396 lines
13 KiB
HTML
396 lines
13 KiB
HTML
<!doctype html public "-//w3c//dtd html 4.0 transitional//en"
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"http://www.w3.org/TR/REC-html40/loose.dtd">
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<html>
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<head>
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<meta http-equiv="Content-Type"
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content="text/html; charset=iso-8859-1">
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<meta name="Author"
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content="David Turner">
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<title>FreeType Glyph Conventions</title>
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</head>
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<body text="#000000"
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bgcolor="#FFFFFF"
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link="#0000EF"
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vlink="#51188E"
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alink="#FF0000">
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<h1 align=center>
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FreeType Glyph Conventions
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</h1>
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<h2 align=center>
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Version 2.1
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</h2>
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<h3 align=center>
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Copyright 1998-2000 David Turner (<a
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href="mailto:david@freetype.org">david@freetype.org</a>)<br>
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Copyright 2000 The FreeType Development Team (<a
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href="mailto:devel@freetype.org">devel@freetype.org</a>)
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</h3>
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<center>
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<table width="65%">
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<tr><td>
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<center>
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<table width="100%"
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border=0
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cellpadding=5>
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<tr bgcolor="#CCFFCC"
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valign=center>
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<td align=center
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width="30%">
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<a href="glyphs-1.html">Previous</a>
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</td>
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<td align=center
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width="30%">
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<a href="index.html">Contents</a>
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</td>
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<td align=center
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width="30%">
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<a href="glyphs-3.html">Next</a>
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</td>
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</tr>
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</table>
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</center>
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<p><hr></p>
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<table width="100%">
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<tr bgcolor="#CCCCFF"
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valign=center><td>
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<h2>
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II. Glyph mutlines
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</h2>
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</td></tr>
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</table>
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<p>This section describes the way scalable representation of glyph images,
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called outlines, are used by FreeType as well as client applications.</p>
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<a name="section-1">
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<h3>
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1. Pixels, points and device resolutions
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</h3>
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<p>Though it is a very common assumption when dealing with computer
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graphics programs, the physical dimensions of a given pixel (be it for
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screens or printers) are not squared. Often, the output device, be it a
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screen or printer, exhibits varying resolutions in both horizontal and
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vertical direction, and this must be taken care of when rendering
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text.</p>
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<p>It is thus common to define a device's characteristics through two
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numbers expressed in <em>dpi</em> (dots per inch). For example, a
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printer with a resolution of 300x600 dpi has 300 pixels per
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inch in the horizontal direction, and 600 in the vertical one. The
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resolution of a typical computer monitor varies with its size
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(15" and 17" monitors don't have the same pixel sizes at
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640x480), and of course the graphics mode resolution.</p>
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<p>As a consequence, the size of text is usually given in
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<em>points</em>, rather than device-specific pixels. Points are a
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simple <em>physical</em> unit, where 1 point = 1/72th of
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an inch, in digital typography. As an example, most Roman books are
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printed with a body text which size is chosen between 10 and
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14 points.</p>
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<p>It is thus possible to compute the size of text in pixels from the
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size in points with the following formula:</p>
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<center>
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<tt>pixel_size = point_size * resolution / 72</tt>
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</center>
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<p>The resolution is expressed in <em>dpi</em>. Since horizontal and
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vertical resolutions may differ, a single point size usually defines a
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different text width and height in pixels.</p>
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<p><em>Unlike what is often thought, the "size of text in pixels" is not
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directly related to the real dimensions of characters when they are
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displayed or printed. The relationship between these two concepts is a
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bit more complex and relate to some design choices made by the font
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designer. This is described in more detail in the next sub-section (see
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the explanations on the EM square).</em></p>
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<a name="section-2">
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<h3>
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2. Vectorial representation
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</h3>
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<p>The source format of outlines is a collection of closed paths called
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<em>contours</em>. Each contour delimits an outer or inner
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<em>region</em> of the glyph, and can be made of either <em>line
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segments</em> or <em>Bézier arcs</em>.</p>
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<p>The arcs are defined through <em>control points</em>, and can be
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either second-order (these are <em>conic</em> Béziers) or
