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<h2>
II. Glyph Outlines
II. Glyph mutlines
</h2>
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<h1>
FreeType Glyph Conventions</h1></center>
<h1 align=center>
FreeType Glyph Conventions
</h1>
<h2 align=center>
Version&nbsp;2.1
</h2>
<h3 align=center>
Copyright&nbsp;1998-2000 David Turner (<a
href="mailto:david@freetype.org">david@freetype.org</a>)<br>
Copyright&nbsp;2000 The FreeType Development Team (<a
href="mailto:devel@freetype.org">devel@freetype.org</a>)
</h3>
<center>
<h2>
version 2.1</h2></center>
<table width="65%">
<tr><td>
<center>
<h3>
Copyright 1998-2000 David Turner (<a href="mailto:david@freetype.org">david@freetype.org</a>)<br>
Copyright 2000 The FreeType Development Team (<a href="devel@freetype.org">devel@freetype.org</a>)</h3></center>
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<h2>
III. Glyph metrics
</h2>
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<table width="100%"><tr valign=center bgcolor="#ccccff"><td><h2>
III. Glyph metrics
</h2></td></tr></table>
<a name="section-1">
<h3>
1. Baseline, pens and layouts
</h3>
<h3><a name="section-1">
1. Baseline, Pens and Layouts
</h3><blockquote>
<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>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 "pen position"
or "origin", is used to locate glyphs.
</p>
<p>Each layout uses a different convention for glyph placement:</p>
<p>Each layout uses a different convention for glyph placement:</p>
<ul>
<li>
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.
</li>
<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>
<p>the distance between two successive pen positions is glyph-specific
and is called the <b>advance width</b>. Note that its value is _always_
positive, even for right-to-left oriented alphabets, like Arabic. This
introduces some differences in the way text is rendered.
</p>
<a name="section-2">
<h3>
2. Typographic metrics and bounding boxes
</h3>
<p>IMPORTANT NOTE:&nbsp; The pen position is always placed on the baseline.
</p>
<p>A various number of face metrics are defined for all glyphs in a
given font.</p>
<p><center><img SRC="Image1.png" height=179 width=458></center></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>&nbsp;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>
<li>with a vertical layout, glyphs are centered around the baseline:</li>
<a name="section-3">
<h3>
3. Bearings and Advances
</h3>
<p><center><img SRC="Image2.png" height=275 width=162></center></p>
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>
<ul>
<li>
<p><em>Left side bearing</em> or <em>bearingX</em></p>
</blockquote><h3><a name="section-2">
2. Typographic metrics and bounding boxes
</h3><blockquote>
<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 one.</p>
</li>
<p>A various number of face metrics are defined for all glyphs in a given
font.</p>
<li>
<p><em>Top side bearing</em> or <em>bearingY</em></p>
<p><b>the ascent</b></p>
<ul>
<p>this is 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 Y axis upwards.
</p>
</ul>
<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>
<p><br><b>the descent</b></p>
<ul><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></ul>
<li>
<p><em>Advance width</em> or <em>advanceX</em></p>
<p><br><b>the linegap</b>
<ul>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></center>
if you use the typographic values.</ul>
Other, simpler metrics are:
<p><b>the glyph's bounding box</b>, also called "<b>bbox</b>"
<ul>this is an imaginary box that encloses all glyphs from the font, 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>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.</ul>
<b>the internal leading</b>
<ul>this concept comes directly from the world of traditional typography.
It represents the amount of space within the "leading" which is reserved
for glyph features that lay outside of the EM square (like accentuation).
