2000-11-09 17:23:23 +01:00
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<!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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2000-11-09 01:01:38 +01:00
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<html>
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2000-11-09 17:23:23 +01:00
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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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2000-11-09 01:01:38 +01:00
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</head>
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<body text="#000000"
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2000-11-09 17:23:23 +01:00
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<h1 align=center>
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FreeType Glyph Conventions
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</h1>
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2000-11-09 01:01:38 +01:00
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2000-11-09 17:23:23 +01:00
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<h2 align=center>
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Version 2.1
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</h2>
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2000-11-09 01:01:38 +01:00
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2000-11-09 17:23:23 +01:00
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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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2000-11-09 01:01:38 +01:00
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<center>
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2000-11-09 17:23:23 +01:00
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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-2.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-4.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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III. Glyph metrics
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</h2>
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</td></tr>
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</table>
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<a name="section-1">
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<h3>
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1. Baseline, pens and layouts
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</h3>
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<p>The baseline is an imaginary line that is used to "guide" glyphs when
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rendering text. It can be horizontal (e.g. Roman, Cyrillic, Arabic,
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etc.) or vertical (e.g. Chinese, Japanese, Korean, etc). Moreover, to
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render text, a virtual point, located on the baseline, called the <em>pen
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position</em> or <em>origin</em>, is used to locate glyphs.</p>
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<p>Each layout uses a different convention for glyph placement:</p>
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<ul>
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<li>
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2000-11-10 23:39:21 +01:00
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<p>With horizontal layout, glyphs simply "rest" on the baseline.
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2000-11-09 17:23:23 +01:00
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Text is rendered by incrementing the pen position, either to the
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right or to the left.</p>
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<p>The distance between two successive pen positions is
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glyph-specific and is called the <em>advance width</em>. Note that
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its value is <em>always</em> positive, even for right-to-left
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oriented alphabets, like Arabic. This introduces some differences
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in the way text is rendered.</p>
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<p><em>The pen position is always placed on the baseline.</em></p>
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<p><center>
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<img src="Image1.png"
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height=179 width=458
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alt="horizontal layout">
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</center></p>
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</li>
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<li>
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<p>With a vertical layout, glyphs are centered around the
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baseline:</p>
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<p><center>
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<img src="Image2.png"
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height=275 width=162
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alt="vertical layout">
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</center></p>
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</li>
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</ul>
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<a name="section-2">
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<h3>
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2. Typographic metrics and bounding boxes
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</h3>
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<p>A various number of face metrics are defined for all glyphs in a
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given font.</p>
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<ul>
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<li>
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<p><em>Ascent</em></p>
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<p>The distance from the baseline to the highest/upper grid
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coordinate used to place an outline point. It is a positive value,
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due to the grid's orientation with the <i>Y</i> axis
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upwards.</p>
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</li>
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<li>
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<p><em>Descent</em></p>
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<p>The distance from the baseline to the lowest grid coordinate used
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to place an outline point. This is a negative value, due to the
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grid's orientation.</p>
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</li>
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<li>
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<p><em>Linegap</em></p>
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<p>The distance that must be placed between two lines of text. The
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baseline-to-baseline distance should be computed as:
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<center><p>
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<tt>ascent - descent + linegap</tt>
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</p></center>
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<p>if you use the typographic values.</p>
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</li>
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</ul>
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<p>Other, simpler metrics are:</p>
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<ul>
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<li>
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<p><em>The glyph's bounding box</em>, also called <em>bbox</em></p>
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<p>This is an imaginary box that encloses all glyphs from the font,
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usually as tightly as possible. It is represented by four fields,
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namely <tt>xMin</tt>, <tt>yMin</tt>, <tt>xMax</tt>, and
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<tt>yMax</tt>, that can be computed for any outline. Their values
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can be in font units (if measured in the original outline) or in
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fractional/integer pixel units (when measured on scaled
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outlines).</p>
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<p>Note that if it wasn't for grid-fitting, you wouldn't need to
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know a box's complete values, but only its dimensions to know how
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big is a glyph outline/bitmap. However, correct rendering of hinted
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glyphs needs the preservation of important grid alignment on each
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glyph translation/placement on the baseline.</p>
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</li>
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<li>
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<p><em>Internal leading</em></p>
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<p>This concept comes directly from the world of traditional
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typography. It represents the amount of space within the
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<em>leading</em> which is reserved for glyph features that lay
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outside of the EM square (like accentuation). It usually can be
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computed as:</p>
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<center><p>
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<tt>internal leading = ascent - descent - EM_size</tt>
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</p></center>
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</li>
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<li>
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<p><em>External leading</em></p>
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<p>This is another name for the line gap.</p>
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</li>
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</ul>
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<a name="section-3">
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<h3>
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3. Bearings and Advances
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</h3>
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Each glyph has also distances called <em>bearings</em> and
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<em>advances</em>. Their definition is constant, but their values
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depend on the layout, as the same glyph can be used to render text
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either horizontally or vertically:
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<ul>
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<li>
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<p><em>Left side bearing</em> or <em>bearingX</em></p>
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<p>The horizontal distance from the current pen position to the
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glyph's left bbox edge. It is positive for horizontal layouts, and
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in most cases negative for vertical one.</p>
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</li>
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<li>
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<p><em>Top side bearing</em> or <em>bearingY</em></p>
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<p>The vertical distance from the baseline to the top of the glyph's
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bbox. It is usually positive for horizontal layouts, and negative
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for vertical ones</p>
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</li>
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<li>
