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FreeType Glyph Conventions
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<center><h2>
version 2.1
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<center><h3>
Copyright 1998-2000 David Turner (<a href="mailto:[email protected]">[email protected]</a>)<br>
Copyright 2000 The FreeType Development Team (<a href="[email protected]">[email protected]</a>)
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<table width="100%"><tr valign=center bgcolor="#CCCCFF"><td><h2>
VI. FreeType Outlines
</h2></td></tr></table>
<p>The purpose of this section is to present the way FreeType
manages vectorial outlines, as well as the most common operations that
can be applied on them.
</p>
<h3><a name="section-1">
1. FreeType outline description and structure :
</h3><blockquote>
<h4>
a. Outline curve decomposition :
</h4>
<p>An outline is described as a series of closed contours in the
2D plane. Each contour is made of a series of line segments and bezier
arcs. Depending on the file format, these can be second-order or third-order
polynomials. The former are also called quadratic or conic arcs, and they
come from the TrueType format. The latter are called cubic arcs and mostly
come from the Type1 format.
</p>
<p>Each arc is described through a series of start, end and control points.
Each point of the outline has a specific tag which indicates wether it
is used to describe a line segment or an arc. The tags can take the
following values :
</p>
<center><table CELLSPACING=5 CELLPADDING=5 WIDTH="80%">
<tr VALIGN=TOP><td>
<p><b>FT_Curve_Tag_On </b></p>
</td>
<td VALIGN=TOP>
<p>Used when the point is "on" the curve. This corresponds to
start and end points of segments and arcs. The other tags specify what
is called an "off" point, i.e. one which isn't located on the contour itself,
but serves as a control point for a bezier arc.</p>
</td>
</tr>
<tr>
<td>
<p><b>FT_Curve_Tag_Conic</b></p>
</td>
<td>
<p>Used for an "off" point used to control a conic bezier arc.</p>
</td>
</tr>
<tr>
<td>
<p><b>FT_Curve_Tag_Cubic</b></p>
</td>
<td>
<p>Used for an "off" point used to control a cubic bezier arc.</p>
</td>
</tr>
</table></center>
<p>The following rules are applied to decompose the contour's points into
segments and arcs :
</p>
<ul>
<li>two successive "on" points indicate a line segment joining them.</li>
<li>one conic "off" point amidst two "on" points indicates a conic bezier
arc, the "off" point being the control point, and the "on" ones the
start and end points.</li>
<li>
Two successive cubic "off" points amidst two "on" points indicate a cubic
bezier arc. There must be exactly two cubic control points and two on
points for each cubic arc (using a single cubic "off" point between two
"on" points is forbidden, for example).
</li>
<li>
finally, two successive conic "off" points forces the rasterizer to create
(during the scan-line conversion process exclusively) a virtual "on" point
amidst them, at their exact middle. This greatly facilitates the definition
of successive conic bezier arcs. Moreover, it's the way outlines are
described in the TrueType specification.
</li>
</ul>
<p><br>Note that it is possible to mix conic and cubic arcs in a single
contour, even though no current font driver produces such outlines.
<br> </ul>
<center><table>
<tr>
<td>
<blockquote><img SRC="points_segment.png" height=166 width=221></blockquote>
</td>
<td>
<blockquote><img SRC="points_conic.png" height=183 width=236></blockquote>
</td>
</tr>
<tr>
<td>
<blockquote><img SRC="points_cubic.png" height=162 width=214></blockquote>
</td>
<td>
<blockquote><img SRC="points_conic2.png" height=204 width=225></blockquote>
</td>
</tr>
</table></center>
<h4>
b. Outline descriptor :</h4>
<p>A FreeType outline is described through a simple structure,
called <tt>FT_Outline</tt>, which fields are :</p>
<center><table CELLSPACING=3 CELLPADDING=3 BGCOLOR="#CCCCCC">
<tr>
<td>
<p><b><tt>n_points</tt></b></p>
</td>
<td>
<p>the number of points in the outline</p>
</td>
</tr>
<tr>
<td>
<p><b><tt>n_contours</tt></b></p>
</td>
<td>
<p>the number of contours in the outline</p>
</td>
</tr>
<tr>
<td>
<p><b><tt>points</tt></b></p>
</td>
<td>
<p>array of point coordinates</p>
</td>
</tr>
<tr>
<td>
<p><b><tt>contours</tt></b></p>
</td>
<td>
<p>array of contour end indices</p>
</td>
</tr>
<tr>
<td>
<p><b><tt>tags</tt></b></p>
</td>
<td>
<p>array of point flags</p>
</td>
</tr>
</table></center>
<p>Here, <b><tt>points</tt></b> is a pointer to an array of
<tt>FT_Vector</tt> records, used to store the vectorial coordinates of each
outline point. These are expressed in 1/64th of a pixel, which is also
known as the <i>26.6 fixed float format</i>.
