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FreeType Glyph Conventions
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Version 2.1
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Copyright 1998-2000 David Turner (<a
href="mailto:david@freetype.org">david@freetype.org</a>)<br>
Copyright 2000 The FreeType Development Team (<a
href="mailto:devel@freetype.org">devel@freetype.org</a>)
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<h2>
III. Glyph metrics
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<a name="section-1">
<h3>
1. Baseline, pens and layouts
</h3>
<p>The baseline is an imaginary line that is used to "guide" glyphs when
rendering text. It can be horizontal (e.g. Roman, Cyrillic, Arabic,
etc.) or vertical (e.g. Chinese, Japanese, Korean, etc). Moreover, to
render text, a virtual point, located on the baseline, called the <em>pen
position</em> or <em>origin</em>, is used to locate glyphs.</p>
<p>Each layout uses a different convention for glyph placement:</p>
<ul>
<li>
<p>With horizontal layout, glyphs simply "rest" on the baseline.
Text is rendered by incrementing the pen position, either to the
right or to the left.</p>
<p>The distance between two successive pen positions is
glyph-specific and is called the <em>advance width</em>. Note that
its value is <em>always</em> positive, even for right-to-left
oriented alphabets, like Arabic. This introduces some differences
in the way text is rendered.</p>
<p><em>The pen position is always placed on the baseline.</em></p>
<p><center>
<img src="Image1.png"
height=179 width=458
alt="horizontal layout">
</center></p>
</li>
<li>
<p>With a vertical layout, glyphs are centered around the
baseline:</p>
<p><center>
<img src="Image2.png"
height=275 width=162
alt="vertical layout">
</center></p>
</li>
</ul>
<a name="section-2">
<h3>
2. Typographic metrics and bounding boxes
</h3>
<p>A various number of face metrics are defined for all glyphs in a
given font.</p>
<ul>
<li>
<p><em>Ascent</em></p>
<p>The distance from the baseline to the highest/upper grid
coordinate used to place an outline point. It is a positive value,
due to the grid's orientation with the <i>Y</i> axis
upwards.</p>
</li>
<li>
<p><em>Descent</em></p>
<p>The distance from the baseline to the lowest grid coordinate used
to place an outline point. This is a negative value, due to the
grid's orientation.</p>
</li>
<li>
<p><em>Linegap</em></p>
<p>The distance that must be placed between two lines of text. The
baseline-to-baseline distance should be computed as:
<center><p>
<tt>ascent - descent + linegap</tt>
</p></center>
<p>if you use the typographic values.</p>
</li>
</ul>
<p>Other, simpler metrics are:</p>
<ul>
<li>
<p><em>The glyph's bounding box</em>, also called <em>bbox</em></p>
<p>This is an imaginary box that encloses all glyphs from the font,
usually as tightly as possible. It is represented by four fields,
namely <tt>xMin</tt>, <tt>yMin</tt>, <tt>xMax</tt>, and
<tt>yMax</tt>, that can be computed for any outline. Their values
can be in font units (if measured in the original outline) or in
fractional/integer pixel units (when measured on scaled
outlines).</p>
<p>Note that if it wasn't for grid-fitting, you wouldn't need to
know a box's complete values, but only its dimensions to know how
big is a glyph outline/bitmap. However, correct rendering of hinted
glyphs needs the preservation of important grid alignment on each
glyph translation/placement on the baseline.</p>
</li>
<li>
<p><em>Internal leading</em></p>
<p>This concept comes directly from the world of traditional
typography. It represents the amount of space within the
<em>leading</em> which is reserved for glyph features that lay
outside of the EM square (like accentuation). It usually can be
computed as:</p>
<center><p>
<tt>internal leading = ascent - descent - EM_size</tt>
</p></center>
</li>
<li>
<p><em>External leading</em></p>
<p>This is another name for the line gap.</p>
</li>
</ul>
<a name="section-3">
<h3>
3. Bearings and Advances
</h3>
Each glyph has also distances called <em>bearings</em> and
<em>advances</em>. Their definition is constant, but their values
depend on the layout, as the same glyph can be used to render text
either horizontally or vertically:
<ul>
<li>
<p><em>Left side bearing</em> or <em>bearingX</em></p>
<p>The horizontal distance from the current pen position to the
glyph's left bbox edge. It is positive for horizontal layouts, and
in most cases negative for vertical ones.</p>
</li>
<li>
<p><em>Top side bearing</em> or <em>bearingY</em></p>
<p>The vertical distance from the baseline to the top of the glyph's
bbox. It is usually positive for horizontal layouts, and negative
for vertical ones.</p>
</li>
<li>
<p><em>Advance width</em> or <em>advanceX</em></p>
<p>The horizontal distance the pen position must be incremented (for
left-to-right writing) or decremented (for right-to-left writing) by
after each glyph is rendered when processing text. It is always
positive for horizontal layouts, and null for vertical ones.</p>
