368 lines
13 KiB
HTML
368 lines
13 KiB
HTML
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<title>FreeType Glyph Conventions</title>
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<center><h1>
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FreeType Glyph Conventions
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</h1></center>
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<center><h2>
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version 2.1
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</h2></center>
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<center><h3>
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Copyright 1998-2000 David Turner (<a href="mailto:david@freetype.org">david@freetype.org</a>)<br>
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Copyright 2000 The FreeType Development Team (<a href="devel@freetype.org">devel@freetype.org</a>)
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</h3></center>
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<center><table width=650><tr><td>
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<center><table width="100%" border=0 cellpadding=5><tr bgcolor="#CCFFCC" valign=center>
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<a href="glyphs-1.html">Previous</a>
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<a href="index.html">Contents</a>
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<td align=center width="30%">
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<a href="glyphs-3.html">Next</a>
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<table width="100%" cellpadding=4><tr bgcolor="#CCCCFF" valign=center><td><h2>
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II. Glyph Outlines
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</h2></td></tr></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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<h3><a name="section-1">
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1. Pixels, Points and Device Resolutions :
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</h3><blockquote>
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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 the horizontal and vertical
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directions, and this must be taken care of when rendering text.
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</p>
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<p>It is thus common to define a device's characteristics through two numbers
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expressed in <b>dpi</b> (dots per inch). For example, a printer with a
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resolution of 300x600 dpi has 300 pixels per inch in the horizontal
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direction, and 600 in the vertical one. The resolution of a typical computer
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monitor varies with its size (a 15" and 17" monitors don't have the same
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pixel sizes at 640x480), and of course the graphics mode resolution.
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</p>
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<p>As a consequence, the size of text is usually given in <b>points</b>,
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rather than device-specific pixels. Points are a simple <i>physical</i>
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unit, where 1 point = 1/72th of an inch, in digital typography. As an
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example, most roman books are printed with a body text which size is
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chosen between 10 and 14 points.</p>
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<p>It is thus possible to compute the size of text in pixels from the size
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in points through the following computation :</p>
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<center>
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<p><tt>pixel_size = point_size * resolution / 72</tt></center>
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<p>Where resolution is expressed in <em>dpi</em>. Note that because the
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horizontal and vertical resolutions may differ, a single point size
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usually defines different text width and height in pixels.</p>
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<p><b>IMPORTANT NOTE:</b>
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<br><i>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're displayed
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or printed. The relationship between these two concepts is a bit more complex
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and relate to some design choice made by the font designer. This is described
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in more details the next sub-section (see the explanations on the EM square).
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</i></p>
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</blockquote><h3><a name="section-2">
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2. Vectorial representation :
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</h3><blockquote>
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<p>The source format of outlines is a collection of closed paths
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called <b>contours</b>. Each contour delimits an outer or inner <i>region</i>
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of the glyph, and can be made of either <b>line segments</b> or <b>bezier
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arcs</b>.</p>
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<p>The arcs are defined through <b>control points</b>, and can be either
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second-order (these are "conic" beziers) or third-order ("cubic" beziers) polynomials, depending on
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the font format. Note that conic beziers are usually called "quadratic"
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beziers in the literature. Hence, each point of the outline has an
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associated <b>flag</b> indicating its type (normal or control point).
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And scaling the points will scale the whole outline.
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</p>
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<p>Each glyph's original outline points are located on a grid of indivisible
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units. The points are usually stored in a font file as 16-bit integer grid
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coordinates, with the grid origin's being at (0,0); they thus range from
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-16384 to 16383. (even though point coordinates can be floats in other
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formats such as Type 1, we'll restrict our analysis to integer ones, driven
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by the need for simplicity..).
