyph">
      <tt>FT_Render_Glyph</tt></a> as in:</p>

      <div class="pre">
  error = FT_Render_Glyph( face-&gt;glyph,   <span class="comment">/* glyph slot  */</span>
                           render_mode ); <span class="comment">/* render mode */</span>
      </div>

      <p>The parameter <tt>render_mode</tt> is a set of bit flags used to
      specify how to render the glyph image.  Set it to
      <tt>FT_RENDER_MODE_NORMAL</tt> to render a high-quality anti-aliased
      (256 gray levels) bitmap, as this is the default.  You can
      alternatively use <tt>FT_RENDER_MODE_MONO</tt> if you want to generate
      a 1-bit monochrome bitmap.</p>

      <p>Once you have a bitmapped glyph image, you can access it directly
      through <tt>glyph-&gt;bitmap</tt> (a simple bitmap descriptor), and
      position it through <tt>glyph-&gt;bitmap_left</tt> and
      <tt>glyph-&gt;bitmap_top</tt>.</p>

      <p>Note that <tt>bitmap_left</tt> is the horizontal distance from the
      current pen position to the leftmost border of the glyph bitmap, while
      <tt>bitmap_top</tt> is the vertical distance from the pen position (on
      the baseline) to the topmost border of the glyph bitmap.  <em>It is
      positive to indicate an upwards distance</em>.</p>

      <p>The next section will give more details on the contents of a glyph
      slot and how to access specific glyph information (including
      metrics).</p>

      <h4>
        c.&nbsp;Using other charmaps
      </h4>

      <p>As said before, when a new face object is created, it will look for
      a Unicode charmap and select it.  The currently selected charmap is
      accessed via <tt>face-&gt;charmap</tt>.  This field is NULL when no
      charmap is selected, which typically happens when you create a new
      <tt>FT_Face</tt> object from a font file that doesn't contain a
      Unicode charmap (which is rather infrequent today).</p>

      <p>There are two ways to select a different charmap with
      FreeType&nbsp;2.  The easiest is when the encoding you need already
      has a corresponding enumeration defined in <tt>FT_FREETYPE_H</tt>, for
      example <tt>FT_ENCODING_BIG5</tt>.  In this case, you can simply call
      <a href="../reference/ft2-base_interface.html#FT_Select_CharMap">
      <tt>FT_Select_CharMap</tt></a> as in:</p>

      <div class="pre">
  error = FT_Select_CharMap(
            face,               <span class="comment">/* target face object */</span>
            FT_ENCODING_BIG5 ); <span class="comment">/* encoding           */</span>
      </div>

      <p>Another way is to manually parse the list of charmaps for the face;
      this is accessible through the fields <tt>num_charmaps</tt> and
      <tt>charmaps</tt> (notice the &lsquo;s&rsquo) of the face object.  As
      you could expect, the first is the number of charmaps in the face,
      while the second is <em>a table of pointers to the charmaps</em>
      embedded in the face.</p>

      <p>Each charmap has a few visible fields used to describe it more
      precisely.  Mainly, one will look at <tt>charmap-&gt;platform_id</tt>
      and <tt>charmap-&gt;encoding_id</tt> that define a pair of values
      that can be used to describe the charmap in a rather generic way.</p>

      <p>Each value pair corresponds to a given encoding.  For example, the
      pair (3,1) corresponds to Unicode.  The list is defined in the
      TrueType specification but you can also use the file
      <tt>FT_TRUETYPE_IDS_H</tt> which defines several helpful constants to
      deal with them.</p>

      <p>To select a specific encoding, you need to find a corresponding
      value pair in the specification, then look for it in the charmaps
      list.  Don't forget that there are encodings which correspond to
      several value pairs due to historical reasons.  Here some code to do
      it:</p>

      <div class="pre">
  FT_CharMap  found = 0;
  FT_CharMap  charmap;
  int         n;


  for ( n = 0; n &lt; face-&gt;num_charmaps; n++ )
  {
    charmap = face-&gt;charmaps[n];