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<html><head><title>Anti-Grain Geometry - Scanlines and Scanline
Renderers</title>
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<table width="640px"><tbody><tr><td><h1>Scanlines and Scanline Renderers</h1></td></tr></tbody></table>
<table class="toc" width="640px"><tbody><tr><td>
<div style="margin-left: 2em; padding: 3px; font-size: 14px;"><a
href="#toc0001"><b>Scanline Containers</b></a>
<div style="margin-left: 2em; font-size: 12px;"><a
href="#toc0002">Introduction</a></div>
<div style="margin-left: 2em; font-size: 12px;"><a
href="#toc0003">Interfaces</a>
<div style="margin-left: 2em; font-size: 12px;"><a
href="#toc0004">Filling Interface (put)</a></div>
<div style="margin-left: 2em; font-size: 12px;"><a
href="#toc0005">Iterating Interface (get)</a></div></div></div>
<div style="margin-left: 2em; padding: 3px; font-size: 14px;"><a
href="#toc0006"><b>Scanline Renderers</b></a></div>
</td></tr></tbody></table>
<h2>Scanline Containers<a name="toc0001"></a></h2>
<h3>Introduction<a name="toc0002"></a></h3>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>The
low level renderers operate with
simplest data and they are very simple too. In fact, the pixel format
renderers are not the obligatory part of the library and can be
replaced or rewritten. For example, if you have an API with similar
functionality, but hardware accelerated, it will be better to use it
instead of pure software blending (the low level renderers are
essentially alpha-blenders). It's also possible to use Intel SSE/SSE2
to write optimized renderers. All other rendering functionality in
AGG is based on these simple classes.</p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>To
draw Anti-Aliased primitives one shoud <b>rasterize</b> them first.
The primary rasterization technique in AGG is scanline based. That is,
a polygon is converted into a number of horizontal scanlines and then
the scanlines are being rendered one by one. Again, the scanline
rasterizer is not the only class that can produce scanlines. It can
be some container or even your own super-ultra-mega rasterizer.</p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>To
transfer information from a rasterizer to the scanline renderer
there scanline containers are used.
A scanline consists of a number of horizontal, non-intersecting spans.
All spans must be ordered by X. It means that there is no sorting
operation provided, the order must be perserved when adding spans
to the scanline. If the order is not guaranteed it can result in
undefined behaviour.</p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>In <b>AGG</b>
there are three types of scanline containers:
</p><ul type="disc">
<li><code>scanline_u</code> - unpacked scaline container</li>
<li><code>scanline_p</code> - packed scanline container</li>
<li><code><a
href="http://www.antigrain.com/__code/include/agg_scanline_bin.h.html#scanline_bin">scanline_bin</a></code>
- container for binary, “aliased” scanlines</li></ul><p></p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>First
two containers can keep <b><nobr>Anti-Aliasing</nobr></b> information,
the third one cannot.</p></td></tr></tbody></table>
<table class="warning" width="640px"><tbody><tr><td><b>IMPORTANT!</b><br>
All the scanline containers are optimized for speed, not for
memory usage. In fact, it allocates memory for the worst case,
so there is some overhead. It's not critical when you use only
few scanline containers, but it's not a good idea to use an
array of scanlines to store the whole shape because it will take
more memory than the resulting image.
</td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>The
difference between <b>packed</b> and <b>unpacked</b> scanline
containers is that the <b>unpacked</b> scaline always keeps the
coverage values for all pixels including the ones that are
fully covered by a polygon. In the <b>packed</b> scanline
all pixels with the same coverage value are merged into <b>solid</b>
spans.
</p></td></tr></tbody></table><table width="640px"><tbody><tr><td><center><img
src="scanlines.agdoc_files/scanline_u.gif" title="" border="0"><br><i></i></center></td></tr></tbody></table>
<table width="640px"><tbody><tr><td><center><img
src="scanlines.agdoc_files/scanline_p.gif" title="" border="0"><br><i></i></center></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>It
can seem it's always better to use the packed version, but in
practice it's not so. The scanline_p works faster when rendering
large solid polygons, that is when the area of the polygon is
much larger than the perimeter in the meaning of the number
of pixels. But when renering text it's definitely better to use
the scanline_u because of much less number of processed spans
The difference is about three times and the number of spans
is also important for the overall performance. Besides, in most
of the span generators, such as gradients, Gouraud shader, or
image transformers, the number of spans becomes even more critical,
and so, the scanline_p is not used there.</p></td></tr></tbody></table>
<h3>Interfaces<a name="toc0003"></a></h3>
<h4>Filling Interface (put)<a name="toc0004"></a></h4>
<h4>Iterating Interface (get)<a name="toc0005"></a></h4>
<br><h2>Scanline Renderers<a name="toc0006"></a></h2>
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<tbody><tr><td><center><span class="authors">
Copyright <span class="larger">©</span> 2002-2006
<a href="http://www.antigrain.com/mcseem/index.html"><b>Maxim Shemanarev</b></a>
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<tr><td><center><span class="authors">
Web Design and Programming
<a href="http://www.antigrain.com/mcseem/index.html"><b>Maxim Shemanarev</b></a>
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