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<p><em>Advance width</em> or <em>advanceX</em></p>
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<p>The horizontal distance the pen position must be incremented (for
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left-to-right writing) or decremented (for right-to-left writing) by
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after each glyph is rendered when processing text. It is always
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positive for horizontal layouts, and null for vertical ones.</p>
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</li>
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<li>
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<p><em>Advance height</em> <em>advanceY</em></p>
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<p>The vertical distance the pen position must be decremented by
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after each glyph is rendered. It is always null for horizontal
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layouts, and positive for vertical layouts.</p>
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</li>
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<li>
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<p><em>Glyph width</em></p>
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<p>The glyph's horizontal extent. For unscaled font coordinates, it
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is <tt>bbox.xMax-bbox.xMin</tt>. For scaled glyphs, its computation
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requests specific care, described in the grid-fitting chapter
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below.</p>
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</li>
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<li>
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<p><em>Glyph height</em>
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<p>The glyph's vertical extent. For unscaled font coordinates, it is
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<tt>bbox.yMax-bbox.yMin</tt>. For scaled glyphs, its computation
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requests specific care, described in the grid-fitting chapter
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below.</p>
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</li>
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<li>
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<p><em>Right side bearing</em></p>
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<p>Only used for horizontal layouts to describe the distance from
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the bbox's right edge to the advance width. It is in most cases a
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non-negative number:</p>
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<p><center>
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<tt>advance_width - left_side_bearing - (xMax-xMin)</tt>
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</center></p>
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</li>
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</ul>
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<p>Here is a picture giving all the details for horizontal metrics:
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<center><p>
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<img src="Image3.png"
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height=253 width=388
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alt="horizontal glyph metrics">
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</p></center>
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<p>And here is another one for the vertical metrics:</p>
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<center><p>
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<img src="Image4.png"
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height=278 width=294
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alt="vertical glyph metrics">
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</p></center>
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<a name="section-4">
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<h3>
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4. The effects of grid-fitting
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</h3>
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<p>Because hinting aligns the glyph's control points to the pixel grid,
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this process slightly modifies the dimensions of character images in
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ways that differ from simple scaling.</p>
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<p>For example, the image of the lowercase "m" letter sometimes fits a
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square in the master grid. However, to make it readable at small pixel
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sizes, hinting tends to enlarge its scaled outline in order to keep its
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three legs distinctly visible, resulting in a larger character
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bitmap.</p>
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<p>The glyph metrics are also influenced by the grid-fitting process:
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<ul>
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<li>
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The image's width and height are altered. Even if this is only by
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one pixel, it can make a big difference at small pixel sizes.
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</li>
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<li>
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The image's bounding box is modified, thus modifying the bearings.
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</li>
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<li>
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The advances must be updated. For example, the advance width must
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be incremented if the hinted bitmap is larger than the scaled one,
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to reflect the augmented glyph width.
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</li>
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</ul>
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<p>This has some implications:</p>
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<ul>
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<li>
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Because of hinting, simply scaling the font ascent or descent might
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not give correct results. A possible solution is to keepthe ceiling
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of the scaled ascent, and floor of the scaled descent.
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</li>
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<li>
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There is no easy way to get the hinted glyph and advance widths of a
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range of glyphs, as hinting works differently on each outline. The
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only solution is to hint each glyph separately and record the
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returned values. Some formats, like TrueType, even include a table
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of pre-computed values for a small set of common character pixel
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sizes.
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</li>
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<li>
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Hinting depends on the final character width and height in pixels,
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which means that it is highly resolution-dependent. This property
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makes correct WYSIWYG layouts difficult to implement.
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</li>
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</ul>
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<em>
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<p>Performing 2D transformations on glyph outlines is very easy with
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FreeType. However, when using translation on a hinted outlines, one
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should aways take care of <b>exclusively using integer pixel
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distances</b> (which means that the parameters to the
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<tt>FT_Translate_Outline()</tt> API should all be multiples
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of 64, as the point coordinates are in 26.6 fixed float
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format).</p>
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<p>Otherwise, the translation will simply <em>ruin the hinter's
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work</em>, resulting in a very low quality bitmaps!</p>
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</em>
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<a name="section-5">
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<h3>
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5. Text widths and bounding box
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</h3>
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<p>As seen before, the "origin" of a given glyph corresponds to the
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position of the pen on the baseline. It is not necessarily located on
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one of the glyph's bounding box corners, unlike many typical bitmapped
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font formats. In some cases, the origin can be out of the bounding box,
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in others, it can be within it, depending on the shape of the given
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glyph.</p>
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<p>Likewise, the glyph's "advance width" is the increment to apply to
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the pen position during layout, and is not related to the glyph's
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"width", which really is the glyph's bounding width.
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<p>The same conventions apply to strings of text. This means that:
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<ul>
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<li>
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The bounding box of a given string of text doesn't necessarily
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contain the text cursor, nor is the latter located on one of its
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corners.
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</li>
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<li>
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The string's advance width isn't related to its bounding box
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|
dimensions. Especially if it contains beginning and terminal spaces
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or tabs.
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</li>
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<li>
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|
Finally, additional processing like kerning creates strings of text
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|
whose dimensions are not directly related to the simple
|
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|
juxtaposition of individual glyph metrics. For example, the advance
|
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|
|
width of "VA" isn't the sum of the advances of "V" and "A" taken
|
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|
separately.
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</li>
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</ul>
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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-2.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-4.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>
|
2000-11-09 01:01:38 +01:00
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</body>
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</html>
|
