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<h1>
FreeType Glyph Conventions</h1></center>
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<h2>
version 2.1</h2></center>
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<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>)</h3></center>
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<table width="100%"><tr valign=center bgcolor="#CCCCFF"><td><h2>
VII. FreeType Bitmaps
</h2></td></tr></table>
<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 relationships between vectorial and pixel coordinates is
explained.
</p>
<h3><a name="section-1">
1. Vectorial versus pixel coordinates :
</h3><blockquote>
<p>This sub-section explains the differences 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>In the pixel case, as we use the <i>Y upwards</i> convention, the coordinate
[0,0] always refers to the <i>lower left pixel</i> of a bitmap, while coordinate
[width-1, rows-1] to its <i>upper right pixel</i>.
<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>The pixels themselves are indeed <i>square boxes</i> of the 2D plane,
which centers lie in half pixel coordinates. For example, the <i>lower
left pixel</i> of a bitmap is delimited by the <i>square</i> (0,0)-(1,1),
its center being at location (0.5,0.5).
<p>This introduces some differences when computing distances. For example,
the "<i>length</i>" 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 if 10.
<center><img SRC="grid_1.png" height=390 width=402></center>
</blockquote><h3><a name="section-2">
2. FreeType bitmap and pixmap descriptor :
</h3><blockquote>
<p>A bitmap or pixmap is described through a single structure,
called <tt>FT_Bitmap</tt>, defined in the file
<tt><freetype/ftimage.h></tt>. It is a simple descriptor whose fields are:</p>
<center><table CELLSPACING=3 CELLPADDING=5 BGCOLOR="#CCCCCC" width="80%" >
<caption><tt>FT_Bitmap</tt></caption>
<tr>
<td><b>rows</b></td>
<td>the number of rows, i.e. lines, in the bitmap</td>
</tr>
<tr>
<td><b>width</b></td>
<td>the number of horizontal pixels in the bitmap</td>
</tr>
<tr>
<td><b>pitch</b></td>
<td>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><b>buffer</b></td>
<td>a typeless pointer to the bitmap pixel bufer</td>
</tr>
<tr>
<td><b>pixel_mode</b></td>
<td>an enumeration used to describe the pixel format of the bitmap.
Examples are: <tt>ft_pixel_mode_mono</tt> for 1-bit monochrome bitmaps
and <tt>ft_pixel_mode_grays</tt> for 8-bit anti-aliased "gray" values
</td>
<tr>
<td><b>num_grays</b></td>
<td>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></center>
<p>Note that the sign of the <b><tt>pitch</tt></b> fields determines wether
the rows in the pixel buffer are stored in ascending or descending order.
</p>
<p>Remember that FreeType uses the <i>Y upwards</i> convention in the 2D
plane. Which means that a coordinate of (0,0) always refer to the
<i>lower-left corner</i> of a bitmap.
</p>
<p>When the pitch is positive, the rows are stored in decreasing vertical
position, which means that the first bytes of the pixel buffer are part
of the <i>upper</i> bitmap row.
</p>
<p>On the opposite, when the pitch is negative, the first bytes of the
pixel buffer are part of the <i>lower</i> 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>
<p>The two conventions are detailed by this graphics:</p>
<center><table>
<tr>
<td><img SRC="up_flow.png" height=261 width=275></td>
<td><img SRC="down_flow.png" height=263 width=273></td>
</tr>
</table></center>
<p>The <em>positive pitch</em> convention is very often used, though
some systems might need otherwise.</p>
</blockquote><h3><a name="section-3">
3. Converting outlines into bitmaps and pixmaps :
</h3><blockquote>
<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. These are :</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,h</tt>] will be mapped to a 2D rectangle window delimited by
(0,0)-(<tt>w,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>
<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
[w,h]
</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>
</ul>
<center><img SRC="clipping.png" height=151 width=539></center>
<p><br>
<br>
<br>
<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 <i>standalone</i> glyph bitmap
is thus to :
</p>
<ul>
<li><p>
Compute 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 transform has been applied to the outline after the load, 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), -ROUND(yMin) )</tt>.
</p></li>
<li><p>
Call the rendering function (it can be <tt>FT_Outline_Render</tt> for
example).
</p></li>
<p><br>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 correspond to the current text cursor/character origin.</blockquote>
</blockquote>
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