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<h1><a href="http://freetype.org/index.html">FreeType</a> Glyph
Conventions / VII</h1>
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<h2>VII. FreeType Bitmaps</h2>
<p>The purpose of this section is to present the way
FreeType manages bitmaps and pixmaps, and how they relate
to the concepts previously defined. The relationship
between vectorial and pixel coordinates is explained.</p>
<h3 id="section-1">1. Vectorial versus pixel
coordinates</h3>
<p>This sub-section explains the difference between
vectorial and pixel coordinates. To make things clear,
brackets will be used to describe pixel coordinates,
e.g. ‘[3,5]’, while parentheses will be used
for vectorial ones, e.g. ‘(-2, 3.5)’.</p>
<p>In the pixel case, as we use the <em>Y upwards</em>
convention; the coordinate [0, 0] always refers to
the <em>lower left pixel</em> of a bitmap, while
coordinate [width-1, rows-1] to its <em>upper right
pixel</em>.</p>
<p>In the vectorial case, point coordinates are expressed in
floating units, like (1.25, -2.3). Such a position
doesn't refer to a given pixel, but simply to an
immaterial point in the 2D plane.</p>
<p>The pixels themselves are indeed <em>square boxes</em> of
the 2D plane, whose centers lie in half pixel coordinates.
For example, the lower left pixel of a bitmap is delimited
by the square (0, 0)-(1, 1), its center being at
location (0.5, 0.5).</p>
<p>This introduces some differences when computing
distances. For example, the <em>length</em> in pixels of
the line [0, 0]-[10, 0] is 11. However,
the vectorial distance between (0, 0)-(10, 0)
covers exactly 10 pixel centers, hence its length
is 10.</p>
<p align="center">
<img src="grid_1.png"
height="390"
width="402"
alt="bitmap and vector grid">
</p>
<h3 id="section-2">2. The <code>FT_Bitmap</code> descriptor</h3>
<p>In FreeType, a bitmap or pixmap is described through a
single structure
called <a href="../reference/ft2-basic_types.html#FT_Bitmap"><code>FT_Bitmap</code></a>.
The fields we are interested in are:</p>
<table class="vertical-space">
<caption>
<b><code>FT_Bitmap</code></b>
</caption>
<tr>
<td valign="top">
<code>rows</code>
</td>
<td valign="top">
the number of rows, i.e., lines, in the bitmap
</td>
</tr>
<tr>
<td valign="top">
<code>width</code>
</td>
<td valign="top">
the number of horizontal pixels in the bitmap
</td>
</tr>
<tr>
<td valign="top">
<code>pitch</code>
</td>
<td valign="top">
its absolute value is the number of bytes per bitmap
line; it can be either positive or negative depending
on the bitmap's vertical orientation
</td>
</tr>
<tr>
<td valign="top">
<code>buffer</code>
</td>
<td valign="top">
a typeless pointer to the bitmap pixel buffer
</td>
</tr>
<tr>
<td valign="top">
<code>pixel_mode</code>
</td>
<td valign="top">
an enumeration used to describe the pixel format of
the bitmap; examples
are <code>ft_pixel_mode_mono</code> for 1-bit
monochrome bitmaps
and <code>ft_pixel_mode_grays</code> for 8-bit
anti-aliased ‘gray’ values
</td>
</tr>
<tr>
<td valign="top">
<code>num_grays</code>
</td>
<td valign="top">
this is only used for ‘gray’ pixel modes,
it gives the number of gray levels used to describe
the anti-aliased gray levels (256 by default with
FreeType 2)
</td>
</tr>
</table>
<p>Note that the sign of the <code>pitch</code> field
determines whether the rows in the pixel buffer are stored
in ascending or descending order.</p>
<p>Remember that FreeType uses the <em>Y upwards</em>
convention in the 2D plane, which means that a coordinate
of (0, 0) always refer to the <em>lower-left
corner</em> of a bitmap.</p>
<p>If the pitch is positive, the rows are stored in
decreasing vertical position; the first bytes of the pixel
buffer are part of the <em>upper</em> bitmap row.</p>
<p>On the opposite, if the pitch is negative, the first
bytes of the pixel buffer are part of the <em>lower</em>
bitmap row.</p>
<p>In all cases, one can see the pitch as the byte increment
needed to skip to the <em>next lower scanline</em> in a
given bitmap buffer.</p>