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third-order (<em>cubic</em> Béziers) polynomials, depending on
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the font format. Note that conic Béziers are usually called
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<em>quadratic</em> Béziers in the literature. Hence, each point
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of the outline has an associated flag indicating its type (normal or
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control point). And scaling the points will scale the whole
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outline.</p>
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<p>Each glyph's original outline points are located on a grid of
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indivisible units. The points are usually stored in a font file as
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16-bit integer grid coordinates, with the grid origin's being at (0,0);
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they thus range from -16384 to 16383. (Even though point
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coordinates can be floats in other formats such as Type 1, we will
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restrict our analysis to integer values for simplicity).</p>
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<p><em>The grid is always oriented like the traditional mathematical
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two-dimensional plane, i.e., the <i>X</i> axis from the left to the
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right, and the <i>Y</i> axis from bottom to top.</em></p>
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<p>In creating the glyph outlines, a type designer uses an imaginary
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square called the <em>EM square</em>. Typically, the EM square can be
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thought of as a tablet on which the character are drawn. The square's
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size, i.e., the number of grid units on its sides, is very important for
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two reasons:</p>
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<ul>
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<li>
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<p>It is the reference used to scale the outlines to a given text
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dimension. For example, a size of 12pt at 300x300 dpi
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corresponds to 12*300/72 = 50 pixels. This is the
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size the EM square would appear on the output device if it was
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rendered directly. In other words, scaling from grid units to
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pixels uses the formula:</p>
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<p><center>
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<tt>pixel_size = point_size * resolution / 72</tt><br>
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<tt>pixel_coord = grid_coord * pixel_size / EM_size</tt>
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</center></p>
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</li>
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<li>
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<p>The greater the EM size is, the larger resolution the designer
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can use when digitizing outlines. For example, in the extreme
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example of an EM size of 4 units, there are only 25 point
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positions available within the EM square which is clearly not
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enough. Typical TrueType fonts use an EM size of 2048 units;
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Type 1 PostScript fonts have a fixed EM size of 1000 grid
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units but point coordinates can be expressed as floating values.</p>
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</li>
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</ul>
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<p>Note that glyphs can freely extend beyond the EM square if the font
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designer wants so. The EM is used as a convenience, and is a valuable
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convenience from traditional typography.</p>
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<p>Grid units are very often called <em>font units</em> or <em>EM
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units</em>.</p>
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<p><em>As said before, <tt>pixel_size</tt> computed in the above formula
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does not relate directly to the size of characters on the screen. It
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simply is the size of the EM square if it was to be displayed. Each
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font designer is free to place its glyphs as it pleases him within the
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square. This explains why the letters of the following text have not
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the same height, even though they are displayed at the same point size
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with distinct fonts:</em>
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<p><center>
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<img src="body_comparison.png"
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height=40 width=580
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alt="Comparison of font heights">
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</center></p>
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<p>As one can see, the glyphs of the Courier family are smaller than
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those of Times New Roman, which themselves are slightly smaller than
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those of Arial, even though everything is displayed or printed at a size
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of 16 points. This only reflects design choices.</p>
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<a name="section-3">
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<h3>
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3. Hinting and Bitmap rendering
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</h3>
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<p>The outline as stored in a font file is called the "master" outline,
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as its points coordinates are expressed in font units. Before it can be
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converted into a bitmap, it must be scaled to a given size/resolution.
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This is done through a very simple transformation, but always creates
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undesirable artifacts, e.g. stems of different widths or heights in
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letters like "E" or "H".</p>
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<p>As a consequence, proper glyph rendering needs the scaled points to
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be aligned along the target device pixel grid, through an operation
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called <em>grid-fitting</em>, and often <em>hinting</em>. One of its
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main purposes is to ensure that important widths and heights are
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respected throughout the whole font (for example, it is very often
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desirable that the "I" and the "T" have their central vertical line of
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the same pixel width), as well as to manage features like stems and
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overshoots, which can cause problems at small pixel sizes.</p>
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<p>There are several ways to perform grid-fitting properly; most
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scalable formats associate some control data or programs with each glyph
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outline. Here is an overview:</p>
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<ul>
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<li>
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<p><em>explicit grid-fitting</em></p>
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<p>The TrueType format defines a stack-based virtual machine, for
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which programs can be written with the help of more than
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200 opcodes (most of these relating to geometrical operations).