It usually can be computed as:
<center>
<p><tt>internal leading = ascent - descent - EM_size</tt></center>
</ul>
<b>the external leading</b>
<ul>this is another name for the line gap.</ul>
</ul>
<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>
<h3><a name="section-3">
3. Bearings and Advances</h3>
<li>
<p><em>Advance height</em> <em>advanceY</em></p>
<ul>Each glyph has also distances called "bearings" and "advances". 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:
<p><b>the left side bearing: a.k.a. bearingX</b>
<ul>this is the horizontal distance from the current pen position to the
glyph's left bbox edge. It is positive for horizontal layouts, and most
generally negative for vertical one.</ul>
<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>
<p><br><b>the top side bearing: a.k.a. bearingY</b>
<ul>this is 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</ul>
<li>
<p><em>Glyph width</em></p>
<p><br><b>the advance width: a.k.a. advanceX</b>
<ul>is 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.</ul>
<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>
<p><br><b>the advance height: a.k.a. advanceY</b>
<ul>is 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.</ul>
<li>
<p><em>Glyph height</em>
<p><br><b>the glyph width</b>
<ul>this is simply the glyph's horizontal extent. More simply it is (bbox.xMax-bbox.xMin)
for unscaled font coordinates. For scaled glyphs, its computation requests
specific care, described in the grid-fitting chapter below.</ul>
<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>
<p><br><b>the glyph height</b>
<ul>this is simply the glyph's vertical extent. More simply, it is (bbox.yMax-bbox.yMin)
for unscaled font coordinates. For scaled glyphs, its computation requests
specific care, described in the grid-fitting chapter below.</ul>
<li>
<p><em>Right side bearing</em></p>
<p><br><b>the right side bearing</b>
<ul>is 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.</ul>
<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>
<center><tt>advance_width - left_side_bearing - (xMax-xMin)</tt></center>
<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></center>
<p>Here is a picture giving all the details for horizontal metrics:
<p>And here is another one for the vertical metrics :
<center>
<p><img SRC="Image4.png" height=278 width=294></center>
</ul>
<center><p>
<img src="Image3.png"
height=253 width=388
alt="horizontal glyph metrics">
</p></center>
<h3><a name="section-4">
4. The effects of grid-fitting</h3>
<p>And here is another one for the vertical metrics:</p>
<ul>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>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>The glyph metrics are also influenced by the grid-fitting process. Mainly
because :
<br>&nbsp;
<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>
<center><p>
<img src="Image4.png"
height=278 width=294
alt="vertical glyph metrics">
</p></center>
<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
when the hinted bitmap is larger than the scaled one, to reflect the augmented
glyph width.</li>
</ul>
<a name="section-4">
<h3>
4. The effects of grid-fitting
</h3>
<p><br>Note also that :
<br>&nbsp;
<ul>
<li>
Because of hinting, simply scaling the font ascent or descent might not
give correct results. A simple solution consists in keeping the ceiling
of the scaled ascent, and floor of the scaled descent.</li>
</ul>
<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>
<ul>
<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>
</ul>
<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>
<ul>
<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>
<p>The glyph metrics are also influenced by the grid-fitting process:
<p><br><b>IMPORTANT NOTE:</b>
<br>Performing 2D transforms on glyph outlines is very easy with FreeType.
However, when using translation on a hinted outlines, one should aways
take care of&nbsp; <b>exclusively using integer pixel distances</b> (which
means that the parameters to the FT_Translate_Outline API should all be
multiples of 64, as the point coordinates are in 26.6 fixed float format).
<p><b>Otherwise</b>, the translation will simply <b>ruin the hinter's work</b>,
resulting in a very low quality bitmaps.
<br>&nbsp;
<br>&nbsp;</ul>
<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>
<h3><a name="section-5">
&nbsp;5. Text widths and bounding box :</h3>
<p>This has some implications:</p>
<ul>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>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.
<br>&nbsp;
<p>The same conventions apply to strings of text. This means that :
<br>&nbsp;
<ul>
<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>
</ul>
<ul>
<li>
Because of hinting, simply scaling the font ascent or descent might
not give correct results. A possible solution is to keepthe ceiling
of the scaled ascent, and floor of the scaled descent.
</li>
<ul>
<li>
The string's advance width isn't related to its bounding box's dimensions.
Especially if it contains beginning and terminal spaces or tabs.</li>
</ul>
<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>
<ul>
<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>
</ul>
</ul>
</ul>
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<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_Translate_Outline()</tt> API should all be multiples
of&nbsp;64, as the point coordinates are in 26.6&nbsp;fixed float
format).</p>
</td></tr></table></center>
<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>
<p><hr></p>
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