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<title>FreeType Glyph Conventions / III</title>
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<h1><a href="http://freetype.org/index.html">FreeType</a> Glyph
Conventions / III</h1>
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<h2>III. Glyph Metrics</h2>
<h3 id="section-1">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. Latin, Cyrillic, Arabic, etc.) or
vertical (e.g. Chinese, Japanese, Mongolian, 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 scripts 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 align="center">
<img src="layout.png"
height="179"
width="458"
alt="horizontal layout">
</p>
</li>
<li>
<p>With a vertical layout, glyphs are centered around
the baseline:</p>
<p align="center">
<img src="layout2.png"
height="275"
width="162"
alt="vertical layout">
</p>
</li>
</ul>
<h3 id="section-2">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 class="emph">
<p>Ascent</p>
<p>The distance from the baseline to the highest or
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 class="emph">
<p>Descent</p>
<p>The distance from the baseline to the lowest grid
coordinate used to place an outline point. In
FreeType, this is a negative value, due to the grid's
orientation. Note that in some font formats this is a
positive value.</p>
</li>
<li class="emph">
<p>Linegap</p>
<p>The distance that must be placed between two lines of
text. The baseline-to-baseline distance should be
computed as
<p align="center">
<tt>linespace = ascent - descent + linegap</tt>
</p>
<p>if you use the typographic values.</p>
</li>
</ul>
<p>Other, simpler metrics are:</p>
<ul>
<li class="emph">
<p>Bounding box</p>
<p>This is an imaginary box that encloses all glyphs
from the font, usually as tightly as possible. It is
represented by four parameters,
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 integer (or
fractional) pixel units when measured on scaled
outlines.</p>
<p>A common shorthand for the bounding box is
‘bbox’.</p>
</li>
<li class="emph">
<p>Internal leading</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>
<p align="center">
<tt>internal leading = ascent - descent - EM_size</tt>
</p>
</li>
<li class="emph">
<p>External leading</p>
<p>This is another name for the line gap.</p>
</li>
</ul>
<h3 id="section-3">3. Bearings and Advances</h3>
<p>Each glyph has also distances called <em>bearings</em> and
<em>advances</em>. The actual values depend on the
layout, as the same glyph can be used to render text
either horizontally or vertically:</p>
<ul>
<li class="emph">
<p>Left side bearing</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>
<p>In the FreeType API, this is also called
<tt>bearingX</tt>. Another shorthand is
‘lsb’.</p>
</li>
<li class="emph">
<p>Top side bearing</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>
<p>In the FreeType API, this is also called
<tt>bearingY</tt>.</p>
</li>
<li class="emph">
<p>Advance width</p>
<p>The horizontal distance to increment (for
left-to-right writing) or decrement (for right-to-left
writing) the pen position after a glyph has been
rendered when processing text. It is always positive
for horizontal layouts, and zero for vertical
ones.</p>
<p>In the FreeType API, this is also called
<tt>advanceX</tt>.</p>
</li>
<li class="emph">
<p>Advance height</p>
<p>The vertical distance to decrement the pen position
after a glyph has been rendered. It is always zero
for horizontal layouts, and positive for vertical
layouts.</p>
<p>In the FreeType API, this is also called
<tt>advanceY</tt>.</p>
</li>
<li class="emph">
<p>Glyph width</p>
<p>The glyph's horizontal extent. For unscaled font
coordinates, it is</p>
<p align="center">
<tt>glyph width = bbox.xMax - bbox.xMin</tt>
</p>
<p>For scaled glyphs, its computation requests specific
care, described in the grid-fitting chapter below.</p>
</li>
<li class="emph">
<p>Glyph height</p>
<p>The glyph's vertical extent. For unscaled font
coordinates, it is</p>
<p align="center">
<tt>glyph height = bbox.yMax - bbox.yMin</tt>
</p>
<p>For scaled glyphs, its computation requests specific
care, described in the grid-fitting chapter below.</p>
</li>
<li class="emph">
<p>Right side bearing</p>
<p>Only used for horizontal layouts to describe the
distance from the bbox's right edge to the advance
width. In most cases it is a non-negative number:</p>
<p align="center">
<tt>right side bearing = advance_width -
left_side_bearing - (xMax-xMin)</tt>
</p>
<p>A common shorthand for this value is
‘rsb’.</p>
</li>
</ul>
<p>Here is a picture giving all the details for horizontal metrics:
<p align="center">
<img src="metrics.png"
height="253"
width="388"
alt="horizontal glyph metrics">
</p>
<p>And here is another one for the vertical metrics:</p>
<p align="center">
<img src="metrics2.png"
height="278"
width="294"
alt="vertical glyph metrics">
</p>
<h3 id="section-4">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 horizontally in order
to keep its three legs distinctly visible, resulting in a
wider character bitmap.</p>
<p>The glyph metrics are also influenced by the grid-fitting
process:</p>
<ul>
<li>
<p>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.</p>
</li>
<li>
<p>The image's bounding box is modified, thus modifying
the bearings.</p>
</li>
<li>
<p>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.</p>
</li>
</ul>
<p>This has some implications:</p>
<ul>
<li>
<p>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.</p>
</li>
<li>
<p>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 (for example in a cache). Some formats, like
TrueType, even include a table of pre-computed values
for a small set of common character pixel sizes.</p>
</li>
<li>
<p>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.</p>
</li>
</ul>
<p>Performing 2D transformations on glyph outlines is very
easy with FreeType. However, when using translation on
hinted outlines, one should always take care
of <b>exclusively using integer pixel distances</b> (which
means that the parameters to the
<tt>FT_Outline_Translate</tt> API function should all be
multiples of 64, as the point coordinates are in
26.6 fixed-point format). Otherwise, the translation
will simply <em>ruin the hinter's work</em>, resulting in
very low quality bitmaps!</p>
<h3 id="section-5">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>
<p>The same conventions apply to strings of text. This
means that:</p>
<ul>
<li>
<p>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.</p>
</li>
<li>
<p>The string's advance width isn't related to its
bounding box dimensions. Especially if it contains
beginning and terminal spaces or tabs.</p>
</li>
<li>
<p>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.</p>
</li>
</ul>
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<div class="updated">
<p>Last update: 07-Dec-2014</p>
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