</li>
<li>
<p><em>Advance height</em> <em>advanceY</em></p>
<p>The vertical distance the pen position must be decremented by
after each glyph is rendered. It is always null for horizontal
layouts, and positive for vertical layouts.</p>
</li>
<li>
<p><em>Glyph width</em></p>
<p>The glyph's horizontal extent. For unscaled font coordinates, it
is <tt>bbox.xMax-bbox.xMin</tt>. For scaled glyphs, its computation
requests specific care, described in the grid-fitting chapter
below.</p>
</li>
<li>
<p><em>Glyph height</em>
<p>The glyph's vertical extent. For unscaled font coordinates, it is
<tt>bbox.yMax-bbox.yMin</tt>. For scaled glyphs, its computation
requests specific care, described in the grid-fitting chapter
below.</p>
</li>
<li>
<p><em>Right side bearing</em></p>
<p>Only used for horizontal layouts to describe the distance from
the bbox's right edge to the advance width. It is in most cases a
non-negative number:</p>
<p><center>
<tt>advance_width - left_side_bearing - (xMax-xMin)</tt>
</center></p>
</li>
</ul>
<p>Here is a picture giving all the details for horizontal metrics:
<center><p>
<img src="Image3.png"
height=253 width=388
alt="horizontal glyph metrics">
</p></center>
<p>And here is another one for the vertical metrics:</p>
<center><p>
<img src="Image4.png"
height=278 width=294
alt="vertical glyph metrics">
</p></center>
<a name="section-4">
<h3>
4. The effects of grid-fitting
</h3>
<p>Because hinting aligns the glyph's control points to the pixel grid,
this process slightly modifies the dimensions of character images in
ways that differ from simple scaling.</p>
<p>For example, the image of the lowercase "m" letter sometimes fits a
square in the master grid. However, to make it readable at small pixel
sizes, hinting tends to enlarge its scaled outline in order to keep its
three legs distinctly visible, resulting in a larger character
bitmap.</p>
<p>The glyph metrics are also influenced by the grid-fitting process:
<ul>
<li>
The image's width and height are altered. Even if this is only by
one pixel, it can make a big difference at small pixel sizes.
</li>
<li>
The image's bounding box is modified, thus modifying the bearings.
</li>
<li>
The advances must be updated. For example, the advance width must
be incremented if the hinted bitmap is larger than the scaled one,
to reflect the augmented glyph width.
</li>
</ul>
<p>This has some implications:</p>
<ul>
<li>
Because of hinting, simply scaling the font ascent or descent might
not give correct results. A possible solution is to keep the ceiling
of the scaled ascent, and floor of the scaled descent.
</li>
<li>
There is no easy way to get the hinted glyph and advance widths of a
range of glyphs, as hinting works differently on each outline. The
only solution is to hint each glyph separately and record the
returned values. Some formats, like TrueType, even include a table
of pre-computed values for a small set of common character pixel
sizes.
</li>
<li>
Hinting depends on the final character width and height in pixels,
which means that it is highly resolution-dependent. This property
makes correct WYSIWYG layouts difficult to implement.
</li>
</ul>
<em>
<p>Performing 2D transformations on glyph outlines is very easy with
FreeType. However, when using translation on a hinted outlines, one
should aways take care of <b>exclusively using integer pixel
distances</b> (which means that the parameters to the
<tt>FT_Outline_Translate()</tt> API should all be multiples
of 64, as the point coordinates are in 26.6 fixed float
format).</p>
<p>Otherwise, the translation will simply <em>ruin the hinter's
work</em>, resulting in a very low quality bitmaps!</p>
</em>
<a name="section-5">
<h3>
5. Text widths and bounding box
</h3>
<p>As seen before, the "origin" of a given glyph corresponds to the
position of the pen on the baseline. It is not necessarily located on
one of the glyph's bounding box corners, unlike many typical bitmapped
font formats. In some cases, the origin can be out of the bounding box,
in others, it can be within it, depending on the shape of the given
glyph.</p>
<p>Likewise, the glyph's "advance width" is the increment to apply to
the pen position during layout, and is not related to the glyph's
"width", which really is the glyph's bounding width.
<p>The same conventions apply to strings of text. This means that:
<ul>
<li>
The bounding box of a given string of text doesn't necessarily
contain the text cursor, nor is the latter located on one of its
corners.
</li>
<li>
The string's advance width isn't related to its bounding box
dimensions. Especially if it contains beginning and terminal spaces
or tabs.
</li>
<li>
Finally, additional processing like kerning creates strings of text
whose dimensions are not directly related to the simple
juxtaposition of individual glyph metrics. For example, the advance
width of "VA" isn't the sum of the advances of "V" and "A" taken
separately.
</li>
</ul>
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