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</p>
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<p><b>IMPORTANT NOTE:</b>
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<br><i>The grid is always oriented like the traditional mathematical 2D
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plane, i.e. the X axis from the left to the right, and the Y axis from
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bottom to top.</i></p>
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<p>In creating the glyph outlines, a type designer uses an imaginary square
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called the "EM square". Typically, the EM square can be thought of as a
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tablet on which the character are drawn. The square's size, i.e., the number
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of grid units on its sides, is very important for two reasons:</p>
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<ul>
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<li><p>
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it is the reference used to scale the outlines to a given text dimension.
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For example, a size of 12pt at 300x300 dpi corresponds to 12*300/72 = 50
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pixels. This is the size the EM square would appear on the output device
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if it was 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><tt>pixel_size = point_size * resolution / 72</tt>
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<br><tt>pixel_coordinate = grid_coordinate * pixel_size / EM_size</tt>
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</center></p>
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<li><p>
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the greater the EM size is, the larger resolution the designer can use
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when digitizing outlines. For example, in the extreme example of an EM
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size of 4 units, there are only 25 point positions available within the
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EM square which is clearly not enough. Typical TrueType fonts use an EM
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size of 2048 units (note: with Type 1 PostScript fonts, the EM size is
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fixed to 1000 grid units. However, point coordinates can be expressed in
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floating values).
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</p></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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<center>
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<p><b>Note : Grid units are very often called "font units" or "EM units".</b></center>
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<p><b>NOTE:</b>
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<br><i>As said before, the pixel_size computed in the above formula
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does not relate directly to the size of characters on the screen. It simply
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is the size of the EM square if it was to be displayed directly. Each font
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designer is free to place its glyphs as it pleases him within the square.
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This explains why the letters of the following text have not the same height,
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even though they're displayed at the same point size with distinct fonts
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:</i>
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<center>
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<p><img SRC="body_comparison.png" height=40 width=580></center>
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<p>As one can see, the glyphs of the Courier family are smaller than those
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of Times New Roman, which themselves are slightly smaller than those of
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Arial, even though everything is displayed or printed at a size of
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16 points. This only reflect design choices.
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</p>
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</blockquote><h3><a name="section-3">
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3. Hinting and Bitmap rendering
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</h3><blockquote>
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<p>The outline as stored in a font file is called the "master"
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outline, as its points coordinates are expressed in font units. Before
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it can be converted into a bitmap, it must be scaled to a given
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size/resolution. This is done through a very simple transform, but always
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creates undesirable artifacts, e.g. stems of different widths or heights
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in letters like "E" or "H".
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</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 called
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"grid-fitting", and often "hinting". One of its main purpose is to ensure
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that important widths and heights are respected throughout the whole font
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(for example, it is very often desirable that the "I" and the "T" have
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their central vertical line of the same pixel width), as well as manage
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features like stems and overshoots, which can cause problems at small pixel
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sizes.
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</p>
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<p>There are several ways to perform grid-fitting properly, for example
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most scalable formats associate some control data or programs with each
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glyph outline. Here is an overview :</p>
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<blockquote>
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<blockquote><b>explicit grid-fitting :</b>
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<blockquote>The TrueType format defines a stack-based virtual machine,
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for which programs can be written with the help of more than 200 opcodes
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(most of these relating to geometrical operations). Each glyph is thus
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made of both an outline and a control program, its purpose being to perform
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the actual grid-fitting in the way defined by the font designer.</blockquote>
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<p><br><b>implicit grid-fitting (also called hinting) :</b>
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<blockquote>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 "hints" which are
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used to describe some important features of the glyph, like the presence
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of stems, some width regularities, and the like. There aren't a lot of
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hint types, and it's up to the final renderer to interpret the hints in
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order to produce a fitted outline.</blockquote>
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<p><br><b>automatic grid-fitting :</b>
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<blockquote>Some formats simply include no control information with each
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glyph outline, apart metrics like the advance width and height. It's then
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up to the renderer to "guess" the more interesting features of the outline
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in order to perform some decent grid-fitting.</blockquote>
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</blockquote>
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</blockquote>
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<center>
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<p><br>The following table summarises the pros and cons of each scheme
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:</center>
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</blockquote>
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<center><table BORDER=0 WIDTH="80%" BGCOLOR="#CCCCCC" >