<table class="vertical-space">
<tr>
<td>
<img src="up_flow.png"
height="261"
width="275"
alt="negative 'pitch'">
</td>
<td>
<img src="down_flow.png"
height="263"
width="273"
alt="positive 'pitch'">
</td>
</tr>
</table>
<p>The ‘positive pitch’ convention is very often
used, though some systems might need the other.</p>
<p>To speed up memory access, <code>pitch</code> is in most
cases a multiple of 16bit, 32bit, or even 64bit. It often
happens that the pitch is thus larger than the necessary
bits (or bytes) for a bitmap or pixmap row; in such cases,
unused bits (or bytes) are at the very right (i.e., the
end) of a row.</p>
<h3 id="section-3">3. Converting outlines into bitmaps and
pixmaps</h3>
<p>Generating a bitmap or pixmap image from a vectorial
image is easy with FreeType. However, one must understand
a few points regarding the positioning of the outline in
the 2D plane before converting it to a bitmap:</p>
<ul>
<li>
<p>The glyph loader and hinter always places the outline
in the 2D plane so that (0, 0) matches its
character origin. This means that the glyph's outline
(and corresponding bounding box), can be placed
anywhere in the 2D plane (see the graphics in
section III).</p>
</li>
<li>
<p>The target bitmap's area is mapped to the 2D plane,
with its lower left corner at (0, 0). This means
that a bitmap or pixmap of dimensions
[<tt>w</tt>, <tt>h</tt>] will be mapped to a 2D
rectangle window delimited by
(0, 0)-(<tt>w</tt>, <tt>h</tt>).</p>
</li>
<li>
<p>When scan-converting the outline, everything that
falls within the bitmap window is rendered, the rest
is ignored.</p>
</li>
<p>A common mistake made by many developers when they
begin using FreeType is believing that a loaded outline
can be directly rendered in a bitmap of adequate
dimensions. The following images illustrate why this is
a problem.</p>
<ul>
<li>
The first image shows a loaded outline in the 2D
plane.
</li>
<li>
The second one shows the target window for a bitmap of
arbitrary dimensions [<tt>w</tt>, <tt>h</tt>].
</li>
<li>
The third one shows the juxtaposition of the outline
and window in the 2D plane.
</li>
<li>
The last image shows what will really be rendered in
the bitmap.
</li>
</ul>
<p align="center">
<img src="clipping.png"
height="151"
width="539"
alt="clipping algorithm">
</p>
</ul>
<p>Indeed, in nearly all cases, the loaded or transformed
outline must be translated before it is rendered into a
target bitmap, in order to adjust its position relative to
the target window.</p>
<p>For example, the correct way of creating
a <em>stand-alone</em> glyph bitmap is as follows:</p>
<ul>
<li>
<p>Get the size of the glyph bitmap. It can be computed
directly from the glyph metrics, or by computing its
bounding box (this is useful when a transformation has
been applied to the outline after loading it, as the
glyph metrics are not valid anymore).</p>
</li>
<li>
<p>Create the bitmap with the computed dimensions.
Don't forget to fill the pixel buffer with the
background color.</p>
</li>
<li>
<p>Translate the outline so that its lower left corner
matches (0, 0). Don't forget that in order to
preserve hinting, one should use integer, i.e.,
rounded distances (of course, this isn't required if
preserving hinting information doesn't matter, like
with rotated text). Usually, this means translating
with a vector
(<tt>-ROUND(xMin)</tt>, <tt>-ROUND(yMin)</tt>).</p>
</li>
<li>
<p>Call the rendering function (it can be
<a href="../reference/ft2-outline_processing.html#FT_Outline_Render"><code>FT_Outline_Render</code></a>,
for example).</p>
</li>
</ul>
<p>In the case where one wants to write glyph images
directly into a large bitmap, the outlines must be
translated so that their vectorial position corresponds to
the current text cursor or character origin.</p>
</div>
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<div class="updated">
<p>Last update: 02-May-2017</p>
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<a href="index.html">FreeType Glyph Conventions</a>
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