</p>
<p><b><tt>contours</tt></b> is an array of point indices used to delimit
contours in the outline. For example, the first contour always starts at
point 0, and ends a point <b><tt>contours[0]</tt></b>. The second contour
starts at point "<b><tt>contours[0]+1</tt></b>" and ends at
<b><tt>contours[1]</tt></b>, etc..
</p>
<p>Note that each contour is closed, and that <b><tt>n_points</tt></b>
should be equal to "<b><tt>contours[n_contours-1]+1</tt></b>" for a valid
outline.
</p>
<p>Finally, <b><tt>tags</tt></b> is an array of bytes, used to store each
outline point's tag.
</p>
</blockquote><h3><a name="section-2">
2. Bounding and control box computations :
</h3><blockquote>
<p>A <b>bounding box</b> (also called "<b>bbox</b>") is simply
the smallest possible rectangle that encloses the shape of a given outline.
Because of the way arcs are defined, bezier control points are not
necessarily contained within an outline's bounding box.
</p>
<p>This situation happens when one bezier arc is, for example, the upper
edge of an outline and an off point happens to be above the bbox. However,
it is very rare in the case of character outlines because most font designers
and creation tools always place on points at the extrema of each curved
edges, as it makes hinting much easier.
</p>
<p>We thus define the <b>control box</b> (a.k.a. the "<b>cbox</b>") as
the smallest possible rectangle that encloses all points of a given outline
(including its off points). Clearly, it always includes the bbox, and equates
it in most cases.
</p>
<p>Unlike the bbox, the cbox is also much faster to compute.</p>
<center><table>
<tr>
<td><img SRC="bbox1.png" height=264 width=228></td>
<td><img SRC="bbox2.png" height=229 width=217></td>
</tr>
</table></center>
<p>Control and bounding boxes can be computed automatically through the
functions <b><tt>FT_Get_Outline_CBox</tt></b> and <b><tt>FT_Get_Outline_BBox</tt></b>.
The former function is always very fast, while the latter <i>may</i> be
slow in the case of "outside" control points (as it needs to find the extreme
of conic and cubic arcs for "perfect" computations). If this isn't the
case, it's as fast as computing the control box.
<p>Note also that even though most glyph outlines have equal cbox and bbox
to ease hinting, this is not necessary the case anymore when a
transform like rotation is applied to them.
</p>
</blockquote><h3><a name="section-3">
3. Coordinates, scaling and grid-fitting :
</h3><blockquote>
<p>An outline point's vectorial coordinates are expressed in the
26.6 format, i.e. in 1/64th of a pixel, hence coordinates (1.0, -2.5) is
stored as the integer pair ( x:64, y: -192 ).
</p>
<p>After a master glyph outline is scaled from the EM grid to the current
character dimensions, the hinter or grid-fitter is in charge of aligning
important outline points (mainly edge delimiters) to the pixel grid. Even
though this process is much too complex to be described in a few lines,
its purpose is mainly to round point positions, while trying to preserve
important properties like widths, stems, etc..
</p>
<p>The following operations can be used to round vectorial distances in
the 26.6 format to the grid :
</p>
<center>
<p><tt>round(x) == (x+32) & -64</tt>
<br><tt>floor(x) == x &
-64</tt>
<br><tt>ceiling(x) == (x+63) & -64</tt></center>
<p>Once a glyph outline is grid-fitted or transformed, it often is interesting
to compute the glyph image's pixel dimensions before rendering it. To do
so, one has to consider the following :
<p>The scan-line converter draws all the pixels whose <i>centers</i> fall
inside the glyph shape. It can also detect "<b><i>drop-outs</i></b>", i.e.
discontinuities coming from extremely thin shape fragments, in order to
draw the "missing" pixels. These new pixels are always located at a distance
less than half of a pixel but one cannot predict easily where they'll appear
before rendering.
<p>This leads to the following computations :
<br>
<ul>
<li>
compute the bbox</li>
</ul>
<ul>
<li>
grid-fit the bounding box with the following :</li>
</ul>
<ul><p>
<ul><tt>xmin = floor( bbox.xMin )</tt>
<br><tt>xmax = ceiling( bbox.xMax )</tt>
<br><tt>ymin = floor( bbox.yMin )</tt>
<br><tt>ymax = ceiling( bbox.yMax )</tt>
</p></ul>
<li>
return pixel dimensions, i.e.
<tt>width = (xmax - xmin)/64</tt> and <tt>height = (ymax - ymin)/64</tt>
</li>
</ul>
<p><br>By grid-fitting the bounding box, one guarantees that all the pixel
centers that are to be drawn, <b><i>including those coming from drop-out
control</i></b>, will be <b><i>within</i></b> the adjusted box. Then the
box's dimensions in pixels can be computed.
<p>Note also that, when <i>translating</i> a <i>grid-fitted outline</i>,
one should <b><i>always</i></b> use <b><i>integer distances</i></b> to
move an outline in the 2D plane. Otherwise, glyph edges won't be aligned
on the pixel grid anymore, and the hinter's work will be lost, producing
<b><i>very
low quality </i></b>bitmaps and pixmaps..</blockquote>
</blockquote>
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