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<tr BGCOLOR="#999999">
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<td>
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<blockquote>
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<center><b><font color="#000000">Grid-fitting scheme</font></b></center>
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</blockquote>
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</td>
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<td>
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<blockquote>
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<center><b><font color="#000000">Pros</font></b></center>
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</blockquote>
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</td>
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<td>
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<blockquote>
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<center><b><font color="#000000">Cons</font></b></center>
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</blockquote>
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</td>
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</tr>
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<tr>
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<td>
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<blockquote>
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<center><b><font color="#000000">Explicit</font></b></center>
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</blockquote>
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</td>
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<td>
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<blockquote>
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<center><b><font color="#000000">Quality</font></b>
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<br><font color="#000000">excellence at small sizes is possible. This is
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very important for screen display.</font>
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<p><b><font color="#000000">Consistency</font></b>
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<br><font color="#000000">all renderers produce the same glyph bitmaps.</font></center>
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</blockquote>
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</td>
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<td>
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<blockquote>
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<center><b><font color="#000000">Speed</font></b>
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<br><font color="#000000">intepreting bytecode can be slow if the glyph
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programs are complex.</font>
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<p><b><font color="#000000">Size</font></b>
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<br><font color="#000000">glyph programs can be long</font>
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<p><b><font color="#000000">Technicity</font></b>
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<br><font color="#000000">it is extremely difficult to write good hinting
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programs. Very few tools available.</font></center>
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</blockquote>
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</td>
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</tr>
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<tr>
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<td>
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<blockquote>
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<center><b><font color="#000000">Implicit</font></b></center>
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</blockquote>
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</td>
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<td>
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<blockquote>
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<center><b><font color="#000000">Size</font></b>
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<br><font color="#000000">hints are usually much smaller than explicit
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glyph programs.</font>
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<p><b><font color="#000000">Speed</font></b>
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<br><font color="#000000">grid-fitting is usually a fast process</font></center>
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</blockquote>
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</td>
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<td>
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<blockquote>
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<center><b><font color="#000000">Quality</font></b>
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<br><font color="#000000">often questionable at small sizes. Better with
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anti-aliasing though.</font>
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<p><b><font color="#000000">Inconsistency</font></b>
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<br><font color="#000000">results can vary between different renderers,
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or even distinct versions of the same engine.</font></center>
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</blockquote>
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</td>
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</tr>
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<tr>
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<td>
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<blockquote>
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<center><b><font color="#000000">Automatic</font></b></center>
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</blockquote>
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</td>
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<td>
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<blockquote>
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<center><b><font color="#000000">Size</font></b>
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<br><font color="#000000">no need for control information, resulting in
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smaller font files.</font>
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<p><b><font color="#000000">Speed</font></b>
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<br><font color="#000000">depends on the grid-fitting algo.Usually faster
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than explicit grid-fitting.</font></center>
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</blockquote>
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</td>
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<td>
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<blockquote>
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<center><b><font color="#000000">Quality</font></b>
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<br><font color="#000000">often questionable at small sizes. Better with
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anti-aliasing though</font>
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<p><b><font color="#000000">Speed</font></b>
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<br><font color="#000000">depends on the grid-fitting algo.</font>
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<p><b><font color="#000000">Inconsistency</font></b>
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<br><font color="#000000">results can vary between different renderers,
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or even distinct versions of the same engine.</font></center>
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</blockquote>
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</td>
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</tr>
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</table></center>
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</blockquote>
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<center><table width="100%" border=0 cellpadding=5><tr bgcolor="#CCFFCC" valign=center>
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<td align=center 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 width="30%">
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<a href="index.html">Contents</a>
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</td>
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<td align=center width="30%">
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<a href="glyphs-3.html">Next</a>
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</td>
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</tr></table></center>
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</td></tr></table></center>
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</body>
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</html>
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