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Each glyph is thus made of both an outline and a control program to
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perform the actual grid-fitting in the way defined by the font
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designer.</p>
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</li>
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<li>
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<p><em>implicit grid-fitting (also called hinting)</em></p>
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<p>The Type 1 format takes a much simpler approach: Each glyph
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is made of an outline as well as several pieces called
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<em>hints</em> which are used to describe some important features of
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the glyph, like the presence of stems, some width regularities, and
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the like. There aren't a lot of hint types, and it is up to the
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final renderer to interpret the hints in order to produce a fitted
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outline.</p>
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</li>
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<li>
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<p><em>automatic grid-fitting</em></p>
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<p>Some formats simply include no control information with each
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glyph outline, apart metrics like the advance width and height. It
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is then up to the renderer to "guess" the more interesting features
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of the outline in order to perform some decent grid-fitting.</p>
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</li>
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</ul>
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<p>The following table summarises the pros and cons of each scheme.</p>
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<center>
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<table width="90%"
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bgcolor="#CCCCCC"
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cellpadding=5>
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<tr bgcolor="#999999">
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<td>
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<center>
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<b>grid-fitting scheme</b>
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</center>
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</td>
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<td>
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<center>
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<b>advantages</b>
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</center>
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</td>
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<td>
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<center>
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<b>disadvantages</b>
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</center>
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</td>
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</tr>
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<tr>
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<td valign=top>
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<center>
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<b>explicit</b>
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</center>
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</td>
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<td valign=top>
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<p><b>Quality.</b> Excellent results at small sizes are possible.
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This is very important for screen display.</p>
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<p><b>Consistency.</b> All renderers produce the same glyph
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bitmaps.</p>
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</td>
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<td valign=top>
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<p><b>Speed.</b> Intepreting bytecode can be slow if the glyph
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programs are complex.</p>
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<p><b>Size.</b> Glyph programs can be long.</p>
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<p><b>Technicity.</b>
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It is extremely difficult to write good hinting
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programs. Very few tools available.</p>
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</td>
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</tr>
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<tr>
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<td valign=top>
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<center>
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<b>implicit</b>
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</center>
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</td>
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<td valign=top>
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<p><b>Size.</b> Hints are usually much smaller than explicit glyph
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programs.</p>
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<p><b>Speed.</b>
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Grid-fitting is usually a fast process.</p>
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</td>
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<td valign=top>
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<p><b>Quality.</b> Often questionable at small sizes. Better with
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anti-aliasing though.</p>
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<p><b>Inconsistency.</b> Results can vary between different
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renderers, or even distinct versions of the same engine.</p>
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</td>
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</tr>
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<tr>
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<td valign=top>
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<center>
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<b>automatic</b>
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</center>
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</td>
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<td valign=top>
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<p><b>Size.</b> No need for control information, resulting in
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smaller font files.</p>
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<p><b>Speed.</b> Depends on the grid-fitting algorithm. Usually
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faster than explicit grid-fitting.</p>
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</td>
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<td valign=top>
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<p><b>Quality.</b> Often questionable at small sizes. Better with
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anti-aliasing though.</p>
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<p><b>Speed.</b> Depends on the grid-fitting algorithm.</p>
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<p><b>Inconsistency.</b> Results can vary between different
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renderers, or even distinct versions of the same engine.</p>
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</td>
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</tr>
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</table>
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</center>
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<p><hr></p>
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<center>
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<table width="100%"
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border=0
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cellpadding=5>
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<tr bgcolor="#CCFFCC"
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valign=center>
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<td align=center
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width="30%">
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<a href="glyphs-1.html">Previous</a>
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</td>
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<td align=center
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width="30%">
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<a href="index.html">Contents</a>
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</td>
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<td align=center
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width="30%">
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<a href="glyphs-3.html">Next</a>
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</td>
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</tr>
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</table>
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</center>
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</td></tr>
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</table>
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</center>
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</body>
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</html>
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